JP3025891B2 - Thin film superconductor and method of manufacturing the same - Google Patents

Description

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

As (prior art) high-temperature superconductors, although such niobium nitride (NbN) and germanium niobium (Nb ₃ Ge) is known as A15 type binary compounds, superconducting transition temperature is at most 23 ° K of these materials Met. On the other hand, perovskite compounds
Higher transition temperatures are expected and Ba-La-Cu-O based high-temperature superconductors have been proposed [JGBednorz 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 superconductors and magnetic materials.

(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.

Conventionally, in order to form a thin film exhibiting good superconducting properties in this Bi system, it is necessary to perform heat treatment at least at 700 ° C or heating during the formation. It was considered extremely difficult to obtain a periodic laminated structure with a 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 main component containing at least bismuth (Bi), copper (Cu), and alkaline earth (Group IIa), It has a structure in which an oxide superconducting thin film having a layered crystal structure and a magnetic thin film whose main component is a perovskite oxide are alternately laminated.

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. An oxide superconducting thin film having a layered crystal structure,
This is a method for producing a thin film superconductor obtained by alternately laminating magnetic thin films made of perovskite oxide.

Here, the alkaline earth refers to at least one or two or more of the Group IIa elements. The perovskite oxide is RFeO ₃ (R = Y, Sm, Eu, Gd, Tb, Dy, Ho, E
r, Tm, Yb, Lu) or MMnO ₃ (M = Bi, La _0.7 Ca _0.3 , La
_0.7 Sr _0.3 , La _0.7 Ba _0.3 , La _0.6 , Pb _0.4 , La _0.7 Cd _0.3 ) or AMnO ₆ (A = Gd ₂ Co, Ba ₂ Fe, Ca ₂ Fe).

(Operation) In the first invention, a Bi-based superconducting thin film and a perovskite-type oxide having a crystal structure covered with a stable Bi ₂ O ₂ oxide film layer or a layer mainly composed of the Bi ₂ O ₂ oxide film layer are used. By taking a structure in which magnetic thin films made of a material are alternately stacked, it is possible to stack with less mutual diffusion between the superconducting thin film and the magnetic thin film. 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 a perovskite oxide are periodically laminated. By producing thin films by controlling at the molecular level,
This is to obtain a laminate of a Bi-based superconducting thin film and a magnetic thin film.

(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 vapor 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, causing 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 a perovskite oxide thin film was suitable as a magnetic thin film. Although the reason for this is not clear, perovskite oxides
The lattice matching with the Bi-based superconductor is extremely excellent,
Also, it is considered that the interface with the Bi-based superconductor is very stable even in the high-temperature heat treatment.

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

In order to further understand the contents of the first invention, a specific embodiment will be described with reference to FIG.

(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.
An -O target, 12 a Bi-Mn-O target, 13 a shutter, 14 an aperture, 15 a substrate, and 16 a substrate heating heater. 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 made to be a Bi-Sr-Ca-Cu-O target or a Bi-Mn-O
Can be stopped on the target. Thus, Bi-Sr-Ca-Cu-O → Bi-Mn-O → Bi-Sr-Ca-Cu
Sputter deposition can be performed in a cycle of -O->Bi-Mn-O-> Bi-Sr-Ca-Cu-O. Bi-Sr-Ca-Cu-O
FIG. 2 conceptually shows how the film and the Bi-Mn-O film are stacked. In the figure, 21 is a Bi-Mn-O film, 22 is Bi-Sr-Ca
3 shows a -Cu-O film. By controlling the input power to the targets 11 and 12, and the sputtering time of each target, the Bi-Mn-O film 21, Bi-Sr
-The thickness of the Ca-Cu-O film 22 can be changed. Base 15
Is heated to about 700 ° C. by a 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, Bi-Mn-O: 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 Bi: Sr: Ca: Cu = 2: 2: 2: 3, and the composition ratio of the elements of the Bi-Mn-O film 21 is Bi: Mn: Targets 11, 12 so that O = 1: 1: 3
The composition ratio of the elements was adjusted. Bi-Sr-Ca-Cu-O film 22
Is formed on the substrate 15 without being laminated with the Bi-Mn-O film 21, that is, the characteristics of the Bi-Sr-Ca-Cu-O film 22 itself cause a superconducting transition at 110 ° K and a temperature of 100 ° K And the resistance value became zero. When only the BiMnO ₃ film was formed and the magnetization was measured, the value was the same as the bulk value. The thicknesses of the BiMnO ₃ film and the Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _y film were each set to 500 °, and were stacked one by one. FIG. 3 shows the temperature characteristics of the resistance value of this film. The superconducting transition temperature (onset temperature) was 110 ° K, which was not different from the case where the BiMnO ₃ film was not 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 perpendicularly to the film surface of the laminated film to magnetize the magnetic film and then remove the external magnetic field. The critical current density is about 30 at each temperature relative to the value before applying the magnetic field.
% Has increased. FIG. 5 shows a temperature characteristic of electric resistance in a state where an external magnetic field is applied. Compared with the result of the Bi—Sr—Ca—Cu—O film itself without the BiMnO ₃ film, 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. The reasons for the enhancement of these critical current densities and the upper critical magnetic field are not clear, but BiMnO ₃
It is considered that the magnetization or spin of the film affected the superconducting mechanism of the Bi-Sr-Ca-Cu-O film. Further, it was found that when the BiMnO ₃ film and the Bi—Sr—Ca—Cu—O film were formed alone, a crystalline thin film was obtained when the film thickness was 100 ° or more and 50 ° or more, respectively. In FIG. 2, Bi-Sr-Ca-C
The thickness of the u-O film 22 is 100Å, 300Å, 500Å, and the number of repetitions
In Fig. 6, the characteristics at 20 times are shown in Fig. 6.
61, 62 and 63. In characteristic 61, the zero resistance temperature is about 30
It was found that the properties of ΔK and Bi—Sr—Ca—Cu—O film 22 deteriorated. The reason for this is that the Bi-Sr-Ca-Cu-O film 22
Films 21 and 22 by interdiffusion of elements with Bi-Mn-O film 21
The crystallinity may be destroyed. 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 substrate 15 without periodic lamination with the Bi-Mn-O film 21. No laminating effect with the thin Bi-Mn-O film 21 was confirmed. However, in property 62,
Critical current density is about
It improved by 30% to 3.2 million A / o at 77 ゜ K. The critical magnetic field was improved by about 20% from the original Bi-Sr-Ca-Cu-O film. At 4.2 ° K, the value when a magnetic field was applied in the direction parallel to the c-axis was 30 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 Bi-Mn-O film 21 or a plurality of Bi- It is considered that the superconducting mechanism in the Bi-Sr-Ca-Cu-O film 22 caused some change by laminating the Sr-Ca-Cu-O 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 lower by about 100 ° C. than in the above embodiment, it is found that the same result as in the above embodiment can be obtained. Was.

Further, the oxides of Bi and the 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 are periodically stacked in the order of -Cu-O → Ca-Cu-O → Sr-Cu-O → Bi-O, and further evaporated in vacuum using a Mn target, and the layers are stacked, (Example A Bi-Sr-Ca-Cu-O superconducting thin film and a Bi-Mn-O magnetic thin film having extremely good controllability, stable film quality, and an extremely flat film surface can be obtained by the method of manufacturing a laminated structure shown in 1). Was found.

Furthermore, Bi-O, Sr-Cu-O, Ca-Cu-O, Bi-Mn-
When O is evaporated from separate evaporation sources and the Bi-Sr-Ca-Cu-O superconducting thin film and the Bi-Mn-O magnetic thin film are periodically laminated, m (Bi-Sr-Ca-Cu −O) · n
It has been found that a thin film having a periodic structure of (Bi-Mn-O) can be formed. Here, m and n each represent a positive integer of at least one or more. Furthermore, this m (Bi-Sr-Ca-Cu-
The O) .n (Bi-Mn-O) thin film is shown in (Example 1).
Compared to a 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 Bi-Mn-O, It was also found that the crystallinity was excellent and the characteristics of the critical current density and the upper critical magnetic field were superior. Further, the Bi-Sr-Ca-Cu-O superconducting thin film and the Bi-Mn
Both the -O magnetic thin films were found to have extremely thin film 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 movement of the elements in the direction perpendicular to the substrate surface is prevented. 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 Bi-Mn-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, they have found that a good laminated film can be formed by laminating different thin layers on the Bi-based oxide superconductor by sputtering. High oxygen gas pressure and sputtering discharge during sputtering
Formation of a phase with a critical temperature of 100 ° K or higher, and Bi-Mn-
This is probably because it is convenient for forming the O 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 be manufactured by a method in which the substrate is moved periodically on 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. Thus, the periodic lamination of the Bi-based oxide can be relatively easily produced by sputtering using 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 a Mn 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. The substrate 77 is heated to about 600 ° C. by a heater 78, and argon and oxygen (5: 1)
Each target was sputtered in a gas of a mixed atmosphere of 3 Pa. Sputter current of each target, Bi: 30m
The experiment was performed with A, SrCu: 80 mA, CaCu: 300 mA, and Mn: 80 mA.
Sputter in the cycle of Bi → SrCu → CaCu → SrCu → Bi, Bi
-The composition ratio of the elements of the Sr-Ca-Cu-O film is Bi: Sr: Ca: Cu: O
By adjusting the sputtering time of each target such that = 2: 2: 2: 3 and performing the above cycle 20 times, a phase having a critical temperature of 110 ° K or more can be produced. Even in this state, the Bi-Sr-Ca-Cu-O film shows a superconducting transition of 110 ° K or more. The temperature was 115 ゜ K, and the temperature at which the resistance value became zero became 100 ゜ K. The phase whose superconducting transition temperature exceeds 100 K is metal element Bi-Sr-Cu-Ca-Cu
-It is said that it is composed of oxide layers arranged in the order of -Ca-Cu-Sr-Bi, and it is considered that the production method of the present invention was very useful in producing this structure. .

In addition, it has been found that when Bi-Mn-O is formed alone, the film has a perovskite-type crystal structure with a thickness of at least 70 ° or more.

The stacking of Bi->SrCu->CaCu-> SrCu is defined as one cycle, and the cycle is stacked for n cycles. −O) · d
A (Bi-Mn-O) 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 Bi-Mn-O thin films while changing the film thickness d were examined. At this time, Bi-Sr-Ca-Cu-O thin film / Bi-Mn-O
The number of times of lamination of the thin film was set to 10 times. In FIG. 8, d = 7
Characteristics 81, 82, and 83 show the temperature changes of the resistance of the multilayer film obtained at 0, 200, and 1000 °, respectively. In FIG. 8, d = 200
In the case of Å, the highest superconducting transition temperature and zero resistance temperature, that is, characteristic 82, were obtained. The superconducting transition temperature and the zero resistance temperature of the characteristic 82 are equivalent to those of the Bi—Sr—Ca—Cu—O film. Critical current density at 360
10,000 A / o, 45% higher than the value of the thin film without the magnetic thin film laminated. The upper critical magnetic field is Bi-Sr-Ca-
It is about 30% higher than the original Cu-O film. At 4.2 ゜ K, the value when a magnetic field was applied in the direction parallel to the c-axis was 33 Tesla, and 490 Tesla in the direction perpendicular to the c-axis. Although the detailed reason for this effect is still unknown, the Bi-Sr-Ca-Cu-O film and the Bi-Mn-O film are periodically laminated by the method described in the present embodiment to obtain Bi. -Sr-Ca-Cu-
O film and Bi-Mn-O film are epitaxially grown, so there is no influence of interdiffusion of elements at the lamination interface, and the crystallinity is also excellent through the thin Bi-Mn-O film with excellent crystallinity. Bi-Sr-Ca-Cu-O film
It is considered that some change was caused in the superconducting mechanism in the -Sr-Ca-Cu-O film.

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 base, 16,78 heater, 21 Bi-Mn-O film, 22
...... Bi-Sr-Ca-Cu-O film, 61, 62, 63, 81, 82, 83 ... Temperature characteristics of resistance of thin film.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroshi Ichikawa 1006 Kazuma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Inventor Kentaro Seto 1006 Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Inventor Kiyotaka Wasa 1006, Oaza Kadoma, Kadoma City, Osaka Pref. Document JP-A-63-318014 (JP, A) JP-A-3-105807 (JP, A)

Claims (4)

(57) [Claims] An oxide superconducting thin film having a layered perovskite structure in which a main component contains at least bismuth (Bi), copper (Cu) and alkaline earth (Group IIa), and at least one or more perovskite oxides A thin film superconductor having a structure in which magnetic thin films are alternately stacked. Here, the perovskite oxide is RFeO ₃ (R = Y, S
m, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) or MMnO ₃ (M = B
i, La _0.7 Ca _0.3 , La _0.7 Sr _0.3 , La _0.7 Ba _0.3 , La _0.6 Pb _0.4 , La
An oxide magnetic material represented by _0.7 Cd _0.3 ) or AMnO ₆ (A = Gd ₂ Co, Ba ₂ Fe, Ca ₂ Fe) and a composite oxide magnetic material containing at least two or more thereof are shown. 2. A layered perovskite formed by periodically stacking an oxide containing at least bismuth (Bi) and an oxide containing at least copper (Cu) and an alkaline earth (Group IIa) on a substrate. A method for producing a thin film superconductor, comprising alternately laminating an oxide superconducting thin film having a structure and a magnetic thin film made of a perovskite oxide. Here, the perovskite oxide is RFeO ₃ (R = Y, S
m, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) or MMnO ₃ (M = B
i, La _0.7 Ca _0.3 , La _0.7 Sr _0.3 , La _0.7 Ba _0.3 , La _0.6 Pb _0.4 , La
An oxide magnetic material represented by _0.7 Cd _0.3 ) or AMnO ₆ (A = Gd ₂ Co, Ba ₂ Fe, Ca ₂ Fe) and a composite oxide magnetic material containing at least two or more thereof are shown. 3. The method for producing a thin film superconductor according to claim 2, wherein the evaporation of the substances formed by lamination is performed by at least two or more kinds of evaporation sources. 4. The method for producing a thin film superconductor according to claim 2, wherein the evaporation of the substance formed by lamination is performed by sputtering.