JP5297299B2 - Magneto-optical property measuring apparatus and magneto-optical property measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magneto-optical characteristic measuring device and method, measuring accurately, inexpensively and simply magneto-optical characteristics of an anisotropic magnetic field of a ferromagnetic material or the like. <P>SOLUTION: This magneto-optical characteristic measuring device 1, is a mode-locked laser for generating an optical pulse train having a highly repeated period synchronizing with a period of optical excitation precession of magnetization of a sample F by a laser light source 2, and irradiating the sample therewith. An external magnetic field application means 4 applies a prescribed external magnetic field to the sample by an electromagnet. A polarized light detector 5 detects reflected light acquired by reflection of the optical pulse train by the sample, and outputs a polarized component as a magneto-optical signal. A control device 6 controls the external magnetic field application means to acquire as a resonance condition, an external magnetic field and the magneto-optical signal when the optical excitation precession of magnetization of the sample is synchronized with the period of the optical pulse train, and calculates an effective internal magnetic field or an anisotropic magnetic field which is a magneto-optical characteristic of the sample and a damping factor based on an LLG equation, by using the period of the optical pulse train in the resonance condition, intensity of the external magnetic field and the magneto-optical signal. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a measuring apparatus for measuring magnetic characteristics of a magnetic material and a measuring method thereof, and more particularly to a magneto-optical characteristic measuring apparatus and a magnetic material for a ferromagnetic material using a resonance phenomenon between light-induced precession and high repetition light pulse excitation. The present invention relates to a method for measuring optical characteristics.

Spintronic devices represented by MRAM (Magnetoresistive Random Access Memory) and spin MOSFET (Metal Oxide Semiconductor Field Effect Transistor) are manufactured based on a new technique called spin injection magnetization reversal. Spin injection magnetization reversal is to control the direction of magnetization in the device by the flow (current) of electrons with uniform spin. In the technical field of the current induced magnetization switching, recently, theory and has been performed so many studies from both experimental, it is strongly demanded to reduce the critical current I _c necessary to control the magnetization ing. If this is realized, dramatic improvements in performance will be expected in spintronic devices, and the range of applications will expand.

From the previous theoretical study, in order to reduce the value of the critical current I _c, a magnetic material having a small anisotropic magnetic field H _ani or a magnetic material having a small parameter α called a damping factor should be selected. Guidance is obtained. Here, the anisotropic magnetic field H _ani is an effective internal magnetic field due to the exchange interaction of magnetic ions, and the damping factor α is a phenomenological parameter that represents the relaxation process of magnetization precession. is there. However, it has not been clarified how the anisotropic magnetic field H _ani and the damping factor α change depending on the substance. For the above reasons, research on the anisotropic magnetic field H _ani and the damping factor α of various substances is underway.

In recent years, for example, as shown in Non-Patent Document 1, research results and the like for determining an anisotropic magnetic field _Hani and a damping factor α using ultrafast time-resolved magneto-optical spectroscopy have been reported. These studies are based on the following principles. When a ferromagnetic material is irradiated with light, a phenomenon occurs in which magnetic anisotropy changes due to a rise in temperature or a change in carrier concentration, and magnetization (magnetization vector) starts precession. Therefore, in these studies, we use the femtosecond time-resolved magneto-optical measurement method to observe the precession of magnetization, analyze the observation data by calculation using a theoretical model, and analyze the anisotropic magnetic field H _ani. And a damping factor α are determined.

  In addition, it is known that when a paramagnetic compound semiconductor is irradiated with circularly polarized light, spin-polarized carriers (carrier spin) are generated. When a magnetic field is applied in the voict arrangement (also referred to as a Voigt arrangement or a forked arrangement), the carrier spin starts precession with a period that reflects the magnitude of the applied magnetic field. This behavior can be observed by ultrafast time-resolved spectroscopy using pulsed light output from a mode-locked pulse laser with a pulse repetition period of approximately 80 [MHz]. The voicing arrangement is an arrangement in which a magnetic field is applied in a direction orthogonal to the light traveling direction.

  When the repetition period of the pulsed light (laser light) is shorter than the relaxation time of the carrier spin of the paramagnetic material, the precession motions of the spins injected by the plural pulsed light excitations interfere with each other. As a result, when the repetition period of the pulsed light is an integral multiple of the period of the precession of the carrier spin, a phenomenon occurs in which the amplitude of the precession of the carrier spin is resonantly amplified (see Non-Patent Document 2). ). This phenomenon is called Resonant Spin Accumulation (RSA).

  On the other hand, the spin behavior in a ferromagnetic material is different from the spin behavior in a paramagnetic material. That is, in the case of a ferromagnetic material, it is necessary to consider the behavior of a large number of collective spins that interact with each other and are oriented in one direction. However, in this case as well, it has been experimentally shown that the spin precession interference occurs as in the case of the paramagnetic material by irradiating the ferromagnetic material with a plurality of pulse lights. It is well described (see Non-Patent Document 3).

Y. Hashimoto et al., "Photoinduced Precession of Magnetization in Ferromagnetic (Ga, Mn) As" Phys. Rev. Lett., 100, 067202 (2008) J.M.Kikkawa and D.D.Awschalom, "Resonant Spin Amplification in n-Type GaAs" Phys. Rev. Lett. Vol.80, No.19, 4313 (1998) Y. Hashimoto et al., "Coherent manipulation of magnetization precession in compressing semiconductor (Ga, Mn) As with successive optical pumping" Appl. Phys. Lett., 93, 202506 (2008)

However, the ferromagnetic resonance method and the ultrafast time-resolved magneto-optical spectroscopy conventionally used for determining the anisotropic magnetic field _Hani and the damping factor α of the ferromagnetic material have the following problems.
For example, ultrafast time-resolved magneto-optical spectroscopy observes changes in the magneto-optical effect induced by a single pulsed light irradiation. Therefore, it cannot be said that the measurement accuracy is sufficient, and it cannot be evaluated depending on the type of the ferromagnetic sample. In addition, large and expensive measuring devices and skilled experimental techniques are essential for experiments. The ferromagnetic resonance method also requires a large and expensive measuring device and a skilled experimental technique for the experiment.

  Therefore, the present invention has been made in view of the above-described problems, and is capable of accurately and inexpensively and easily measuring magneto-optical characteristics such as an anisotropic magnetic field of a ferromagnetic sample. An object of the present invention is to provide an optical property measuring apparatus and a magneto-optical property measuring method.

  In order to achieve the above object, a magneto-optical characteristic device according to claim 1 of the present invention is characterized by a period of photoexcited precession of magnetization of a ferromagnetic sample and a high repetition of irradiation of the sample from a laser light source. A magneto-optical characteristic measuring apparatus that synchronizes the period of a pulsed laser beam and measures the magneto-optical characteristic of the sample, the laser light source generating laser light and irradiating the sample, and an external magnetic field applied to the sample An external magnetic field applying means for applying, a polarization component detecting means for detecting a polarization component of detection light from the sample and outputting it as a magneto-optical signal, and a control device for calculating the magneto-optical characteristics .

  According to the magneto-optical characteristic apparatus configured as described above, the laser light source is a mode-locked laser that generates a high-repetition period optical pulse train that can be synchronized with the period of photoexcited precession of the magnetization of the sample and irradiates the sample. is there. The external magnetic field applying means is provided adjacent to the sample and applies a predetermined external magnetic field to the sample by an electromagnet. The polarization component detecting means detects reflected light reflected by the sample from the irradiated optical pulse train or transmitted light transmitted through the sample by the optical pulse train, and controls the polarization component as a magneto-optical signal. Output to the device. In this polarization component detection means, when detecting reflected light, the polarization component as magnetic characteristics is detected by the Kerr effect principle, and when detecting transmitted light, polarization as magnetic characteristics by the Faraday effect principle is detected. Detect ingredients.

  In the magneto-optical characteristic device having such a configuration, the control device controls the external magnetic field applying unit, and the external magnetic field when the photoexcitation precession of the magnetization of the sample is synchronized with the period of the optical pulse train. And the magneto-optical signal as a resonance condition, and based on an LLG (Landau-Lifshitz-Gilbert) equation using the period of the optical pulse train, the intensity of the external magnetic field, and the magneto-optical signal under the resonance condition Then, an effective internal magnetic field or anisotropic magnetic field, which is a magneto-optical characteristic of the sample, and a damping factor are calculated.

  The magneto-optical characteristic device according to claim 2 of the present invention is the magneto-optical measurement device according to claim 1, wherein the polarization component detection means includes a light quantity detection means, a photoelastic modulator, and a lock-in amplifier. The light amount detecting means detects the intensity of the reflected light reflected by the sample or the transmitted light transmitted through the sample by the light pulse train, and the light The elastic modulator is arranged in the middle of the optical path of the optical pulse train irradiated on the sample from the laser light source, and outputs the optical pulse train by alternately switching to a counterclockwise circular polarization and a clockwise circular polarization at a predetermined frequency. The lock-in amplifier uses a frequency signal obtained by modulating the optical pulse train by the photoelastic modulator as a reference signal, and results of measuring a polarization component from a light intensity signal detected by the light amount detecting means. Examples magneto-optical signal, lock-in detection is output to the control device.

  According to the magneto-optical measurement apparatus having such a configuration, the polarization detection unit includes the light amount detection unit, the photoelastic modulator, and the lock-in amplifier, and indirectly outputs the magneto-optical signal from the time-modulated light intensity. To detect. That is, when the irradiated light pulse train reflects off the sample, or when the light pulse train passes through the sample and enters the light amount detection means, the lock-in amplifier turns counterclockwise according to the principle of circular dichroism. A polarization component as a magnetic characteristic is detected from a difference between circularly polarized light and clockwise circularly polarized light.

The magneto-optical measurement apparatus according to claim 3 is the magneto-optical measurement apparatus according to claim 1 or 2, wherein the external magnetic field applying means is an electromagnet disposed on both sides of the sample. An in-plane magnetic field parallel to the sample surface is generated, and an in-plane magnetic field is applied to the sample surface.
The magneto-optical measurement apparatus according to claim 4 is the magneto-optical measurement apparatus according to claim 1 or 2, wherein the external magnetic field applying unit is an electromagnet disposed on the back side of the sample. A perpendicular magnetic field perpendicular to the sample surface is generated, and a perpendicular magnetic field is applied to the sample surface.

  According to the magneto-optical measurement apparatus having such a configuration, the external magnetic field applying means can be an in-plane magnetic field application method or a plane magnetic field application method depending on the sample form or physical characteristics depending on the sample preparation conditions.

  A magneto-optical measurement apparatus according to claim 5 is the magneto-optical measurement apparatus according to any one of claims 1 to 4, wherein an optical lens is provided in an optical path between the sample and the laser light source. It further has a certain condensing lens.

  The condensing lens is an optical lens that condenses the optical pulse train from the mode-locked laser that is the laser light source and irradiates the sample, and increases the optical excitation intensity in the measurement of the magneto-optical characteristics of the sample. The detection resolution of the optical signal is improved and the spatial resolution is improved.

  In order to achieve the above object, a magneto-optical characteristic measuring method according to claim 6 of the present invention comprises a laser light source, an external magnetic field applying means, a polarization component detecting means, and a control device. A method for measuring a magneto-optical characteristic of a magneto-optical characteristic measuring apparatus for measuring a magneto-optical characteristic of a ferromagnetic sample, which includes the following steps.

  In the method for measuring magneto-optical characteristics, as a first step, a mode-locked laser as the laser light source generates an optical pulse train having a high repetition period that can be synchronized with the period of photoexcited precession of the magnetization of the sample. Irradiate. As a second step, a predetermined external magnetic field is applied to the sample by an electromagnet by the external magnetic field applying means provided adjacent to the sample. As a third step, reflected light reflected by the sample from the irradiated optical pulse train or transmitted light transmitted through the sample by the optical pulse train is detected by the polarization component detecting means, and the polarized component is magnetically detected. Output as an optical signal. Then, as a fourth step, the external magnetic field applying means is controlled by the control device, and the external magnetic field and the magneto-optical signal when the photoexcitation precession of the magnetization of the sample is synchronized with the period of the optical pulse train. As a resonance condition, and as a fifth step, based on the LLG equation, using the control device, each information of the magnitude of the external magnetic field in the resonance condition and the repetition period of the light irradiation, The effective internal magnetic field or anisotropic magnetic field and damping factor of the sample are calculated.

  According to such a process, in the magneto-optical property measurement method, the magneto-optical property of the ferromagnetic material is measured using the magneto-optical measurement device according to any one of claims 1 to 5. Can do.

  According to the magneto-optical property measuring apparatus according to claim 1 of the present invention, the magneto-optical property such as the anisotropic magnetic field of the ferromagnetic sample can be measured accurately, inexpensively and simply.

In addition, the laser light source of the magneto-optical characteristic measuring device is a mode-locked laser that generates a light pulse train with a high repetition period, so that a conventional laser light source with a repetition period of about 80 [MHz], for example, a linear space of about 1 m Compared with a bulk type laser light source such as a CO ₂ gas laser or a Ti sapphire laser that requires a laser, the laser light source device can be remarkably reduced in size, so that the device as a whole magneto-optical characteristic device can be reduced in size. be able to.

  According to the second aspect of the present invention, the magneto-optical characteristic measuring apparatus can be configured such that the polarization component detecting means uses an inexpensive light amount detecting means, for example, a photo diode.

  According to the third aspect of the invention, the magneto-optical characteristic measuring apparatus can apply an in-plane magnetic field to the sample surface. According to the fourth aspect of the present invention, the magneto-optical characteristic measuring apparatus can apply a perpendicular magnetic field to the sample surface. Therefore, by selecting an external magnetic field applying means corresponding to the sample form or physical characteristics depending on the sample preparation conditions, the magnetic characteristic optical characteristics of the sample can be measured.

  According to the fifth aspect of the present invention, the magneto-optical characteristic measuring apparatus can improve the spatial resolution in measuring the magneto-optical characteristic of the ferromagnetic sample.

  According to the sixth aspect of the present invention, in the magneto-optical characteristic measuring method, the magneto-optical characteristic of the ferromagnetic material is accurately and inexpensively measured using the magneto-optical characteristic measuring apparatus according to any one of the first to fifth aspects. It can be easily measured.

It is a block diagram for demonstrating the magneto-optical characteristic measuring apparatus of the 1st Embodiment of this invention. It is a conceptual diagram for demonstrating the precession of magnetization in this invention, Comprising: (a) is an equilibrium state, (b) is immediately after irradiation with pulsed light, (c) is the direction in which magnetization is directed after irradiation with pulsed light. Show. In the present invention, a graph showing the calculation results of the temporal change in the magnetization M when irradiated one pulse light to the sample, (a) shows the slope of H _eff, (b) is 1-M _x component (c) is M _y component, (d), the M _z components, and, (e) respectively show the synthesis change of M _y component / M _z component. In this invention, it is a conceptual diagram for demonstrating the precession of the magnetization M at the time of irradiating the sample F with 2 continuous pulsed light, (a) is the magnetization M after irradiating the 1st pulsed light. (B1) shows the precession of the magnetization M due to the first irradiation under the resonance condition, and (c1) shows the precession of the magnetization M due to the second irradiation under the resonance condition. , (B2) shows the behavior of the magnetization M during the first irradiation under the non-resonant condition, and (c2) shows the behavior of the magnetization M after the second irradiation under the non-resonant condition. 4 is a timing chart conceptually showing interference of magnetization precession in the present invention, where (a) shows precession during non-resonance, (b) shows precession during resonance, and (c) shows a pulse laser. Each output timing is shown. In the present invention, it is a diagram showing calculation results for explaining the behavior of the time course of the magnetization M in the case of two consecutive pulses light resonance condition is irradiated to the sample, (a) shows the slope of H _eff , (b) is 1-M _x component, (c), the M _y component, (d), the M _z components, and, (e) respectively show the synthesis change of M _y component / M _z component . In this invention, it is a figure which shows the calculation result for demonstrating the behavior of the time passage of the magnetization M at the time of irradiating a sample with 2 continuous pulsed light on non-resonant conditions, (a) is a figure of _Heff gradient, (b) is 1-M _x component, (c), the M _y component, (d), the M _z components, and, (e), respectively a synthetic variation of M _y component / M _z component Show. In the present invention, is a diagram showing calculation results for explaining the behavior of the time course of the magnetization M in the case of the 50 present a continuous pulse beam at the resonance condition is irradiated to the sample, (a) shows the slope of H _eff , (b) is 1-M _x component, (c), the M _y component, (d), the M _z components, and, (e) respectively show the synthesis change of M _y component / M _z component . In this invention, it is a figure which shows the calculation result for demonstrating the behavior of the time passage of the magnetization M at the time of irradiating a sample with 50 continuous pulsed light on non-resonant conditions, (a) is a figure of _Heff gradient, (b) is 1-M _x component, (c), the M _y component, (d), the M _z components, and, (e), respectively a synthetic variation of M _y component / M _z component Show. In this invention, it is a figure which shows the calculation result for demonstrating the behavior of magnetization M in the time passage time 5000 ps vicinity of magnetization M at the time of irradiating a sample with 50 continuous pulsed light on resonance conditions, (a) It is 1-M _x component, (b), the M _y component, (c) shows M _z component change, respectively. In the present invention, a .DELTA.M _x, .DELTA.M _y, the calculation result for explaining an effective internal magnetic field H _eff dependent .DELTA.M _z when the number n of the pulsed light that is output continuously 50. In the present invention, a calculation result using a damping factor α with respect to an effective internal magnetic field H _eff of the magnetization M when the number n of continuously output pulse lights is 50, is a calculation result, and (a) is a δM _x component, (b) is .DELTA.M _y component, and (c) is a calculation result for .DELTA.M _z component. In the present invention, a calculation result using a relaxation time τ with respect to the effective internal magnetic field H _eff of the magnetization M when the number n of continuously output pulse lights is 50, is a calculation result, and (a) is a δM _x component, (b) is .DELTA.M _y component, and (c) is a calculation result for .DELTA.M _z component. It is a flowchart which shows an example of the flow of the measuring method of the magneto-optical characteristic of this invention. It is a block diagram which shows the structural example of the control apparatus of the magneto-optical characteristic measuring apparatus of 1st Embodiment. It is a block diagram for demonstrating the magneto-optical characteristic measuring apparatus of the 2nd Embodiment of this invention. It is a block diagram for demonstrating the magneto-optical characteristic measuring apparatus of the 3rd Embodiment of this invention. It is a block diagram for demonstrating the magneto-optical characteristic measuring apparatus of the 4th Embodiment of this invention.

  First to fourth embodiments of a magneto-optical property measuring apparatus and a magneto-optical property measuring method according to the present invention will be described with reference to FIGS. In the following, for convenience of explanation, as a first embodiment, (1) an outline of a magneto-optical characteristic measuring device, (2) an outline and calculation result of a theoretical model, and (3) an overall flow of a method for measuring a magneto-optical characteristic (4) After describing the configuration example of the control device of the magneto-optical measurement apparatus, the second to fourth embodiments will be sequentially described.

(First embodiment)
(1) Outline of Magneto-optical Property Measuring Device FIG. 1 is a configuration diagram for explaining a magneto-optical property measuring device 1 according to a first embodiment of the present invention. The magneto-optical characteristic measuring apparatus 1 measures the magneto-optical characteristic of the sample F including the anisotropic magnetic field H _ani by irradiating the ferromagnetic sample F with laser light having a predetermined repetition period t _rep. is there.
As shown in FIG. 1, the magneto-optical characteristic measuring apparatus 1 includes a laser light source 2, an optical system 3, an external magnetic field applying unit 4, a polarization detector 5, and a control device 6.

The laser light source 2 emits laser light of a continuous optical pulse train having a repetition period t _{rep that} can be synchronized with the period T _{M of the} precession of magnetization of the ferromagnetic sample F as pump light. Here, the repetition frequency ν _L (= 1 / t _rep ) of the laser light source 2 is set in advance, for example, in the range of 100 [MHz] to 1 [THz] according to the sample of the magneto-optical characteristic measurement target. . The repetition period t _rep of the laser light source 2 is preferably smaller than the period T _{M of the} precession of magnetization of the sample F.

In the present embodiment, a mode-locked laser having a repetition frequency ν _L of, for example, 3 to 5 [GHz] is used as the laser light source 2. In this case, the laser light source 2 may be a dedicated device that outputs laser light of an optical pulse train having a constant repetition frequency of 3 [GHz], a mode that outputs a repetition frequency of 3 [GHz], and 5 [GHz]. It is good also as a general purpose apparatus which can switch the mode output in [GHz].

  As shown in FIG. 1, the optical system 3 can also include a condensing lens 10 composed of an optical lens, so that the laser beam of the optical pulse train from the laser light source 2 can be condensed and irradiated onto the sample F. Configured.

The external magnetic field applying means 4 applies a predetermined magnetic field to the sample F fixed to the substrate 12, and is composed of an electromagnet. As will be described later, the external magnetic field applying unit 4 applies a magnetic field to the sample F to control the period of magnetization precession in the sample F. In the present embodiment, the direction of the magnetic field (external magnetic field H _ext ) applied by the external magnetic field applying means 4 is parallel to the direction of the anisotropic magnetic field H _ani of the sample F. That is, as an example, when the direction of the effective internal magnetic field H _eff of the sample F is the in-plane direction, the external magnetic field applying unit 4 is in-plane parallel to the surface of the sample F as shown in FIG. It arrange | positions so that a magnetic field can be applied. Incidentally, in response to changes in the magnitude of the external magnetic field H _ext to be applied to the sample F (value), the size of the effective internal magnetic field H _eff in Sample F was changed, is not applied external magnetic field H _ext is the sample F In this case, the magnitude of the effective internal magnetic field H _eff is equal to the magnitude of the anisotropic magnetic field H _ani .

  The substrate 12 is provided, for example, for fixing a thin film-like sample F of about several to several tens [nm]. If the sample F can be fixed, its size, shape, material, and the like are limited. It is not something. For example, when the sample F is laminated on the substrate 12 by sputtering or the like, a general substrate material such as silicon (Si) or glass can be used. At this time, the thickness and size of the substrate 12 can be set to several [mm].

  The polarization detector (polarization component detection means) 5 receives the reflected light reflected by the sample F from the laser beam of the optical pulse train from the laser light source 2, detects the polarization component, and detects the detected magnetic component indicating the polarization component. An optical signal (Magneto-Optical signal: hereinafter referred to as MO signal) is output to the control device 6.

The control device 6 is composed of, for example, a general personal computer. The control device 6 acquires the MO signal detected by the polarization detector 5 and analyzes the acquired data using a theoretical model based on the LLG (Landau-Lifshitz-Gilbert) equation, thereby allowing the magneto-optic of the sample F to be analyzed. As characteristics, the value of the effective internal magnetic field H _{eff and} the value of the damping factor α are calculated.

(2) Outline and Calculation Results of Theoretical Model Next, the outline and calculation results of the theoretical model in the magneto-optical measurement of the present invention will be described with reference to FIGS. Here, sections of 2.1 precession of magnetization, interference of 2.2 precession of magnetization, and 2.3 MO signal will be sequentially described.

(2.1) Precession of magnetization First, the precession of magnetization will be described as a premise of a theoretical model. A conceptual diagram for explaining the theoretical model is shown in FIG. In FIG. 2, the plane on which the thin film-like sample F is fixed is shown as the xy plane, and the normal direction of the plane is shown as the z-axis. That is, the irradiation direction of the pulsed light emitted from the laser light source 2 is the z-axis direction. 2A, the magnetization vector M (hereinafter simply referred to as magnetization M) and the effective internal magnetic field vector H _eff (hereinafter simply referred to as effective internal magnetic field H _eff ) of the sample F are illustrated. It is assumed that it is oriented in the x-axis direction.

That is, the magnetization M is represented by the formula (1), and similarly, the effective internal magnetic field H _eff is represented by the formula (2). M _s in equation (1) represents the magnitude of the x component of magnetization (saturated magnetization), and H _{0 in} equation (2) represents the magnitude of the x component of the effective internal magnetic field.

In this equilibrium state, when the magnetic material is irradiated with light, the magnetic anisotropy changes due to a temperature rise of the substance or a change in carrier concentration. Here, as shown in FIG. 2B, a change in magnetic anisotropy caused by the pulsed light occurs immediately after the irradiation of the pulsed light emitted from the laser light source 2 to the sample F, and the effective internal magnetic field H _eff Is inclined by an angle θ in the z-axis direction. Therefore, immediately after the irradiation with the pulsed light, the relational expression indicating the effective internal magnetic field is rewritten from the above expression (2) to expression (3). At this time, a torque F represented by the formula (4) is applied to the spin in a direction perpendicular to the plane including the magnetization M and the anisotropic magnetic field _Hani . In Equation (4), the symbol “x” represents the outer product, and γ represents the gyromagnetic ratio.

Therefore, the magnetization M starts precession due to the torque F expressed by the equation (4). After the irradiation with the pulsed light, the precession of the magnetization M shows a circular motion with the effective internal magnetic field H _eff as the central axis, as shown in FIG. This time change of the precession of the magnetization M can be well reproduced using the equation (5) called LLG equation (see Non-Patent Document 1).

The precession of the magnetization M induced by the pulsed light irradiation becomes smaller as time passes. This is called damping, and its behavior is expressed using a damping factor α (Gilbert damping constant) shown in Equation (5). If the damping factor α is 0, the period T _{M of the} precession movement of the magnetization _M is obtained using the equation (6). In Equation (6), ν _M represents the precession frequency (1 / T _M ), h represents the Planck constant, μ _B represents the Bohr magneton, and g represents the g factor.

It can be seen that the precession period T _M (= 1 / ν _M ) of the magnetization M depends on the g factor and the value of the effective internal magnetic field H _eff ignoring the constant part shown in the equation (6). The period T _M strongly depends on the type of the sample F, the experimental conditions, and the like, and it has been confirmed through experiments so far that it is approximately picoseconds to subnanoseconds (see Non-Patent Document 1).

Here, in the description of the outline of this theoretical model, the following two conditions are introduced for the sake of simplicity.
Condition 1: The angle θ shown in the above equation (2), that is, the angle θ at which the effective internal magnetic field H _eff is tilted in the z-axis direction is not relaxed in the considered time (for example, picoseconds to subnanoseconds). And In other words, the angle θ is constant regardless of time, and is not a function of time t. In other words, it is assumed that the relaxation time τ of change in the direction of the effective internal magnetic field H _eff due to light irradiation is infinite (τ = ∞).
Condition 2: A damping factor α is set to 0.

When the relaxation time τ of the precession of the magnetization M is shorter than the repetition period t _{rep of the} pulsed light irradiated to the magnetic material, interference of the precession of the magnetization M occurs. The behavior of the magnetization M at this time is such that the change in the anisotropic magnetic field induced by continuous pulsed light irradiation (change due to a plurality of pulsed lights) changes in the anisotropic magnetic field induced by individual pulsed light irradiation ( It can be calculated by assuming that it is a linear sum of changes due to one pulsed light. This is derived from Non-Patent Document 3.

For example, FIG. 3 shows the calculation results under conditions 1 and 2 for the magnetization M when the sample is irradiated with one pulsed light. In the calculation, it is assumed that H _eff is inclined by 0.1 radians in the z-axis direction by light irradiation as shown in FIG. Under such conditions, indicating 1-M _x when irradiated with one pulse light, M _y, and the time variation of M _z, in FIGS 3 (b) to FIG. 3 (d). Each shows a remarkable vibration structure reflecting the precession of magnetization M. Mx, since about three orders of magnitude as compared with the M _y and M _z less, comparing the change in the M _y component and M _z component in FIG. 3 (e). By H _eff is inclined in the z-axis direction by the light irradiation, it can be seen that a circular motion magnetization M is a central axis H _eff.

(2.2) Interference of magnetization precession Here, in order to explain the interference of precession of magnetization M, it is assumed that two continuous pulse lights are irradiated to a magnetic body by computer simulation. In this calculation, it is assumed that the laser repetition frequency ν _L (= 1 / t _rep ) is 5 [GHz]. That is, the laser repetition period t _rep is 200 [ps].

Further, when the period T _M (= 1 / ν _M ) of the precession of the magnetization M of the magnetic substance is synchronized with the repetition period t _rep (= 1 / ν _L ) of the laser, an effective effect is generated in the magnetic substance. The internal magnetic field H _eff is expressed as “magnetic field H _res where resonance occurs”, and is expressed by Expression (7). This is also called a resonance condition. At this time, the magnetic field H _{res at which} resonance occurs is expressed as in Expression (8).

In Expression (8), when the value of the repetition frequency ν _L is 5 [GHz] and the value of the g factor is “2”, the value of the magnetic field H _{res at which} resonance occurs is estimated to be 178.5 [mT]. .

The expression (8) is similar to the above expression (6), but the repetition frequency ν _L (= 1 / t _rep ) of the laser fixed as 5 [GHz], for example, and a predetermined value associated therewith. It is a relational expression with an effective internal magnetic field H _eff (magnetic field H _{res at which} resonance occurs) fixed to a value.

On the other hand, the above-described equation (6) is a relational expression between the period T _M (= 1 / ν _M ) of the precession of the magnetization M and the effective internal magnetic field H _eff, and both vary. Therefore, the behavior of the magnetization M can be controlled by changing the effective internal magnetic field H _eff as the variable parameter.

Here, a conceptual diagram of the magnetization M when two continuous pulse lights are irradiated will be described with reference to FIG. 4 (a), 4 (b1), and 4 (c1) show the precession of the magnetization M when two continuous pulse lights are irradiated in the case of the resonance condition (7). Shows movement behavior. 4 (a), 4 (b2), and 4 (c2), in the case where the resonance condition is deviated, here, in Equation (7), the condition that H _eff = H _res / 2 is satisfied. Shows the behavior of precession of magnetization M at.

The behavior of the magnetization M under the resonance condition is as shown in FIG. 4A. After the first pulsed light is irradiated, the magnetization M starts precession with H _eff as the central axis. Then, as shown in FIG. 4 (b1), the magnetization M makes a precession around H _eff and makes a round. Next, when the second pulse light is irradiated under the resonance condition, that is, H _eff = H _res , the angle formed between the magnetization M and H _eff is as shown in FIG. 4 (c1). This is twice the angle in the case of precession due to (FIG. 4A), and the amplitude of precession is also doubled.

Further, when deviating from the resonance condition, the behavior of the magnetization M when H _eff = H _res / 2 is changed from the state of FIG. 4A to the magnetization M centered on H _eff as shown in FIG. Start precession around the axis, but stop precession after half a lap. Under the condition deviating from the resonance condition (non-resonant condition), when the second pulsed light is irradiated, as shown in FIG. 4C2, the magnetization M and H _eff become parallel, and the precession of the magnetization M is Disappear.

Here, the synchronization between the precession of the magnetization M and the pulse repetition period t _rep will be described with reference to FIG. FIG. 5 is a timing chart conceptually showing the synchronization between the magnetization precession and the pulse repetition period. In FIG. 5A, for simplicity, the y component of the magnetization precession when one pulse of light is irradiated is shown as a sine waveform. In this example, the initial value of the period T _M of the magnetization precession is 400 [ps]. The value of the laser repetition period t _rep was set to 200 [ps] as shown in FIG. For example, the first (n = 1) pulse (pulse laser) is output at time 0 [ps], the second (n = 2) pulse is output at time 200 [ps], and the third (n = 3). ) Is output at time 400 [ps]. The time width of the pulse waveform is about femtoseconds.

As shown in FIG. 5A, if the initial value (400 [ps]) of the precession period T _M is different from the value (200 [ps]) of the repetition period t _rep , Since the precession is not synchronized with the pulse repetition period t _rep , the precession of the magnetization M does not resonate with the pulse laser even if a plurality of pulse lights are irradiated (non-resonance).

Next, by applying a predetermined external magnetic field in a state where the value of the laser repetition period t _rep is fixed at 200 [ps], as shown in FIG. The value of the period T _M was changed to 200 [ps]. In the case of the example shown in FIG. 5B, the value of the precession period T _M (200 [ps]) and the value of the repetition period t _rep (200 [ps]) are the same, and The precession is synchronized with the pulse repetition period t _rep . At this time, for example, when the second pulse (n = 2) is irradiated, the precession generated in the first time interferes with the precession generated in the second time. Then, resonance occurs, and the precession amplitude of the magnetization M is approximately doubled compared to the case where the first (n = 1) pulse is irradiated. Similarly, when the third pulse (n = 3) is irradiated by resonance, the amplitude is approximately three times the initial amplitude.

The waveform shown in FIG. 5 is an example for conceptually explaining the synchronization between the precession of the magnetization M and the pulse repetition period t _rep , and is different from the actual waveform. However, as a result of actual confirmation by computer simulation, the following was concluded.

Under the resonance condition, the precession of the magnetization M has the following characteristics. That is, in the case where the condition that the value of the effective internal magnetic field H _eff of the magnetic material is equal to the value of the magnetic field H _res where resonance with the pulsed light occurs in the magnetic material, the precession amplitude of the magnetization M is It increases in proportion to the number of pulse lights irradiated as pump light.
On the other hand, when the resonance condition is not satisfied (in the case of a non-resonance condition), the precession of the magnetization M induced by the irradiation with a plurality of pulsed light cancels each other, and the amplitude of the precession becomes small.
Therefore, in this embodiment, the behavior of the magnetization M excited by the continuous pulse light is evaluated by observing the MO signal indicated by the reflected light of the pulse light.

With respect to the resonance condition and the non-resonance condition, the behavior of the magnetization M over time is shown in FIGS. 6 and 7, respectively.
FIGS. _{6A to 6E} show the time course from the irradiation of the first pulse light to the second pulse light under the resonance condition H _eff = H _res . The vertical axis and the horizontal axis in each figure are the same as those in FIG. 3 showing the first magnetization M. However, it was assumed that H _eff was inclined by 1 milliradian in the z-axis direction by light irradiation. The calculation result continues precession in the same manner as the behavior of the magnetization M under the resonance condition described with reference to FIGS. 4 (a), 4 (b1), and 4 (c1).

FIGS. 7A to _7E show the time course from the irradiation of the first pulse light to the second pulse light under the condition deviating from the resonance condition, H _eff = H _res / 2. Yes. The calculation result shows that the precession of the magnetization M is similar to the behavior of the magnetization M when deviating from the resonance condition described with reference to FIGS. 4 (a), 4 (b2), and 4 (c2). Absent.

That is, as explained in the above equation (6), it can be seen from the calculation result that the behavior of the magnetization M can be controlled by changing the effective internal magnetic field H _eff .

Furthermore, it was found from the calculation results that the behavior of the magnetization M is the same when the number of pulse lights to be irradiated is increased, for example, when 50 continuous pulse lights are irradiated. FIG. 8 and FIG. 9 show the calculation result of the behavior of the magnetization M when the number of irradiated pulse lights is 50, and the case where the resonance condition H _eff = H _res and the condition deviated from the resonance condition (non-resonance condition). ) And H _eff = H _res / 2, respectively.

  Under resonance conditions, the amplitude of precession of the magnetization M increases approximately in proportion to the number of pulsed lights. On the other hand, in the case of a condition deviating from the resonance condition, the precession of the magnetization M induced by irradiation with a plurality of pulsed light cancels each other, and the amplitude of the precession becomes small.

Further, referring to FIG. 10, the behavior of the magnetization M at the elapsed time of 5000 ps in the elapsed time of irradiation of 50 continuous pulse lights in the resonance condition, that is, when the repetition period of the laser pulse light is 200 ps. Will be explained. Figure 10 (a) to FIG. 10 (c), 1-M _x, M _y magnetization M, and shows the time variations of the M _z.

Time as a characteristic of a change in the behavior of the magnetization M in the resonance condition, illustrated 1-M _x of the magnetization M at time (time) of the pulse light is irradiated, M _y, and the value of M _z (by the arrow The value at is substantially zero. The same applies to the case of irradiation with the first and second pulsed light under the resonance condition described with reference to FIGS.
That is, under the resonance condition, it can be said that the magnetization M is close to the direction in the equilibrium state.

(2.3) MO signal In this section, 2.3.1 Relationship between effective internal magnetic field and MO signal, 2.3.2 Relationship between external magnetic field and MO signal, 2.3.3 Damping factor and relaxation time Each theme related to the MO signal will be described.
(2.3.1) Relationship between Effective Internal Magnetic Field and MO Signal The behavior of the magnetization M excited by continuous pulsed light will be evaluated by observing the MO signal indicated by the reflected light of the pulsed light. An example of the observed MO signal Θ is defined by equation (9a).

  Here, n is an integer indicating the number of pulsed light output from the laser light source 2 (n = 0, 1, 2,...).

Both Θ _x δM _x ⁿ and Θ _y δM _y ⁿ shown in the equation (9b) are the in-plane magnetization components observed through MLD (Magnetic Linear Dichroism) and the longitudinal Kerr effect. It represents a change. Further, Θ _z δM _z ⁿ shown in the equation (9b) represents a change in the perpendicular magnetization component observed through the polar Kerr effect. Further, Θ _x , Θ _y , and Θ _z depend on experimental conditions such as the type and temperature of the sample F, and represent weights to be evaluated from the polarization dependence and wavelength dependence of the observed MO signal Θ. Further, δM _x ⁿ , δM _y ⁿ , and δM _z ⁿ are not directly observed, but are calculated by the equations (9c) to (9e).

On the right side of formula (9c) ~ formula (9e), t _rep indicates the repetition period of the pulse _{(1 / ν L), M} x n (nt rep), M y n (nt rep), M z n ( nt _rep ) indicates the magnitudes of the x, y, and z components of the magnetization M at the time when the nth pulse is received, and M _s indicates the saturation magnetization.

Further, .DELTA.M _x ⁿ respectively shown on the left side of formula (9c) ~ formula ^{_{(9e), δM y n,}} δM z n is, n-th x-component of the magnetization M by the pulse, y components, indicates a change in the z component Yes. Here, following the method of Non-Patent Document 3, the change in the anisotropic magnetic field induced by continuous pulsed light irradiation is a linear sum of the change in the anisotropic magnetic field induced by individual pulsed light irradiation. As shown in equations (10a) to (10c), the anisotropic magnetic field changes δM _x , δM _y , δM _z induced by n consecutively output pulses are expressed as MO signals. It is calculated as an amount corresponding to each component.

As an example, FIG. 11 shows the magnetic field dependence of δM _x , δM _y , and δM _z calculated by setting the number n of continuously output pulse lights to “50”. In the graph of FIG. 11, the horizontal axis indicates the effective internal magnetic field H _eff , and the vertical axis indicates the magnitudes of δM _x , δM _y , and δM _{z in} an arbitrary unit (au). . Here, the value of the repetition frequency ν _L is set to 5 [GHz], and the above two conditions (damping factor α = 0, relaxation time τ = ∞) are imposed.

Incidentally, .DELTA.M _x is about three orders of magnitude smaller than that .DELTA.M _y and .DELTA.M _z. Further, although n = 50 is set in order to reduce the calculation load, the number of pulses n is not limited to 50. Furthermore, when the pulse light is continuously irradiated, the angle θ of the magnetization inclination continues to increase, but after a sufficient time has passed, a certain quasi-equilibrium state is reached in order to relax. This quasi-equilibrium state can also be obtained by calculation.

From the graph of FIG. 11, it can be seen that when the value of the effective internal magnetic field H _eff is approximately 178.5 [mT], the values of δM _x , δM _y , and δM _z are “0”. This is because, under the condition where the resonance condition is satisfied, the magnetization M is in the same direction as the direction of the equilibrium state as shown in FIG. Further, the value of the effective internal magnetic field H _eff at this time is the same as the value of the magnetic field H _{res at which} resonance occurs, which is estimated by setting the value of the g factor to “2” in the above equation (8). . On the other hand, in the range of the effective internal magnetic field H _eff when the resonance condition is not satisfied, as shown in FIG. 11, the values of δM _x , δM _y , and δM _z have a constant value almost independent of the effective internal magnetic field H _eff. taking it. That is, it can be concluded that the values of δM _x , δM _y , and δM _z (amount corresponding to each component of the MO signal) show significant changes in the vicinity of the magnetic field that satisfies the resonance condition.

(2.3.2) Relationship between external magnetic field and MO signal The effective internal magnetic field H _eff of the ferromagnetic material in the external magnetic field H _ext is described by the equation (11). In this formula (11), H _dem indicates a demagnetizing field. Here, it is assumed that the anisotropic magnetic field H _ani is sufficiently larger than the demagnetizing field H _dem and the equation (12) in which H _dem = 0 is used. At this time, the anisotropic magnetic field H _ani is obtained from the equation (13).

The magnetic field H _{res at which} resonance occurs is determined by the laser repetition period t _rep (= 1 / ν _L ) from the relational expression of the equation (8). Therefore, if the value of the effective internal magnetic field H _eff is equal to the value of the magnetic field H _{res at which} resonance occurs, that is, when the resonance condition is satisfied, if the external magnetic field H _ext can be determined, the anisotropic magnetic field H _ani can be determined. Equations (12) and (13) suggest. In the present embodiment, by treating the direction of the external magnetic field H _{ext and} the direction of the anisotropic magnetic field H _ani as being parallel, the magnetic field that is a vector in equation (13) is handled as a scalar. .

Determining the external magnetic field H _ext satisfying the resonance condition can be realized by systematically observing the MO signal with respect to the external magnetic field H _ext . That is, when the sample F is irradiated with a continuous pulse with a predetermined value of the external magnetic field H _ext applied, the polarization signal detector 5 detects the MO signal and the control device 6 analyzes it, and the resonance condition must be satisfied. For example, the same operation may be repeated by changing the value of the external magnetic field H _ext .

For example, when an external magnetic field is not applied to the sample F and the vibration frequency of the precession of the magnetization M is ν _M1 , the above equation (6) can be rewritten as the equation (14). If the frequency of precession of magnetization M when resonating with pulsed light in the external magnetic field H _ext is ν _M2 , the above equation (6) can be rewritten as equation (15).

(2.3.3) Relationship between Damping Factor / Relaxation Time and MO Signal The above two conditions (damping factor α = 0, relaxation time τ = ∞) were introduced for the sake of simplicity. However, in practice, for example, it is known that the relaxation time τ of a ferromagnetic metal is several microseconds to several milliseconds, and the relaxation time τ of a ferromagnetic semiconductor is sub-nanoseconds. Further, it is known that the actual damping factor α is in the order of 0.1 to 0.001 for both ferromagnetic metals and ferromagnetic semiconductors. In addition to the anisotropic magnetic field H _ani , the determination of the damping factor α is also an important issue in the magnetic properties of the ferromagnetic material.

  Therefore, the dependence of the MO signal on the damping factor α and the relaxation time τ will be described.

FIG. 12A to FIG. 12C show the magnetic field dependency and α dependency of δM _x , δM _y , and δM _z calculated with the number of pulses n output continuously being “50”. In the graphs of FIGS. 12A to 12C, the horizontal axis indicates the effective internal magnetic field H _eff , and the vertical axis indicates the magnitudes of δM _x , δM _y , and δM _z in arbitrary units. Yes. Here, assuming that the value of the repetition frequency ν _L is 5 [GHz], only the relaxation time τ = ∞ of the above two conditions is imposed, and the value of the damping factor α is 0, 0.001, 0.01, Calculations were made for each of the cases 0.1 and 1.

As shown in FIGS. 12A to 12C, it can be seen that the graphs showing the magnetic field dependence of δM _x , δM _y , and δM _z each have a characteristic shape. Specifically, when the value of the damping factor α is 0, the shape is the same as the graph shown in FIG. That is, the graph is substantially flat in the range of the non-resonance condition before and after the effective internal magnetic field H _eff is around the resonance condition (approximately 178.5 [mT]). Convex or concave protrusions are formed. The protrusion has a line width indicating a predetermined magnetic field range in the graph. First, even if the value of the damping factor α is increased from 0 to 0.001, there is no significant change in δM _x , δM _y , δM _z (amount corresponding to each component of the MO signal). That is, in the graph, the line width when the value of α is 0 is almost the same as the line width when the value of α is 0.001, but this is due to insufficient calculation accuracy.

On the other hand, in the range of the value of the damping factor α indicated by a general ferromagnetic material (in the order of 0.1 to 0.001), the line width tends to increase as the value of the damping factor α is increased. That is, in the shape of the graph, it can be concluded that the line width particularly strongly depends on the value of the damping factor α. This is because the value of the damping factor α of the sample F is determined by analyzing the magnetic field dependence of the MO signal Θ (amount corresponding to δM _x , δM _y , δM _z ) observed when the sample F is used. It suggests that it can be determined.

Further, FIG. 13A to FIG. 13C show the magnetic field dependence and τ dependence of δM _x , δM _y , δM _z calculated with the number of pulses n output continuously being “50”. In the graphs of FIGS. 13A to 13C, the horizontal axis indicates the effective internal magnetic field H _eff , and the vertical axis indicates the magnitudes of δM _x , δM _y , and δM _z in arbitrary units. Yes. Here, assuming that the value of the repetition frequency ν _L is 5 [GHz], only the damping factor α = 0.01 of the above two conditions is imposed, and the value of the relaxation time τ is 10 ps, 30 ps, 100 ps, 1 ns, And it calculated about each case when it was set as infinity.

As shown in FIGS. 13A to 13C, it can be seen that the graphs showing the magnetic field dependence of δM _x , δM _y , and δM _z each have a characteristic shape. From this result, when the relaxation time τ is sufficiently longer than the repetition period (200 ps) of the laser beam as in a magnetic metal, the MO signal Θ hardly depends on τ. However, it can be seen that the MO signal Θ depends on the relaxation time τ when the relaxation time τ is equal to or shorter than the repetition frequency of the laser light, such as a ferromagnetic semiconductor.

  Therefore, when measuring a sample with a sufficiently long τ such as a ferromagnetic metal, the contribution of the relaxation time τ can be ignored, but the relaxation time τ is a sub-nanosecond such as a ferromagnetic semiconductor. When a sample is to be measured, it is necessary to consider the length of the relaxation time τ. However, in the latter case, it is conceivable to improve the experimental accuracy by directly determining the value of the relaxation time τ using an ultrafast time-resolved magneto-optical spectroscopy method.

(3) Overall Flow of Magneto-Optical Characteristic Measuring Method Next, an overall flow of the magneto-optical characteristic measuring method will be described with reference to FIG. 14 as a flowchart (refer to FIG. 1 as appropriate).

The magneto-optical property measuring apparatus 1 uses the control device 6 to search for a resonance condition in which the precession of the magnetization M of the sample F resonates with a plurality of continuous pulses as a resonance condition (steps S1 to S7). ) And an anisotropic magnetic field calculation step (step S8) for _obtaining the anisotropic magnetic field _Hani of the sample F when the resonance condition is searched.

In this resonance condition search stage, as a series of processes repeated until a predetermined condition is satisfied, the process by the external magnetic field applying means 4 (step S1), the process by the laser light source 2 (step S2), and the process by the polarization detector 5 ( Step S3) and processing (steps S4, S5) by the control device 6 are executed in cooperation. That is, in step S1, the external magnetic field application unit 4 changes the external magnetic field _Hext in a predetermined order and applies it to the sample F. In step S2, the laser light source 2 irradiates the sample F with a plurality of continuous pulses of laser light. In step S3, the polarization detector 5 detects the magneto-optical effect (polarization component) indicated by the reflected light of the sample F reflected from the laser beam.

In step S4, the control device 6 acquires the polarization component detected by the polarization detector 5 as the MO signal Θ. In step S5, the control device 6 determines whether or not the predetermined condition is satisfied by determining whether or not the absolute value of the MO signal Θ of the sample is minimized with the current value of the external magnetic field H _ext. The process for determining is executed. When the absolute value of the MO signal Θ is not minimum (step S5: No), the control device 6 executes processing (steps S6 and S1) for changing the external magnetic field _Hext . On the other hand, when the absolute value of the MO signal Θ is minimized (step S5: Yes), the control device 6 executes a process (step S7) for _obtaining the value of the external magnetic field H _ext as a resonance condition.

In the anisotropic magnetic field calculation step (step S8), when the resonance condition is acquired by the control device 6, the magnitude H _ext of the external magnetic field when the absolute value of the minimum MO signal Θ is detected under this resonance condition. And the anisotropic magnetic field _Hani of the sample F is calculated _| required based on a LLG equation using each information of repetition period _trep .

(4) Configuration Example of Control Device of Magneto-Optical Characteristic Measuring Device Next, a configuration example of the control device 6 of the magneto-optical property measuring device 1 will be described with reference to FIG. 15 (refer to FIG. 1 as appropriate).
Here, the control device 6 includes a laser light source control unit 61, an external magnetic field control unit 62, an MO signal acquisition unit 63, an MO signal storage unit 64, an MO signal determination unit 65, and an anisotropic magnetic field calculation unit 66. And a fitting data storage unit 67 and an attenuation parameter calculation unit 68.

  The laser light source control unit 61 controls the laser light source 2 to switch on / off of the pulse laser.

The external magnetic field control unit 62 controls the external magnetic field application unit 4 to switch on / off the magnetic field or to apply a magnetic field having a value of a predetermined external magnetic field H _ext to the sample F.
The external magnetic field control unit 62 changes the external magnetic field _Hext in a predetermined order.
Then, the MO signal acquisition unit 63 acquires the MO signal Θ from the polarization detector 5 and stores it in the MO signal storage unit 64. The MO signal storage unit 64 includes, for example, a general memory or a hard disk. The MO signal storage unit 64 also stores the value of the external magnetic field H _ext from the external magnetic field control unit 62.

The MO signal determination unit 65 supervises the laser light source control unit 61 and the external magnetic field control unit 62 and refers to the data of the MO signal Θ stored in the MO signal storage unit 64 to compare the data. In this case, it is determined whether or not the absolute value of the MO signal Θ of the current sample is minimized. When the MO signal determination unit 65 determines that the absolute value of the MO signal Θ is minimized as a result of the determination, the MO signal determination unit 65 outputs the current value of the external magnetic field H _ext to the anisotropic magnetic field calculation unit 66. Further, when the MO signal determination unit 65 determines that the absolute value of the MO signal Θ is not the minimum as a result of the determination, the MO signal determination unit 65 cooperates with the laser light source control unit 61 and the external magnetic field control unit 62 in the above-described step S6. The resonance condition search phase is continued.

The anisotropic magnetic field calculation unit 66 calculates the magnetic field H _{res at} which resonance occurs according to the above equation (8) based on the value of the laser repetition period t _rep (= 1 / ν _L ).
Further, the anisotropic magnetic field calculation unit 66 uses the calculated magnetic field H _res where the resonance occurs and the value of the external magnetic field H _ext acquired as the resonance condition from the MO signal determination unit 65 according to the above equation (12). The value of the effective internal magnetic field H _eff is determined.
Then, the anisotropic magnetic field calculation unit 66 uses the determined value of the effective internal magnetic field H _eff and the information of the external magnetic field H _ext acquired from the MO signal storage unit 64 to calculate the sample according to the above equation (13). An anisotropic magnetic field _{Hani of} F is calculated.
The anisotropic magnetic field calculation unit 66 displays the calculated anisotropic magnetic field H _ani on a display (not shown) and outputs the determined value of the effective internal magnetic field H _eff to the attenuation parameter calculation unit 68.

The fitting data storage unit 67 stores the magnetic field dependence and α dependence data (hereinafter referred to as fitting data) of the MO signal, which is obtained in advance from the theoretical calculation by the LLG equation. And hard disk. If it is possible to calculate the optimum damping factor α for the range of the magnetic field when the absolute value of the MO signal Θ is minimum with respect to the effective internal magnetic field H _eff , the fitting data is stored in advance. It may be provided as a conversion table, or may be information such as a predetermined function used for calculation.

The attenuation parameter calculation unit 68 acquires the value of the effective internal magnetic field H _eff from the anisotropic magnetic field calculation unit 66, refers to the data of the MO signal Θ stored in the MO signal storage unit 64, and the fitting data storage unit By fitting with the fitting data stored in 67, the value of the damping factor α specific to the sample is calculated as the magnetic characteristic of the sample.

  Subsequently, magneto-optical characteristic measuring apparatuses according to second to fourth embodiments of the present invention will be described with reference to FIGS. 16 to 18. In addition, in each figure, about the site | part which has the function and effect | action similar to the magneto-optical characteristic measuring apparatus 1 of 1st Embodiment, the same code | symbol is attached | subjected and description may be abbreviate | omitted depending on the case.

(Second Embodiment)
As shown in FIG. 16, the magneto-optical property measuring apparatus 1 </ b> A of the second embodiment is arranged so that the external magnetic field applying unit 4 </ b> A can apply a perpendicular magnetic field to the sample F, The configuration is the same as that of the magneto-optical characteristic measuring apparatus 1 shown in FIG. Thereby, the sample F in which the direction of the effective internal magnetic field H _{eff is} the _{perpendicular} direction can also be set as the measurement object.
It should be noted that the external magnetic field applying means has both configurations so as to correspond to a mode in which a perpendicular magnetic field is applied and a mode in which an in-plane magnetic field is applied, or the arrangement is switched so as to serve both modes. You may make it comprise so that.

(Third embodiment)
As shown in FIG. 17, the magneto-optical property measuring apparatus 1 </ b> B of the third embodiment receives the transmitted light transmitted through the sample F by the laser light of the optical pulse train from the laser light source 2 by the polarization detector 5. The magneto-optical characteristic measuring apparatus shown in FIG. 1 except that the polarization component can be detected and the concave portion 13 is formed by the substrate 12B on which the sample F is fixed and the back surface of the sample F. 1 is the same configuration.

  In this case, the MO signal is observed through the Faraday effect instead of the Kerr effect, but the MO signal can be defined in the same manner as in the first embodiment. Further, the size and shape of the recess 13 are arbitrary, and it is not necessary to be a through hole of the substrate 12B. For example, the substrate material may be peeled away leaving a thin region where the beam can be transmitted from the center. The substrate 12B may be fixed by sandwiching a thin film sample F from the front surface and the back surface.

(Fourth embodiment)
As shown in FIG. 18, the magneto-optical characteristic measuring apparatus 1C of the fourth embodiment has the same configuration as the magneto-optical characteristic measuring apparatus 1 shown in FIG. 1 except that it includes a polarization component detecting means 5C. is there. The polarization component detection means 5C corresponds to the polarization detector 5 shown in FIG. 1, and includes a PD 51, a photoelastic modulator 52, and a lock-in amplifier 53.

  The PD 51 is a light amount detection means, and is composed of, for example, a general photodiode. The PD 51 detects the intensity of the reflected light reflected by the sample F. The detected light intensity signal of the reflected light is input to the lock-in amplifier 53.

  The photoelastic modulator 52 is disposed between the laser light source 2 and the sample F, for example. The photoelastic modulator 52 switches the laser light of the optical pulse train from the laser light source 2 alternately to the left-handed circularly polarized light and the right-handed circularly polarized light at a predetermined frequency and outputs the laser light toward the sample F through the condenser lens 10. As a result, the output beam becomes a beam exhibiting circular dichroism.

  Further, the photoelastic modulator 52 inputs a signal of this modulation frequency to the lock-in amplifier 53 as a reference signal. The modulation frequency can be set to 50 [kHz], for example. Further, the arrangement of the photoelastic modulator 52 is not limited to that shown in FIG. 18 and may be arranged in the middle of the optical path of the pulsed light irradiated from the laser light source 2 to the sample F.

The lock-in amplifier 53 outputs the result of measuring the polarization component from the light intensity signal detected by the PD 51 to the control device 6 as an MO signal using the reference signal input from the photoelastic modulator 52. In this case, the MO signal is observed through circular dichroism instead of the Kerr effect, but the MO signal can be defined in the same manner as in the first embodiment.
According to the magneto-optical characteristic measuring apparatus 1C, the polarization component detecting means 5C can be configured using the inexpensive PD 51.

  As described above, the magneto-optical measurement apparatus according to each embodiment has been described. However, the magneto-optical measurement apparatus of the present invention is not limited to these, and can be implemented in various ways without changing the gist thereof. For example, in the magneto-optical characteristic measuring apparatus having the polarization component detection means 5C as in the fourth embodiment, a configuration in which a perpendicular magnetic field is applied or the polarization component of transmitted light that has passed through the sample F is detected. It can also be configured to do.

1, 1A, 1B, 1C Magneto-optical property measuring device 2 Laser light source (mode-locked laser)
3 Optical system 4, 4A External magnetic field application means (electromagnet)
5 Polarization detector (polarization component detection means)
5C Polarization component detection means 51 PD (light quantity detection means)
52 Photoelastic Modulator 53 Lock-in Amplifier 6 Control Device 61 Laser Light Source Control Unit 62 External Magnetic Field Control Unit 63 MO Signal Acquisition Unit 64 MO Signal Accumulation Unit 65 MO Signal Determination Unit 66 Anisotropic Magnetic Field Calculation Unit 67 Fitting Data Storage Unit 68 Attenuation parameter calculation unit 10 Condensing lens (optical lens)
12, 12B Substrate F Sample

Claims (6)

Magneto-optical property measurement that measures the magneto-optical properties of a sample by synchronizing the period of photoexcited precession of magnetization of a ferromagnetic sample with the cycle of a high-repetition pulsed laser beam irradiated to the sample from a laser light source A device,
The laser light source that generates laser light and irradiates the sample, external magnetic field applying means that applies an external magnetic field to the sample, and polarization that detects the polarization component of detection light from the sample and outputs it as a magneto-optical signal Comprising a component detection means and a control device for calculating the magneto-optical characteristics,
The laser light source is a mode-locked laser that irradiates the sample by generating a high repetition period optical pulse train that can be synchronized with the period of photoexcited precession of magnetization of the sample,
The external magnetic field applying means is provided adjacent to the sample and applies a predetermined external magnetic field to the sample with an electromagnet,
The polarization component detection means detects reflected light reflected from the sample by the irradiated optical pulse train or transmitted light transmitted through the sample by the optical pulse train, and outputs the polarization component as a magneto-optical signal. ,
The control device controls the external magnetic field applying means to acquire an external magnetic field and the magneto-optical signal when the photoexcitation precession of magnetization of the sample is synchronized with a period of the optical pulse train as a resonance condition, Based on the LLG (Landau-Lifshitz-Gilbert) equation using the period of the optical pulse train under the resonance condition, the intensity of the external magnetic field, and the magneto-optical signal, the effective internal magnetic field that is the magneto-optical characteristic of the sample Alternatively, a magneto-optical characteristic measuring apparatus that calculates an anisotropic magnetic field and a damping factor. The polarization component detection unit includes a light amount detection unit, a photoelastic modulator, and a lock-in amplifier.
The light amount detection means detects the intensity of the reflected light that the irradiated light pulse train reflects from the sample, or the intensity of the transmitted light that the light pulse train transmits through the sample,
The photoelastic modulator is disposed in the middle of the optical path of the optical pulse train irradiated on the sample from the laser light source, and alternately switches the optical pulse train to a counterclockwise circular polarization and a clockwise circular polarization at a predetermined frequency. Output,
The lock-in amplifier uses the frequency signal obtained by modulating the optical pulse train by the photoelastic modulator as a reference signal, and locks the result of measuring the polarization component from the light intensity signal detected by the light amount detection means as the magneto-optical signal. 2. The magneto-optical characteristic measuring apparatus according to claim 1, wherein in-detection is performed and output to the control apparatus. The external magnetic field applying means includes
3. The in-plane magnetic field parallel to the sample surface is generated from electromagnets disposed on both sides of the sample, and the in-plane magnetic field is applied to the sample surface. Magneto-optical property measuring device. The external magnetic field applying means includes
3. The surface perpendicular magnetic field perpendicular to the sample surface is generated from an electromagnet disposed on the back side of the sample, and the surface perpendicular magnetic field is applied to the sample surface. Magneto-optical property measuring device. 5. The magneto-optical measurement apparatus according to claim 1, further comprising a condensing lens that is an optical lens in an optical path between the sample and the laser light source. A method of measuring a magneto-optical characteristic of a magneto-optical characteristic measuring apparatus that includes a laser light source, an external magnetic field applying unit, a polarization component detecting unit, and a control device, and that measures a magneto-optical characteristic of a ferromagnetic sample. ,
Irradiating the sample by generating a high-repetition period optical pulse train that can be synchronized with the period of photoexcitation precession of magnetization of the sample by a mode-locked laser that is the laser light source;
Applying a predetermined external magnetic field to the sample by an electromagnet by the external magnetic field applying means provided adjacent to the sample;
A step of detecting reflected light reflected by the sample by the irradiated optical pulse train or transmitted light transmitted by the optical pulse train through the sample by the polarization component detecting means and outputting the polarized component as a magneto-optical signal. When,
A step of controlling the external magnetic field applying means by the control device to acquire an external magnetic field and the magneto-optical signal as resonance conditions when the photoexcited precession of magnetization of the sample is synchronized with the period of the optical pulse train. When,
Based on the LLG equation, the control device uses each information of the magnitude of the external magnetic field under the resonance condition and the repetition period of the light irradiation, and the effective internal magnetic field or anisotropic magnetic field of the sample and the damping factor. And a method for measuring magneto-optical characteristics.

Images