JP7137837B2 - microwave sensor - Google Patents

Description

The present invention relates to microwave sensors, and more particularly to microwave sensors for detecting microwave magnetic fields emitted by spin torque oscillators (STO).

The microwave-assisted writing method is promising for magnetic heads for next-generation hard disks. This method is a method that applies microwaves to a recording medium to enable more efficient writing of recording bits on the magnetic medium (for example, Patent Document 1). The source of this microwave must be installed in or very close to the magnetic head to locally irradiate the microwave onto the recording bit where information is to be written. Due to such demands, a spin torque oscillator (hereinafter referred to as STO) using spin torque oscillation in a magnetoresistive element attracts attention as a promising microwave transmission source for microwave assisted writing.

The STO is attracting attention as a promising microwave source, but there is a problem that there is no simple method for measuring whether or not a local microwave (microwave magnetic field) is really emitted. The difficulty in detecting the microwave magnetic field is that the size of the STO element itself is extremely small (a few hundred nanometers in diameter at most), and the spatial spread of the microwave magnetic field generated from the STO is about the same size. In addition, microwave magnetic fields arise from the attenuation of the distance from the source of the magnetic field with the cube of the distance. Therefore, a very small sensor capable of detecting the microwave magnetic field generated from the STO and a measurement method corresponding to the sensor are required.

Patent Document 2 discloses a measuring device for an in-plane high-frequency magnetic field generated by a microwave-assisted magnetic head. In this measuring device, a magnetic sensor having a tunnel magnetoresistive (TMR) element measures an in-plane high-frequency magnetic field generated by applying a bias current to an STO provided in a microwave-assisted magnetic head. However, the measuring device of Patent Document 2 does not provide a microwave sensor that directly detects microwaves (microwave magnetic field) emitted by the STO.

JP 2017-224376 A United States Patent Application Publication US2017/0301162

SUMMARY OF THE INVENTION An object of the present invention is to provide a compact microwave sensor for detecting microwaves (microwave magnetic field) emitted from an STO for a microwave assisted magnetic head.

The present invention provides a microwave sensor for detecting the microwave magnetic field emitted by the STO. The microwave sensor has two intersecting signal lines made of a nonmagnetic material and a rectangular parallelepiped shape joined to the intersection of the two signal lines, each side having a length of approximately 100 nm or less, and a width longer than the length. and a magnetic body of The other of the signal lines obtained when the surface of the STO that emits microwaves is opposed to the surface of the rectangular parallelepiped magnetic material with a gap of approximately 10 nm or less, and a current is caused to flow along one of the signal lines. The microwave magnetic field is detected from the voltage signal in the width direction of the .

According to the microwave sensor of the present invention, it is possible to detect a microwave magnetic field (local magnetic field) in the vicinity of the STO without interfering with the microwaves emitted by the STO, which is as small as the STO. Become.

1 is a schematic diagram for explaining microwave-assisted magnetic recording to which the microwave sensor of one embodiment of the present invention is applied; FIG. 1 is a schematic diagram showing the basic configuration of a microwave sensor according to one embodiment of the present invention; FIG. It is a schematic diagram which shows the arrangement|positioning of the microwave sensor of one Embodiment of this invention, and STO. FIG. 4 is a diagram for explaining the magnetic field generating operation of the STO of one embodiment of the present invention; FIG. 4 is a diagram showing changes in abnormal Hall voltage of the microwave sensor of one embodiment of the present invention; FIG. 4 is a diagram showing changes in abnormal Hall voltage of the microwave sensor of one embodiment of the present invention; FIG. 4 is a diagram showing the relationship between the frequency of the microwave magnetic field of the STO and the frequency of the measured voltage of the microwave sensor according to one embodiment of the present invention; FIG. 4 is a diagram showing the relationship between the amplitude of the microwave magnetic field of the STO and the amplitude of the measured voltage of the microwave sensor according to one embodiment of the present invention; It is a figure which shows the relationship between the length of the microwave sensor of one Embodiment of this invention, and the amplitude of a measured voltage.

An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram for explaining microwave-assisted magnetic recording to which the microwave sensor of one embodiment of the present invention is applied. In the microwave-assisted magnetic recording, the magnetization direction 3 of each recording bit 2 of the recording layer 1 is reversed by the switching magnetic field 5 generated by the write pole 4 to perform writing. At this time, a microwave magnetic field (local magnetic field) 7 generated by the STO 6 is applied to the recording layer 1 to excite the precession of the magnetization of the recording bit 2 and assist the magnetization reversal in the perpendicular direction by the light pole 4. do.

In this case, the recording bit size S is approximately 15 nm or less, and the STO having a diameter R of approximately 30 nm or less must be arranged at a distance of approximately 10 nm or less from the surface of the recording layer 1 . The microwave sensor of the present invention detects a microwave magnetic field (local magnetic field) generated by STO used in microwave assisted magnetic recording as imaged in FIG.

FIG. 2 is a schematic diagram showing the basic configuration of a microwave sensor according to one embodiment of the present invention. The microwave sensor 10 is joined to two signal lines 11 and 12 made of a non-magnetic material that intersect and the intersection of the two signal lines 11 and 12, and each side W, l, and _d has a length of about 100 nm or less. and a rectangular parallelepiped magnetic body 13 having a width W longer than the length l. The magnetic body 13 is approximately 100 nm square or less, and has a size as small as the STO to be measured.

The microwave sensor 10 has a planar structure, and has a structure in which the STO can be easily approached from the surface of the magnetic body 13 to a position in the vicinity of about 10 nm or less. An arrow 14 extending from the surface of the magnetic body 13 means the direction of magnetization (magnetization vector), and θ means the initial inclination angle of the magnetization from the vertical direction.

For the two signal lines 11 and 12 made of non-magnetic material, metals or alloys containing Cu, Pt, Pd, Ta, W, etc. can be used. The magnetic body 13 is a ferromagnetic metal or alloy containing one or more of Fe, Co, and Ni, such as Co--Fe--B, Fe--Co, Fe--Ni, Co--Fe--Ni, or any of them. Alloys or intermetallic compounds to which other elements are added, such as Fe--Pt, Fe--Pd, Co--Pt, Co--Pd, Heusler alloys, etc. can be used.

ただし、θ：磁性体１３の磁化の最初の傾き角度、θ_AHE：異常ホール角、ρ_N：信号線１１、１２の非磁性体の抵抗率、ρ_F：磁性体１３の抵抗率、ｄ_N：信号線１１、１２の厚さ、ｄ_F：磁性体１３の厚さ、Ｉ_L：信号線１１に流す電流である。 In the microwave sensor 10 shown in FIG. 2, when a current IL is passed through the signal line 11, a voltage _V is generated in the orthogonal signal line 12 due to the anomalous Hall effect. An anomalous Hall voltage signal (hereinafter referred to simply as a voltage signal) V(θ) due to the anomalous Hall effect is given by the following equation (1).

where θ: the initial inclination angle of the magnetization of the magnetic body 13, θ _AHE : anomalous Hall angle, ρ _N : the resistivity of the non-magnetic material of the signal lines 11 and 12, ρ _F : the resistivity of the magnetic body 13, d _N : thickness of the signal lines 11 and 12 d _F : thickness of the magnetic material 13 I _L : current flowing through the signal line 11 .

When the STO is brought close to the surface of the magnetic body 13 of the microwave sensor 10, the voltage signal V(θ) changes due to the microwave magnetic field generated by the STO. In expression (1), the tilt angle θ of the magnetization of the magnetic body 13 changes (oscillates), and the vibration state of V(θ) changes. In other words, the amplitude voltage and frequency of the voltage signal V(θ) of the microwave sensor 10 change according to the amplitude voltage and frequency of the microwave magnetic field generated by the STO. The microwave sensor 10 estimates the amplitude voltage and frequency of the microwave magnetic field emitted by the STO from the measured amplitude voltage and frequency of the voltage signal V(θ).

3 to 10, the microwave magnetic field generated by the STO can be detected by the voltage signal V(θ) of equation (1) in the microwave sensor 10. Specifically, the amplitude voltage of the voltage signal V(θ) A method for estimating the amplitude voltage and frequency of the microwave magnetic field emitted by the STO from the frequency and frequency will be described. First, parameter values of the microwave sensor 10 and the STO 6 are set as shown in FIGS. i.e.

・Thickness d _N of signal lines 11 and 12: 3 nm, material: Cu
・Current I _L applied to signal line 11: 0.15 mA
・Width W of magnetic body 13: 100 nm, length l: 50 nm, thickness d _F : 20 nm
・Material of the magnetic body 13: CoPt
・Diameter of STO 6: 20 nm, thickness d _STO : 3 nm
- Magnetization M of STO6: 1500 emu/c. c.
・Gap z between the upper surface of the magnetic body 13 and the lower surface of the STO 6: 5 nm

and set.

ここで、Ｍ_STOは図４のＭと同じであり、ｒはＳＴＯ６の半径（２０ｎｍ）を意味し、ψは歳差運動（回転運動）しているＳＴＯ６の傾き角度で、ここでは８５度、すなわちＳＴＯ６の磁化は面内（真横方向）から５度傾いて歳差運動していると仮定している。これらの値を式（２）、（３）に代入して計算すると、図４に示すＳＴＯ６によって作られるダイポール磁場のＨａｃ＝６０５Ｏｅ、Ｈｄｃ＝１０６Ｏｅという値が得られる。 Dipole magnetic field (microwave magnetic field ) can be calculated.

Here, M _STO is the same as M in FIG. 4, r means the radius of STO 6 (20 nm), ψ is the tilt angle of STO 6 that is precessing (rotating), here 85 degrees, That is, it is assumed that the magnetization of the STO 6 is tilted by 5 degrees from the in-plane (right lateral direction) and is precessing. By substituting these values into equations (2) and (3) and calculating, values of Hac=605 Oe and Hdc=106 Oe of the dipole magnetic field produced by the STO 6 shown in FIG. 4 are obtained.

ここで、γは磁気回転比と呼ばれる量で、磁性体の材料にあまり依存せず約１．７６４×１０⁷ｒａｄ／（Ｏｅ ｓ）という値を持つ。またＮ_STO,zは反磁場係数と呼ばれる磁性体の形状を特徴付ける量で、ＳＴＯ６の形状で決まる。図３の半径ｒ＝２０ｎｍ、厚さｄ_STOが３ｎｍの円柱形状では約０．８３３という値を持つ。これらの値を式（４）に代入して計算すると図４に示すＳＴＯ６の磁化の歳差運動の周波数ｆ＝３．４６ＧＨｚが得られる。 Also, the frequency f of the magnetization precession of the STO 6 is given by the following equation (4).

Here, γ is a quantity called a gyromagnetic ratio, and has a value of about 1.764×10 ⁷ rad/(Oes), which does not depend much on the magnetic material. N _STO,z is a quantity that characterizes the shape of the magnetic material, called a demagnetizing coefficient, and is determined by the shape of STO 6 . The cylindrical shape of FIG. 3 with a radius r=20 nm and a thickness d _STO of 3 nm has a value of about 0.833. By substituting these values into the equation (4) and calculating, the precession frequency f=3.46 GHz of the magnetization of the STO 6 shown in FIG. 4 is obtained.

ここで、ｍは検出器の磁化方向の単位ベクトルである。αはダンピング定数と呼ばれ、磁性体では約０．０１程度の大きさである（シミュレーションでは０．０１としている）。 The magnetization dynamics of the magnetic material (CoPt) of the microwave sensor 10 is calculated in consideration of the dipole magnetic field (microwave magnetic field) generated by the STO 6 described above. Specifically, a numerical simulation of the Landau-Lifschitz-Gilbert equation (LLG equation) of magnetization represented by the following equation (5) is performed.

where m is the unit vector of the magnetization direction of the detector. α is called a damping constant, and is about 0.01 in a magnetic material (it is 0.01 in the simulation).

ここで、ｍｘ、ｍｙ、ｍｚは、ｍ＝（ｍｘ、ｍｙ、ｍｚ）のようにベクトルｍの成分を表している。またＮｘ、Ｎｙ、Ｎｚは、マイクロ波センサ１０の磁性体の反磁場係数で、計算例（図３）で設定した長さｌ（ｘ方向５０ｎｍ）、幅Ｗ（ｙ方向１００ｎｍ）、厚さｄ_F（ｚ方向２０ｎｍ）の場合は、Ｎｘ＝０．２６、Ｎｙ＝０．１３、Ｎｚ＝０．６１となる。Ｈ_Kは垂直異方性磁場と呼ばれ、シミュレーションでは１．０×１０⁴Ｏｅとしている。 H is a magnetic field that acts on the magnetization of the magnetic material (CoPt) of the microwave sensor 10 and is given by the following equation (6).

Here, mx, my, mz represent the components of vector m such that m=(mx, my, mz). Nx, Ny, and Nz are demagnetizing coefficients of the magnetic material of the microwave sensor 10, and the length l (50 nm in the x direction), the width W (100 nm in the y direction), and the thickness d set in the calculation example (FIG. 3) _{For F} (20 nm in the z direction), Nx=0.26, Ny=0.13, and Nz=0.61. H _K is called a perpendicular anisotropic magnetic field, and is set to 1.0×10 ⁴ Oe in the simulation.

It is important that the above equation (6) for the magnetic field H includes H _dc , H _ac and the frequency f of the dipole magnetic field (microwave magnetic field) produced by the STO 6 described above. This is because the magnetic field generated by the magnetic material of the STO 6 reaches the magnetic material of the microwave sensor 10 . Since the magnetization of the magnetic material of the microwave sensor 10 that occurs in the presence of this magnetic field H _{dc ,} H _ac and frequency f moves, if the movement is measured, the magnetic field from the STO 6 and the frequency (H _dc , H _ac , f ) can be estimated (estimated).

In the present invention, as described with reference to FIG. 2, the anomalous Hall effect of microwave sensor 10 is used to detect the magnetic field from STO 6 . The anomalous Hall effect is a phenomenon in which the Hall voltage depends on the direction of magnetization of the magnetic material 13 of the microwave sensor 10, specifically on the value of _mz in the above equation (6). Rewriting m _z =cos θ to define the tilt angle θ of the magnetization of the magnetic body 13 from the perpendicular direction, the voltage signal V(θ) is given by the above equation (1).

By substituting the solution obtained by simulating the LLG equation of equation (5) for cos θ (=m _z ) of equation (1), the graph of the oscillation of voltage signal V (μV) shown in FIG. 5 can be obtained. can. From FIG. 5, it can be seen that the magnitude (amplitude) of vibration of the voltage signal V is about 40 nV and can be detected. A voltage signal V of 40 nV can be obtained from the calculation result of FIG.

Furthermore, in order to increase the magnitude (amplitude) of the voltage oscillation to increase the sensitivity, it is necessary to allow more current to flow through the magnetic body 13 in order to increase the anomalous Hall effect. For this reason, tantalum (Ta), which has a higher resistivity ρ _N than Cu, can be used for the non-magnetic signal lines 11 and 12 . FIG. 6 shows the calculation result of the voltage signal (θ) in that case. The voltage signal V increases tenfold to about 400 nV, and the detection sensitivity can be greatly improved. Note that Ta is only an example, and other non-magnetic materials such as Pt and Pd, which have a higher resistivity ρ _N than Cu, can also be used.

While changing the magnetization M of STO 6 from 1000 Oe to 1500 Oe, substituting the solution obtained by simulating the LLG equation of Equation (5) into cos θ (=m _z ) of Equation (1), the oscillation of STO in FIG. A graph showing the relationship between the frequency and the frequency of the voltage signal V and a graph showing the relationship between the STO microwave magnetic field H _ac and the amplitude of the voltage signal V in FIG. 8 were obtained. 7 shows that when the magnetization 16 of the STO 6 oscillates at frequency f, the magnetization 14 of the magnetic body 13 of the microwave sensor 10 oscillates at a frequency 2f, which is about twice the crescent-shaped oscillation trajectory in the figure. indicates that Therefore, by detecting the frequency f1 of the voltage signal V of the microwave sensor 10, it can be estimated that about half (0.5f1) of it is the frequency of the microwave magnetic field of the STO13.

From the graph of FIG. 8, the corresponding STO microwave magnetic field H _ac can be obtained from the oscillation width (amplitude) of the voltage signal V of the microwave sensor 10 . Therefore, by detecting the oscillation width (amplitude) of the voltage signal V of the microwave sensor 10, the STO microwave magnetic field H _ac can be estimated.

The vibration of the crescent shape frequency 2f of the magnetization 14 of the magnetic body 13 of the microwave sensor 10 of FIG. It comes from making value. That is, the difference between the width W and the length l causes motion (vibration) in which θ of the magnetization 14 of the magnetic body 13 in FIG. 2 is not constant. FIG. 9 shows changes in the oscillation width (amplitude) of the voltage signal V of the microwave sensor 10 when the length l of the magnetic body 13 is changed in the range of 10 to 100 nm.

As the length l of the magnetic body 13 is gradually increased from 10 nm, the perpendicular shape anisotropy magnetic field (half magnetic field) increases and the net perpendicular anisotropy decreases, thereby facilitating oscillation of the magnetization 14 . As a result, the oscillation width (amplitude) of the voltage signal V increases and reaches a maximum when the length l is approximately 60 nm. After 60 nm, the shape anisotropy in the length l direction and the width w direction becomes the same, and the precession (rotation) orbit of the magnetization 14 of the magnetic body 13 changes from a crescent shape to a circle (the magnetization inclination angle θ is constant), the oscillatory component disappears from the voltage due to the anomalous Hall effect. As a result, the oscillation width (amplitude) of the voltage signal V becomes smaller as shown in the figure. Therefore, the voltage signal V becomes maximum when the ratio (W:l) of the width W to the length l of the magnetic body 13 is in a relationship of 5:3. Note that even when the length l of the magnetic body 13 is changed in the range of 10 to 100 nm, the relationship between the oscillation frequency of the STO and the frequency of the voltage signal V in FIG. about twice the frequency.

Embodiments of the present invention have been described with reference to the drawings. However, the invention is not limited to these embodiments. Furthermore, the present invention can be implemented with various improvements, modifications, and variations based on the knowledge of those skilled in the art without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY The microwave sensor of the present invention can be widely used as a compact microwave sensor capable of detecting a microwave magnetic field in the vicinity of a minute device that generates a microwave magnetic field such as an STO for a microwave assisted magnetic head. .

REFERENCE SIGNS LIST 1 recording layer 2 recording bit 3, 14, 16 orientation of magnetization (magnetization vector) 4 light pole 5 switching magnetic field 6 spin torque oscillator (STO)
7 Microwave magnetic field (local magnetic field)
10 microwave sensor 11, 12 signal line 13 magnetic body

Claims (5)

A microwave sensor that detects a microwave magnetic field emitted by a spin torque oscillator (STO),
two intersecting signal lines made of a non-magnetic material;
a rectangular parallelepiped magnetic body joined to the intersection of the two signal lines, each side having a length of approximately 100 nm or less, and a width longer than the length;
the surface of the STO that emits microwaves is closely opposed to the surface of the rectangular parallelepiped magnetic body, and the signal line obtained when a current is caused to flow in one of the longitudinal directions of the signal line; the microwave magnetic field can be detected from the other voltage signal in the width direction;
The voltage signal V(θ) is obtained by the following formula:

where θ: the initial inclination angle of the magnetization of the magnetic material, θ _AHE : the anomalous Hall angle, ρ _N : the resistivity of the non-magnetic material of the signal line, ρ _F : the resistivity of the magnetic material, and d _N : the above thickness of the signal line, d _F : thickness of the magnetic material, _IL : current flowing through one of the signal lines,
A microwave sensor , given by 2. The microwave sensor according to claim 1, wherein the ratio (w:l) of the width w to the length l of said magnetic body is 5:3. 3. The microwave sensor of claim 1 or 2 , wherein the frequency of said voltage signal is twice the frequency of said microwave magnetic field. The amplitude (μV) of the voltage signal V(θ) and the microwave magnetic field (Oe) of the STO according to any one of claims 1 to 3, having a relationship shown in the graph of FIG. microwave sensor. The microwave sensor according to any one of claims 1 to 4, wherein the non-magnetic material contains Cu or Ta, and the magnetic material contains CoPt.

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