JPH042181A - Thin film superconductor and manufacture thereof - Google Patents

Abstract

PURPOSE:To improve a critical current density, a critical magnetic field by alternately laminating a layer oxide superconducting thin films each including bismuth, copper and alkaline earth element and magnetic thin films made of EuO. CONSTITUTION:Bi-O, Sr-Cu-O, Ca-Cu-O, EUO are evaporated from separate evaporation sources. When Bi-Sr-Ca-Cu-O superconducting thin films and EuO magnetic thin films are periodically laminated, a thin film having a periodic structure of m(Bi-Sr-Ca-Cu-O).n(EuO) can be formed with extremely high controllability. The thin film has for excellent crystallinity as compared with the thin film obtained by periodically laminating a superconducting thin film obtained by simultaneously depositing Bi-Sr-Ca-Cu-O and an oxide magnetic thin film obtained by simultaneously depositing EuO, and has superior critical current density and upper critical magnetic field characteristics. Further, the temperature of a base necessary to obtain excellent superconducting characteristic and heat treatment temperature can be lowered as compared with that of prior art.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a thin film of an oxide superconductor containing bismuth, which is expected to have a high critical temperature of 100' or more, and a method for manufacturing the same.

(Prior art) Niobium nitride (NbN) and germanium niobium (Nb3Ge) are used as A15 type binary compounds as high-temperature superconductors.
etc., but the superconducting transition temperature of these materials was only 23". On the other hand, perovskite compounds are expected to have an even higher transition temperature, and B a-L
A-Cu-〇-based high-temperature superconductor was proposed [J
, G., Bednorz and K., Mulle.
r.

Zeitschrift Fuyur Physique (Zetsh
rift Fire Physik B)-Co
ndensedMatter, Vol, 64,189-
193 (1986)].

Furthermore, the material of ]B1-8r-Ca-Cu-〇 is 10
It was also discovered that [H
, Maeda, Y., Tanaka, M., Fu
kuto@i and T.

AsanO+Japanese “Journal of Applied Fist Physics”
l of Ap-plied Physics)Vo
l, 27. L209-210 (198g).

The details of the superconducting mechanism of this type of material are not clear, but the transition temperature may be higher than room temperature, and it is expected that it will have more promising properties as a high-temperature superconductor than conventional binary compounds.

Furthermore, higher critical current density and higher critical magnetic field have been conventionally expected by alternately stacking superconductors and magnetic materials.

(Problem to be solved by the invention) However, with current technology, B1-5r-Ca-Cu-0-based materials can only be formed mainly through the process of sintering, so they can only be obtained in the form of ceramic powder or blocks. I can't. On the other hand, when this type of material is to be put to practical use, there is a strong demand for processing it into a thin film, but with conventional techniques, it has been difficult to produce a thin film with good superconducting properties.

In other words, it is known that the B1-8r-Ca-Cu-○ system has several phases with different superconducting transition temperatures, but it is particularly difficult to achieve a phase with a transition temperature of 100'' or higher in the form of a thin film. It was considered extremely difficult to do so.

In addition, in order to form a thin film that exhibits good superconducting properties in this conventional Bi system, heat treatment at at least 700°C or higher or heating during formation is required, and therefore a high critical current density and high critical magnetic field are expected. It is thought to be extremely difficult to obtain a periodic layered structure with magnetic thin films.
It was also considered to be extremely difficult to construct an integrated device using this structure.

An object of the present invention is to eliminate the conventional drawbacks and provide a bismuth-containing oxide superconducting thin film that is expected to have a high critical temperature of 100' or more, and a method for manufacturing the same.

(Means for solving problems) The thin film superconductor of the first invention by the present inventors is as follows.

The main components are at least bismuth (Bi), copper (Cu),
It has a structure in which layered oxide superconducting thin films containing alkaline earth elements (NA group) and magnetic thin films made of EuO are alternately laminated.

Furthermore, the method for producing a thin film superconductor according to the second invention includes an oxide film formed by periodically stacking an oxide containing at least Bi and an oxide containing at least copper and alkaline earth (NA group) on a substrate. Thin film.

This is a method for manufacturing a thin film superconductor obtained by alternately stacking magnetic thin films made of EuO.

Here, alkaline earth refers to at least one or two or more elements of Group IIa elements.

(Function) In the first invention by the present inventors, it is stable.
By adopting a structure in which a Bi-based superconducting thin film and a magnetic thin film made of EuO are alternately laminated, the Bi-based superconducting thin film and the magnetic thin film made of EuO have a crystal structure covered with a 02 oxide film layer or a layer mainly composed of this. Lamination with less mutual diffusion between the superconducting film and the magnetic thin film becomes possible. Furthermore, the critical current density and critical magnetic field in the Bi-based superconducting thin film have been improved due to the interaction between the magnetic moment or spin of the magnetic thin film and the superconductor.

Furthermore, in the second invention, in order to achieve the above structure,
By periodically stacking an oxide containing at least Bi and an oxide containing at least copper and alkaline earth (group IIa) or EuO to form a thin film through control at the molecular level, it is possible to achieve good reproducibility. This has succeeded in producing a laminated layer of a Bi-based superconducting thin film and a magnetic thin film.

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

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

However, when a magnetic thin film is laminated next to a Bi-based superconducting thin film when the substrate temperature is high, or when heat treatment is performed after forming the magnetic thin film, mutual diffusion of elements occurs between the superconducting thin film and the magnetic thin film, resulting in superconducting It was found that the characteristics were significantly deteriorated. In order to prevent mutual diffusion, the crystallinity of the superconducting thin film and the magnetic thin film must be excellent, the lattice matching between the superconducting thin film and the magnetic thin film must be excellent, and the magnetic thin film must be heat-treated at 800 to 950°C. It is considered essential to be stable against

As a result of various studies, we found that EuO is suitable as a magnetic thin film. The reason for this is not clear, but EuO has extremely good lattice matching with Bi-based superconductors, and even during high-temperature heat treatment, EuO
It is thought that the interface with the i-based superconductor is very stable.

Furthermore, it has been found that when Bi-based superconducting thin films and Eu○ thin films are stacked periodically, the inherent critical current density and critical magnetic field of the Bi-based superconducting thin films are improved.

In order to further understand the contents of the first invention, a specific example will be shown using FIG. 1. (Example 1) FIG. It is a schematic diagram of the inside, 11 is B1-8r-C
a-Cu-○ target, 12 is EuO target, 1
3 is a shutter, 14 is an aperture, 15 is a base, 1
6 indicates a heater for heating the substrate. Two targets 11 and 12 produced by press forming a sintered body were used and arranged as shown in FIG.

That is, each target is installed at an angle of approximately 30 degrees so as to focus on the MgO (100) substrate 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 can be stopped on the B1-5r-Ca-Cu-0 target or the EuO target.

In this way, B1-8r-Ca-Cu-〇→E u
o-4B i-S r-Ca-Cu-0-+ EuO→
Sputtering vapor can be performed in a cycle of B1-8r-Ca-Cu-〇. B1-5r-Ca-Cu-○ film,
FIG. 2 conceptually shows how the EuO films are stacked.

In the same figure, 21 is an EuO film, 22 is a B1-8r-C
The a-Cu-◯ film is shown. By controlling the input power to the targets 11 and 12 and the sputtering time of each target, the Eu○ film 21 is deposited on the substrate 15.
, the thickness of the B1-8r-Ca-Cu-0 film 22 can be changed. The substrate 15 is heated to about 700°C with a heater 16, and placed in a 0.5P mixed atmosphere of argon and oxygen (1:1).
Sputtering of each target in the gas of a was performed. After the thin film was prepared, heat treatment was performed at 800'C for 2 hours in an oxygen atmosphere. In this example, the sputtering power for each target was set to B1-8r-Ca-Cu-0: 150w.
, EuO: 100 W, and the sputtering time of targets 11 and 12 was controlled. B1-8r-Ca-Cu-○ film 2
The composition ratio of the two elements is Bi: Sr: Ca: Cu=
The elemental composition ratio of the target 11 was adjusted to be 2:2:2:3. Moreover, the composition ratio of the EuO film is Eu:
The sputtering conditions were optimized so that ○=1:1. B
When the i-S r-Ca-Cu-0 film 22 is formed on the base 15 without being laminated with the EuO film 21, that is, B1-5
The characteristics of the r-Ca-Cu-0 film 22 itself are 110X
A superconducting transition occurs at 100', and the resistance value becomes zero at 100'. Further, when only the EuO film was formed and the magnetization was measured, the axis of easy magnetization was parallel to the film surface.

Moreover, the magnetization value was the same as that of the bulk. EuO film and Bi, 5r2Ca, Cu, 0. The thickness of each film was 500, and one layer was laminated. When the magnetization of this laminated film was measured, it was naturally the same as the value of a film made only of EuO. The temperature characteristics of the resistance of this film are shown in FIG.

The superconducting transition temperature (onset temperature) was 110', which was the same as when no EuO film was laminated.

FIG. 4 shows the temperature dependence of the critical current density measured when an external magnetic field of 10 koe was applied parallel to the film surface of the laminated film to magnetize the magnetic film and then the external magnetic field was removed. The critical current density is approximately 20% larger at each temperature than the value before applying the magnetic field. Figure 5 shows the temperature characteristics of electrical resistance when an external magnetic field is applied.
Comparing the results of the Bi-5r-Ca-Cu-◯ film itself, which is not a laminated film, it was found that in the laminated film, the spread of the superconducting transition temperature region due to the magnetic field is smaller.

This means an improvement in the upper critical magnetic field. The reason for these improvements in critical current density and upper critical magnetic field is not clear, but the magnetization or spin of the EuO film is B1-8.
This is thought to be the result of affecting the superconducting mechanism of the r-Ca-Cu-○ film. In addition, EuO film and B1-
It was found that when the 8r-Ca-Cu-0 film was formed alone, a crystalline thin film was obtained when the film thickness was 100 Å or more and 50 Å or more, respectively. In FIG. 2, EuO film 2
B1-3r-Ca-Cu-0 with the film thickness of 1 as 100 people
The characteristics when the thickness of the film 22 is 100, 300, and 500 and the number of repetitions is 20 are shown in characteristics 61, 62, and 63 in FIG. 6, respectively. In characteristic 61, the zero resistance temperature is about 30' and B1-5r-Ca-C
It was found that the characteristics of the u-0 film 22 deteriorated. The reason for this is that the B1-8r-Ca-Cu-0 film 22 and the EuO
It is considered that the crystallinity of the films 21 and 22 is destroyed due to interdiffusion of elements with the film 21. Furthermore, in characteristic 63,
The B1-8r-Ca-Cu-0 film 22 has almost the same superconducting properties as the original when it is deposited on the substrate 15 without periodic lamination with the EuO film 21, and the lamination effect with the magnetic thin film EuO film 21 is confirmed. It wasn't done. However, in characteristic 62, the critical current density was improved by about 20% compared to the film without laminated magnetic film, reaching 3.2 million A10 at 77'. The critical magnetic field was improved by about 20% compared to the original B1-8r-Ca-Cu-0 film. At 4.2″K,
When a magnetic field is applied in a direction parallel to the C axis, the value is 30 Tesla,
Moreover, in the direction perpendicular to the C axis, it was 450 Tesla. the current,
The detailed reason for these effects is still unknown, but it may be due to the influence of the magnetic moment or spin of the Eu○ film 21, or the influence of the magnetic moment or spin of the EuO film 21, or the
By laminating the r-Ca-Cu-0 film 22, B1-
It is conceivable that some change was caused in the superconducting mechanism in the 5r-Ca-Cu-0 film 22.

In addition, target 11. Alternatively, it has been found that when lead (pb) is added to 12 and sputtered, results equivalent to those of the above example can be obtained even if the temperature of the substrate 15 is about 100° C. lower than that of the above example.

Further, the Bi oxide and the Sr, Ca, and Cu oxides were separately evaporated in vacuum from different evaporation sources.

B 1-0-S r-Cu-0-Ca-Cu on the substrate
-0-S r-Cu-O-Bi-0 is laminated periodically in the order of evaporation in vacuum using a rough Eu○ target and laminated, as shown in (Example 1). This method has much better controllability and stable film quality than the laminated structure fabrication method.
Moreover, the film surface is extremely flat.1-5r-Ca-Cu-
It has been found that a EuO superconducting thin film and a EuO magnetic thin film can be obtained.

Furthermore, B1-0.5r-Cu-0, Ca-Cu-0,
EuO is evaporated from separate sources and B1-8r-Ca
- When Cu-0 superconducting thin films and EuO magnetic thin films are stacked periodically, controllability is extremely good < m (Bi-5r-Ca-C
We have discovered that it is possible to form a thin film with a periodic structure of u-0).n(EuO). Here, m and n each represent a positive integer of at least 1 or more. moreover.

This m(Bi-5r-Ca-Cu-0) ・n(EuO
) thin film.

A thin film obtained by periodically laminating a superconducting thin film obtained by simultaneously depositing B1-5r-Ca-Cu-0 shown in (Example 1) and an oxide magnetic thin film obtained by simultaneously depositing EuO. We also found that it has much better crystallinity and superior critical current density and upper critical magnetic field characteristics. Furthermore, B i-S r produced by the above method
It has been found that the surfaces of both the -Ca-Cu-0 superconducting thin film and the EuO magnetic thin film are extremely flat.

These things are possible because by stacking different elements separately and sequentially, the deposited elements move only in the plane parallel to the substrate surface, but the elements do not move in the direction perpendicular to the substrate surface. This is thought to be due to the fact that there is no such thing.

Furthermore, the temperature of the substrate required to obtain good superconducting properties,
It was also found that the heat treatment temperature was lower than conventional ones.

B1-0, 5r-Cu-0, Ca-Cu-0, Eu-○
There are several possible methods for periodically stacking layers. In general, it is possible to achieve periodic stacking by controlling the opening and closing shutter in front of the evaporation source in an MBE device or multi-source EB evaporation device, or by switching the type of gas during production using the vapor phase growth method. can. However, sputtering deposition has traditionally been considered unsuitable for this type of extremely thin layer stacking. The reason for this is thought to be the incorporation of impurities due to the high gas pressure during film formation and damage caused by high energy particles. However, when different thin layers were laminated by sputtering on this Bi-based oxide superconductor, it was surprisingly discovered that it was possible to produce a good laminated film.

High oxygen gas pressure during sputtering and sputtering discharge
This is thought to be because it is convenient for the formation of a Bi-based phase having a critical temperature of 100'' or higher, and for the formation of a dense EuO film.

There are 1 methods for layering different materials using sputter deposition.
Although there is a method of periodically controlling the discharge position of one sputtering target provided with a composition distribution, this can be achieved relatively easily by using a method of sputtering a plurality of targets having different compositions. In this case, a periodic laminated film can be produced by periodically controlling the amount of sputtering for each of a plurality of targets, or by providing a shutter in front of the target and opening and closing it periodically. Alternatively, it can be manufactured by a method in which the substrate is moved periodically and moved over each target. When laser sputtering or ion beam sputtering is used, periodic laminated films can be realized by periodically moving a plurality of targets and periodically changing the targets irradiated with the beam. By sputtering using such a plurality of targets, a periodic stack of Bi-based oxides can be produced relatively easily.

In order to further understand the content of the invention of cylinder 2, specific examples will be shown below.

(Example 2) FIG. 7 shows a schematic diagram of a four-element magnetron sputtering apparatus used in this example. In FIG. 7, 71 is a Bi metal
72 is a 5rCu alloy target.

73 is a Ca Cu alloy target, 74 is an Eu target, 75 is a shutter, 76 is an aperture, 77 is a substrate, and 78 is a heater for heating the substrate. A total of four targets 71, 72, 73, and 74 were arranged in the same manner as shown in FIG. That is, each target is installed at an angle of about 30° so as to focus on the Mg0 (100) substrate 77. There is a rotating shutter 7 in front of the target.
5, and by driving it with a pulse motor, the rotation of an aperture 76 provided therein is controlled, and the cycle and sputtering time of each target can be set. The substrate 77 was heated to about 600° C. by a heater 78, and sputtering of Bi, 5rCu and CaCu targets was performed in a mixed atmosphere of argon and oxygen (5:1) at 3 Pa.

Further, in order to prevent oxidation of the Eu target during sputtering of the Eu target, only Ar was used as the atmospheric gas. Thereafter, oxygen gas was introduced to oxidize the formed Eu film into EuO. Let the sputtering current of each target be B i
: 30mA, SrCu: 80mA.

CaCu: 300+nA, Eu: 80mA
The experiment was carried out.

Sputtering was performed using a Bi−+5rCu−+CaCu−+5rCu−+Bi cycle, and the elemental composition ratio of the B1−5r−Ca−Cu−0 film was Bi:Sr:Ca:Cu=2:
As a result of adjusting the sputtering time of each target to become 2:2:3 and performing the above cycle for 204 times, the sputtering time was 11.
We were able to fabricate a phase with a critical temperature of 0'' or higher.Even in this state, this B1-5r-Ca-Cu-〇 film showed a superconducting transition of 110X or higher, but it was further developed at 600°C in oxygen. After 1 hour of heat treatment, the reproducibility was very good, and the superconducting transition temperature was 115' and the temperature at which the resistance value became zero was 100' K. Phases with superconducting transition temperatures exceeding 100' K are metallic. The element is B1-8r-Cu-
It is said that it consists of layers of oxides arranged in the order Ca-Cu-Ca-Cu-Sr-Bi, and the manufacturing method of the present invention is not very useful in creating this structure. I think so.

1 layer of Bi-+5rCu-+CaCu-+5rCu
Layer n periods as a period, and layer EuO on top with a film thickness d (people)
Sputter each target so that n(Bi-5r
-Ca-Cu-0) d(EuO) thin film was produced on the substrate 77. Here, n represents a positive integer of 1 or more. n=
10, the superconducting properties of the films obtained by stacking EuO thin films with varying film thicknesses d were investigated. At this time, B1-5r-C
The number of repetitions of lamination of a-Cu-0 thin film/Eu○ thin film is 1
It was set as 0 times. In Figure 8, d = 7o, 200°1000
Characteristics 81, 82, and 83 show the temperature changes in the resistance of the multilayer film obtained for humans. In Figure 8, d=200
For humans, the highest superconducting transition temperature and zero resistance temperature, characteristic 82, were obtained. The superconducting transition temperature and zero resistance temperature of characteristic 82 are B i-S r-Ca-Cu-0
These values are equivalent to those of the original film. Critical current density is 77
"K" was 3.6 million A10, which was 45% higher than the value of a thin film without laminated magnetic thin film.
The upper critical magnetic field is improved by about 30% compared to the original B1-3r-Ca-Cu-0 film. At 4.2″K, the value when applying a magnetic field parallel to the C axis is 33 Tesla, and C
In the direction perpendicular to the axis, it was 490 Tesla. Although the detailed reason for this effect is still unknown, by periodically stacking the B1-5r-Ca-Cu-0 film and the EuO film using the method shown in this example, the B1-8r-Ca -C
Because the u-0 film and the Eu○ film are epitaxially grown, there is no effect of mutual diffusion of elements at the laminated interface, and B1-8r-, which also has excellent crystallinity, is produced through the thin EuO film with excellent crystallinity. It is considered that some change was caused in the superconducting mechanism in the B i-S r-Ca-Cu-◯ film by laminating the Ca-Cu-0 film.

Furthermore, it has been found that when sputtering is performed by adding lead (PB) to target 7I or 74, results equivalent to those of the above embodiment can be obtained even if the temperature of the base 77 is about 100° C. lower than that of the above embodiment. Ta.

(Effects of the Invention) The thin film superconductor of the first invention provides a structure that improves the critical current density and critical magnetic field of the Bi-based thin film superconductor, and the method for manufacturing a thin film superconductor of the second invention is the first
The present invention has great industrial value because it more effectively realizes the invention described above and establishes a low-temperature process that is essential for applications such as devices.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a thin film manufacturing apparatus according to an embodiment of the first invention, FIG. 2 is a structural conceptual diagram of the first invention, and FIGS. 3 and 6 are thin films obtained by the apparatus of FIG. 1. Figure 4 shows the temperature dependence of the critical current density in the thin film obtained with the apparatus shown in Figure 1. Figure 5 shows the temperature dependence of the critical current density in the thin film obtained with the apparatus shown in Figure 1 under an external magnetic field. 7 is a schematic diagram of the thin film manufacturing apparatus in the embodiment of the second invention, and FIG. 8 is a temperature characteristic diagram of the resistance value of the thin film obtained by the apparatus of FIG. 7. . II, 12.71.72.73.74...Sputtering target, 13.75...Shutter 14
．． 76... Aperture, 15.77... MgO base, 16.78... Heater, 21-=EuO film, 22- B1-8r-Ca-Cu-0 film, 61.
62.63.81.82.83...Temperature characteristics of thin film resistance. Patent applicant: Matsushita Electric Industrial Co., Ltd. Matsushita Electric Works Co., Ltd.

Claims (4)

[Claims] (1) The main components are at least bismuth (Bi) and copper (C).
u) and a thin film superconductor characterized by having a structure in which layered oxide superconducting thin films containing alkaline earth elements (group IIa) and magnetic thin films made of EuO are alternately laminated. Here, alkaline earth refers to at least one or two or more elements of 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 alkaline earth (group IIa) on a substrate, and a magnetic thin film made of EuO. , a method for producing a thin film superconductor, characterized in that it is obtained by alternately laminating layers. Here, alkaline earth refers to at least one or two or more elements of group IIa elements. (3) The method for manufacturing a thin film superconductor according to claim (2), characterized in that the evaporation of the laminated material is performed using at least two 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.