JP2741277B2 - Thin film superconductor and method of manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a bismuth-containing oxide superconductor thin film expected to have a high critical temperature of 100 ° K or higher and a method for producing the same.

(Prior art) As high-temperature superconductors, niobium nitride (NbN) and germanium niobium (Nb ₃ Ge) have been known as A15 type binary compounds, but the superconducting transition temperature of these materials is at most 23 ゜ K. Met. On the other hand, perovskite compounds
Higher transition temperatures are expected and Ba-La-Cu-O based high-temperature superconductors have been proposed [JGBendnorz and KAMulle
r, Zetshrif
t Fr Physik B) -Condensed Matter, Vol. 64, 189-19
3 (1986)].

Furthermore, it has been discovered that Bi-Sr-Ca-Cu-O-based materials exhibit a transition temperature of 100 K or higher [H. Maeda, Y. Tanaka,
M.Fukutomi and T.Asano, Japanese Journal
Of Applied Physics (Japanese Journal o
f Applied Physics) Vol.27, L209-210 (1988)]. Although the details of the superconducting mechanism of this type of material are not clear, the transition temperature may be higher than room temperature, and more promising properties are expected as a high-temperature superconductor than conventional binary compounds.

Further, a higher critical current density and a higher critical magnetic field have been conventionally expected by alternately laminating a superconductor and a magnetic material.

(Problems to be Solved by the Invention) However, Bi-Sr-Ca-Cu-O-based materials can be formed only in the process of sintering with the current technology, and are obtained only in the form of ceramic powder or blocks. I can't. On the other hand, when this kind of material is put to practical use, it is strongly desired to process it into a thin film, but it has been difficult to produce a thin film having good superconducting properties with the conventional technology. That is, it is known that there are several phases having different superconducting transition temperatures in the Bi-Sr-Ca-Cu-O system, and in particular, a phase having a transition temperature of 100 ° K or more is achieved in the form of a thin film. It was very difficult to do.

In addition, in order to form a thin film showing good superconductivity in this conventional Bi system, heat treatment of at least 700 ° C. or heating at the time of formation is necessary, so that a high critical current density and a high critical magnetic field are expected It was considered extremely difficult to obtain a periodic laminated structure with a magnetic thin film, and it was also very difficult to construct an integrated device using this structure.

An object of the present invention is to solve the conventional disadvantages and to provide a bismuth-containing oxide superconducting thin film expected to have a high critical temperature of 100 ° K or more and a method for producing the same.

(Means for Solving the Problems) The thin-film superconductor of the first invention by the present inventors has a layered structure in which a main component contains at least bismuth (Bi), copper (Cu), and alkaline earth (IIa group). An oxide superconducting thin film;
It has a structure in which magnetic thin films made of EuO are alternately stacked.

Further, in the method for manufacturing a thin film superconductor according to the second invention, an oxide containing at least Bi and an oxide containing at least copper and an alkaline earth (Group IIa) are periodically laminated on a substrate. This is a method for manufacturing a thin film superconductor obtained by alternately laminating oxide thin films and magnetic thin films made of EuO.

Here, the alkaline earth refers to at least one element or two or more elements among the group IIa elements.

(Operation) In the first invention by the present inventors, stable Bi ₂ O
By taking a structure in which a Bi-based superconducting thin film and a magnetic thin film made of EuO, which have a crystal structure covered together by a ₂ oxide film layer or a layer mainly composed of this, are alternately laminated, Lamination with less interdiffusion between the superconducting film and the magnetic thin film becomes possible. Further, the critical current density and the critical magnetic field of the Bi-based superconducting thin film are improved by the interaction between the magnetic moment or spin of the magnetic thin film and the superconductor.

Further, in the second invention, in order to achieve the above structure, an oxide containing at least Bi and an oxide containing at least copper and an alkaline earth (Group IIa) or EuO are periodically laminated to form a molecule. By producing a thin film under the control of the above, a stack of a Bi-based superconducting thin film and a magnetic thin film was successfully obtained with good reproducibility.

(Example) First, in order to realize a periodic laminated structure of a Bi-based superconducting thin film and a magnetic thin film, the interaction at the interface between the Bi-based superconducting thin film and various magnetic thin films was examined.

Usually, a Bi-based superconducting thin film is obtained by vapor deposition on a substrate heated to 500 to 700 ° C. After deposition, the thin film shows superconducting properties as it is, but is then subjected to a heat treatment at 800 to 950 ° C. to improve the superconducting properties.

However, if the magnetic thin film is laminated next to the Bi-based superconducting thin film or the heat treatment is performed after the formation of the magnetic thin film when the substrate temperature is high, mutual diffusion of elements occurs between the superconducting thin film and the magnetic thin film, and the superconducting thin film is formed. It was found that the characteristics were greatly deteriorated. To prevent mutual diffusion, superconducting thin films and magnetic thin films must have excellent crystallinity,
It is indispensable that the lattice matching between the magnetic thin films is excellent and that the magnetic thin films are stable against heat treatment at 800 to 950 ° C.

As a result of various studies, it was found that EuO is suitable as a magnetic thin film. Although the reason for this is not clear, it is thought that EuO has extremely excellent lattice matching with the Bi-based superconductor and that the interface with the Bi-based superconductor is very stable even at high temperature heat treatment. Can be

Furthermore, it was found that when Bi-based superconducting thin films and EuO thin films were periodically laminated, the intrinsic critical current density and critical magnetic field of Bi-based superconducting thin films were improved.

In order to better understand the contents of the first invention, the first invention
Specific examples will be described with reference to the drawings.

(Example 1) Fig. 1 is a schematic view of the inside of a high-frequency dual magnetron sputtering apparatus used in this example, and 11 is Bi-Sr-Ca-Cu.
-O target, 12 is an EuO target, 13 is a shutter, 14 is an aperture, 15 is a substrate, and 16 is a heater for heating the substrate. Using two targets 11 and 12 produced by press-forming a sintered body, they were arranged as shown in FIG. That is, each target is set to be inclined by about 30 ° so as to focus on the MgO (100) base 15. In front of the target is a rotating shutter 13, in which an aperture 14 is provided. By controlling the rotation of the shutter 13 with a pulse motor, the aperture 14 is controlled.
Can be stopped on the Bi—Sr—Ca—Cu—O target or the EuO target. In this way, Bi-Sr
-Ca-Cu-O->EuO->Bi-Sr-Ca-Cu-O->EuO-> Bi-Sr
The sputtering vapor can be performed in a cycle of -Ca-Cu-O. FIG. 2 conceptually shows how the Bi—Sr—Ca—Cu—O film and the EuO film are stacked. In the figure, 21 is an EuO film, 22 is B
3 shows an i-Sr-Ca-Cu-O film. By controlling the input power to the targets 11 and 12, and the sputtering time of each target, the EuO film 21, Bi-S
The thickness of the r-Ca-Cu-O film 22 can be changed. Substrate 1
5 is heated to about 700 ° C. by the heater 16 and argon / oxygen (1:
1) Each target was sputtered in a mixed atmosphere of 0.5 Pa gas. After forming the thin film, a heat treatment at 800 ° C. was performed for 2 hours in an oxygen atmosphere. In this embodiment, the sputtering power of each target is set to Bi-Sr-Ca-Cu-O: 150
w, EuO: 100 w, and the sputtering time of the targets 11 and 12 was controlled. The composition ratio of the elements of the Bi-Sr-Ca-Cu-O film 22 is B
The element composition ratio of the target 11 was adjusted so that i: Sr: Ca: Cu = 2: 2: 2: 3. Further, the composition ratio of the EuO film is Eu: O =
The sputter conditions were optimized to be 1: 1. Bi-Sr-Ca
-When the Cu-O film 22 is formed on the substrate 15 without being laminated with the EuO film 21, that is, the characteristics of the Bi-Sr-Ca-Cu-O film 22 itself cause a superconducting transition at 110 ° K, At 値 K, the resistance became zero. Also, only the EuO ₃ film is formed,
When the magnetization was measured, the axis of easy magnetization was parallel to the film surface. The value of magnetization was the same as the value of bulk. The thickness of each of the EuO film and the Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _y film was 500
The layers were laminated one by one. When the magnetization of this laminated film was measured, the value was naturally the same as that of the film of EuO alone. FIG. 3 shows the temperature characteristics of the resistance of this film. The superconducting transition temperature (onset temperature) was 110 ° K, which was not different from the case where no EuO film was laminated. FIG. 4 shows the temperature dependence of the critical current density measured in a state where an external magnetic field of 10 kOe was applied in parallel to the film surface of the laminated film and the external magnetic field was removed after the magnetic film was magnetized. The critical current density is about 20% larger at each temperature than the value before applying the magnetic field.
FIG. 5 shows a temperature characteristic of electric resistance in a state where an external magnetic field is applied. Bi-Sr-Ca-C without EuO film
Comparing the results of the u-O film itself, it was found that the superconducting transition temperature region due to the magnetic field was less widened in the laminated film. This means that the upper critical magnetic field is improved. It is not clear why these critical current densities and upper critical magnetic fields are improved, but the magnetization or spin of the EuO film is B
This is considered to be the result of affecting the superconducting mechanism of the i-Sr-Ca-Cu-O film. In addition, EuO film and Bi-Sr-C
When the a-Cu-O film is formed alone, the film thickness is 100 mm each.
It was found that a crystalline thin film was obtained when the temperature was 50 ° or more. In FIG. 2, the thickness of the EuO film 21 is set to 100 °.
When the film thickness of the Bi-Sr-Ca-Cu-O film 22 is 100 mm, 300 mm, 500 mm,
The characteristics when the number of repetitions is 20 are shown in FIG. 6 as characteristics 61, 62, and 63, respectively. In the characteristic 61, it was found that the zero resistance temperature was about 30 ° K and the characteristic of the Bi—Sr—Ca—Cu—O film 22 was deteriorated. The reason for this is that Bi-Sr-Ca
It is conceivable that the crystallinity of the films 21 and 22 may be destroyed due to mutual diffusion of elements between the Cu—O film 22 and the EuO film 21. Further, the characteristic 63 is almost the same as the original superconducting characteristic of the Bi-Sr-Ca-Cu-O film 22 when the film is provided on the base 15 without periodic lamination with the EuO film 21. No laminating effect was observed. However, in the characteristic 62, the critical current density was improved by about 20% as compared with the film without the magnetic film laminated, and became 3.2 million A / o at 77 ° K. The critical magnetic field is Bi-S
The r-Ca-Cu-O film was improved by about 20% from the original one. 4.2 ゜
In K, the value when a magnetic field is applied in the direction parallel to the c-axis is 3
It was 0 Tesla and 450 Tesla in the direction perpendicular to the c-axis.
At present, the detailed reasons for these effects are still unknown, but the influence of the magnetic moment or spin of the EuO film 21 or a plurality of Bi-Sr-Ca
-Bi-Sr-Ca-Cu-O by laminating -Cu-O film 22
It is possible that some change in the superconducting mechanism was caused in the film 22.

It should be noted that, when lead (Pb) is added to the target 11 or 12 and sputtered, even if the temperature of the substrate 15 is about 100 ° C. lower than that of the above-described embodiment, the same result as that of the above-described embodiment is obtained. Was.

Further, Bi oxide and oxides of Sr, Ca and Cu are separately evaporated in vacuum from different evaporation sources, and Bi-O-Sr
In the case where the layers were periodically laminated in the order of -Cu-O-Ca-Cu-O-Sr-Cu-O-Bi-O, and further evaporated in a vacuum using an EuO target, and the layers were laminated (Example Bi-Sr-Ca-Cu-O with very good controllability, stable film quality, and extremely flat film surface than the method for fabricating a laminated structure shown in 1).
It was found that superconducting thin films and EuO magnetic thin films could be obtained.

Further, when Bi-O, Sr-Cu-O, Ca-Cu-O, and EuO are evaporated from separate evaporation sources, and a Bi-Sr-Ca-Cu-O superconducting thin film and a EuO magnetic thin film are periodically laminated. It has been found that a thin film having a periodic structure of m (Bi-Sr-Ca-Cu-O) .n (EuO) can be formed with extremely high controllability. Where m,
n represents a positive integer of at least 1 or more. Further, the m (Bi-Sr-Ca-Cu-O) .n (EuO) thin film is a superconducting thin film obtained by simultaneously depositing Bi-Sr-Ca-Cu-O shown in (Example 1), Compared to a thin film obtained by periodically laminating an oxide magnetic thin film obtained by simultaneously depositing EuO, it has much better crystallinity, and also has superior characteristics of critical current density and upper critical magnetic field. I found it. Further, Bi-Sr-Ca-Cu-O prepared by the above method
It has been found that both the superconducting thin film and the EuO magnetic thin film have extremely flat surfaces.

This is because, by sequentially stacking different elements separately, the deposited elements only move in a plane parallel to the substrate surface, and the elements move in the direction perpendicular to the substrate surface. It is thought that it is not.

Further, they have also found that the temperature of the substrate and the heat treatment temperature necessary for obtaining good superconducting properties are lower than before.

There are several possible methods for periodically stacking Bi-O, Sr-Cu-O, Ca-Cu-O, and Eu-O. In general, periodic stacking can be achieved by controlling the front of the evaporation source with an opening / closing shutter using an MBE device or a multi-source EB evaporation device, or by switching the type of gas when manufacturing by vapor phase growth. it can. However, sputtering deposition has heretofore been unsuitable for stacking very thin layers of this type. It is considered that the reason for this is that impurities are mixed due to the high gas pressure during the film formation and damage is caused by high energy particles. However, when different thin layers were laminated on this Bi-based oxide superconductor by sputtering, it was discovered that a surprisingly good laminated film could be produced. It is considered that the high oxygen gas pressure and the sputter discharge during the sputtering are convenient for the formation of a Bi-based phase having a critical temperature of 100 ° K or more and the formation of an EuO insulating film.

As a method of laminating different materials by sputter deposition, there is a method of periodically controlling the discharge position of one sputtering target having a composition distribution, but a method of sputtering a plurality of targets having different compositions is used. And can be achieved relatively easily. In this case, a periodic laminated film can be manufactured by periodically controlling the amount of sputtering of each of the plurality of targets, or by providing a shutter in front of the targets and periodically opening and closing them. Further, it can also be manufactured by a method in which the substrate is moved periodically over the target by periodically moving the substrate.
In the case of using laser sputtering or ion beam sputtering, a periodic laminated film can be realized by periodically moving a plurality of targets to change the target to be irradiated with a beam periodically. The periodic lamination of the Bi-based oxide can be relatively easily produced by sputtering using such a plurality of targets.

In order to better understand the content of the second invention,
A specific example will be described.

Embodiment 2 FIG. 7 is a schematic diagram of a quaternary magnetron sputtering apparatus used in this embodiment. In FIG. 7, 71 is a Bi target, 72 is a SrCu alloy target, 73 is a CaCu alloy target, 74 is an Eu target, 75 is a shutter, 76 is an aperture, 77 is a substrate, and 78 is a substrate heating heater. 4 in total
The targets 71, 72, 73, 74 were arranged in the same manner as shown in FIG. That is, each target is set to be inclined by about 30 ° so as to focus on the MgO (100) base 77. In front of the target, there is a rotating shutter 75, which is driven by a pulse motor to control the rotation of an aperture 76 provided therein, so that the cycle and sputtering time of each target can be set. Heated to about 600 ° C with heater 78 of substrate 77, argon and oxygen (5: 1)
Bi, SrCu and CaCu targets were sputtered in a mixed atmosphere of 3 Pa gas. In addition, to prevent oxidation of the Eu target during sputtering of the Eu target,
The atmosphere gas was only Ar. Then, oxygen gas was introduced to oxidize the formed Eu to form EuO. The experiment was performed with the sputter current of each target set to Bi: 30 mA, SrCu: 80 mA, CaCu: 300 mA, and Eu: 80 mA. Sputtering in the cycle of Bi → SrCu → CaCu → SrCu → Bi, and for each target where the elemental composition ratio of the Bi-Sr-Ca-Cu-O film is Bi: Sr: Ca: Cu = 2: 2: 2: 3 After adjusting the sputtering time and performing the above cycle for 20 cycles, 110
A phase with a critical temperature of ゜ K or more could be produced.
Even in this state, the Bi-Sr-Ca-Cu-O film showed a superconducting transition of 110 ° K or more.
After a long heat treatment, the reproducibility became very good. The superconducting transition temperature was 115 ゜ K and the temperature at which the resistance value became zero was 100 ゜ K. Phases with a superconducting transition temperature exceeding 100 K
It is also said that it consists of an oxide layer arranged in the order of Bi-Sr-Cu-Ca-Cu-Ca-Cu-Sr-Bi. It is thought that it may be a role.

The stacking of Bi → SrCu → CaCu → SrCu is defined as one cycle, and n cycles are stacked. Then, each target is sputtered with EuO to a thickness d (Å), and n (Bi-Sr-Ca-Cu-O). d (Eu
O) A thin film was formed on the substrate 77. Here, n represents a positive integer of 1 or more. When n = 10, the superconducting characteristics of the films obtained by laminating the EuO thin films while changing the film thickness d were examined. At this time
Bi-Sr-Ca-Cu-O thin film / EuO thin film
10 times. FIG. 8 shows the temperature changes of the resistance of the multilayer film obtained when d = 70, 200, and 1000 °, as characteristics 81, 82, and 83, respectively. In FIG. 8, when d = 200 °, the highest superconducting transition temperature and zero resistance temperature, that is, the characteristic 82 were obtained. The superconducting transition temperature and zero resistance temperature of characteristic 82 are Bi-Sr
-These are equivalent to the original values of the Ca-Cu-O film. The critical current density at 77 ゜ K was 3.6 million A / o, 45% higher than that of the thin film without the magnetic thin film. Also,
The upper critical magnetic field is about twice that of the original Bi-Sr-Ca-Cu-O film.
30% improvement. At 4.2 ゜ K, the value when a magnetic field is applied in the direction parallel to the c-axis is 33 Tesla, and in the direction perpendicular to the c-axis,
It was 490 Tesla. Although the detailed reason for this effect is not yet known, Bi-Sr-Ca
-By stacking the Cu-O film and the EuO film periodically, the Bi-Sr-Ca-Cu-O film and the EuO film are epitaxially grown, so that there is no influence of interdiffusion of elements at the stack interface. Some change in the superconducting mechanism in the Bi-Sr-Ca-Cu-O film by stacking the Bi-Sr-Ca-Cu-O film also having excellent crystallinity via the thin EuO film having excellent crystallinity May have been caused.

Furthermore, it has been found that when lead (Pb) is added to the target 71 or 74 and sputtered, even if the temperature of the substrate 77 is about 100 ° C. lower than that of the above-described embodiment, the same result as that of the above-described embodiment can be obtained. Was.

(Effect of the Invention) The thin-film superconductor of the first invention provides a structure for improving the critical current density and the critical magnetic field of the Bi-based thin-film superconductor, and the method of manufacturing the thin-film superconductor of the second invention Is the first
The invention of the present invention has been realized more effectively, and a low-temperature process essential for application of devices and the like has been established, and the industrial value of the present invention is great.

[Brief description of the drawings]

FIG. 1 is a schematic view of an apparatus for manufacturing a thin film according to an embodiment of the first invention, FIG. 2 is a conceptual diagram of the structure of the first invention, and FIGS. 3 and 6 are thin films obtained by the apparatus of FIG. FIG. 4 shows the temperature dependence of the critical current density in the thin film obtained by the apparatus shown in FIG. 1, and FIG. 5 shows the temperature dependence of the thin film obtained by the apparatus shown in FIG. , FIG. 7 is a schematic view of an apparatus for manufacturing a thin film according to the second embodiment of the present invention, and FIG. 8 is a temperature characteristic view of a resistance value of the thin film obtained by the apparatus of FIG. . 11,12,71,72,73,74 …… Sputtering target, 13,
75 …… Shutter, 14,76 …… Aperture, 15,77 ……
MgO substrate, 16,78 heater, 21 EuO film, 22 Bi
-Sr-Ca-Cu-O films, 61, 62, 63, 81, 82, 83 ... Temperature characteristics of resistance of thin films.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01B 12/06 ZAA H01B 12/06 ZAA 13/00 565 13/00 565D H01F 10/26 H01F 10/26 H01L 39/02 ZAA H01L 39/02 ZAAD (72) Inventor Shinichiro Hatta 1006 Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. 72) Inventor Kiyotaka Wasa 1006 Kazuma Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-63-318014 (JP, A) JP-A-3-105807 (JP, A)

Claims (4)

(57) [Claims] 1. A structure in which a layered oxide superconducting thin film containing at least bismuth (Bi), copper (Cu) and alkaline earth (IIa group) as main components and a magnetic thin film made of EuO are alternately laminated. A thin-film superconductor characterized by having. Here, the alkaline earth refers to at least one element or two or more elements among the group IIa elements. 2. An oxide thin film formed by periodically laminating an oxide containing at least Bi and an oxide containing at least copper and an alkaline earth (Group IIa) on a substrate, and a magnetic thin film made of EuO. A method for manufacturing a thin-film superconductor, comprising alternately laminating thin films. Here, the alkaline earth refers to at least one or two or more of the Group IIa elements. 3. The method of manufacturing a thin film superconductor according to claim 2, wherein the evaporation of the laminated material is performed by at least two or more types of evaporation sources. 4. The method for manufacturing a thin film superconductor according to claim 2, wherein the evaporation of the laminated material is performed by sputtering.