JP3045503B1 - Method for producing single crystal film of superconducting oxide - Google Patents

Abstract

The present invention provides a new method for obtaining a superconducting oxide single crystal film having excellent characteristics having a thickness of a micron order, that is, a thickness of 1 μm or more, which has been extremely difficult with a conventional method. To develop. SOLUTION: A single crystal substrate of a bulk superconducting oxide such as La ₂ CuO ₄ is heat-treated to form a single crystal substrate of a non-superconducting oxide, which is converted into a permanganate aqueous solution at 40 to 200 ° C. By dipping, the surface of the single crystal substrate is chemically oxidized to introduce oxygen into the single crystal substrate to form an oxygen-excess superconducting oxide film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a superconducting oxide single crystal film.

[0002]

2. Description of the Related Art A single crystal of an oxide superconductor is manufactured by a floating zone (FZ) method or a TSFZ (Traveling Solvent) method.
Floating Zone) method. The oxide superconductor film is mainly synthesized by a gas phase method such as the MBE method. In addition, as a method for increasing the carrier density by introducing excess oxygen into the oxide exhibiting semiconductivity and making it superconductive, there are high pressure oxygen treatment, electrochemical or chemical oxidation, oxygen plasma method, etc. The method is known as a method suitable for the production of a film (JP-A-10-273).
No. 317).

[0003] The above-mentioned Japanese Patent Application Laid-Open No. 10-273317 discloses that an oxide thin film having a thickness of 1 μm or less which does not exhibit superconductivity is irradiated with an oxidizing gas having a stronger oxidizing power than its molecular state to make it superconductive. A method is disclosed.

As an electrochemical oxidation method, for example, Japanese Patent Application Laid-Open No. 6-298531 discloses an infinite layer structure represented by MCuO ₂ (M = alkali metal ion) formed on an oxide substrate. For copper oxide thin films,
There is disclosed a method of synthesizing a superconductor having a Tc of 110 ° K or more by introducing excess oxygen into a thin film in a copper oxide at normal pressure by an oxidation reaction in an electrolyte solution. JCGrenier
et al. Physica C202 (1992) include La ₂ C pellets.
The uO ₄ sintered body was placed in an aqueous solution of potassium hydroxide at 25 ° C. for 9 hours.
As a result of performing the electrochemical oxidation over time, the excess oxygen
That La ₂ CuO _{4 + d} ≦ d ≦ 0.09 was obtained,
FCChou, DC Johnston, SWCheong and PCCan
Field, Physica C216 (1993) 66, indicates that the electrochemical oxidation method of La ₂ CuO ₄ bulk single crystal takes one month even for a crystal as small as about 20 mg.

Further, as a chemical oxidation method, Japanese Patent Application Laid-Open No.
JP-A-219303 discloses an intermediate product or a product that has shown superconductivity in an oxide superconductor manufacturing process.
It discloses a method of treating with a solution containing a compound having an oxidizing property, examples of the compound having an oxidizing property include carbonyl compounds, ozone, hydrogen peroxide, organic peroxides, dimethyl sulfoxide, permanganate, and the like. Is illustrated.

[0006]

The crystal structure of the La-based oxide high-temperature superconductor has a layered structure in which CuO ₂ conductive layers and La ₂ O ₂ insulating layers are alternately stacked along the c-axis direction. , A CuO ₂ conductive layer—La ₂
A Josephson junction between the O ₂ insulating layer and the CuO ₂ conductive layer is formed. Therefore, the single crystal of the La-based oxide high-temperature superconductor can be regarded as a microelectronic device composed of countless Josephson junctions. In addition, the Josephson plasma phenomenon was discovered from the single crystal, and application to ultra-high-speed microelectronic devices is expected. However, in order to realize such an electronic device, a single crystal film having a thickness on the order of microns, that is, a thickness of 1 μm or more is indispensable.

[0007] Incidentally, the FZ method and the TSFZ method, which are methods for producing a single crystal of an oxide superconductor, are methods for producing a large single crystal, and it is difficult to form a single crystal film having a thickness of 1 mm or less. It is. In a gas phase method such as a conventional MBE method suitable for the production of a film, a superconductor formed on a substrate is an oriented polycrystal, not a single crystal, and has a film thickness of 0.1 mm. Very thin, 1 μm or less.

A method of forming a single crystal film on a single crystal substrate is also known. However, a magnesium oxide or titanium oxide having a different crystal structure or lattice constant from an oxide superconductor is formed on a single crystal substrate. Strontium acid or the like is used, and a single crystal of a substance which is the same as or a parent of the oxide superconductor is not used. Therefore, a high-quality single crystal cannot be obtained, the crystal orientation of the film cannot be controlled, and only a single-crystal film having a thickness of nanometer class has been formed. In the method of electrochemically oxidizing a non-superconducting single crystal, the excess oxygen amount d can only be increased to a maximum of 0.09, and it is difficult to control the film thickness by oxidizing only the crystal surface layer. At the same time, the crystal surface is eroded and a smooth surface cannot be obtained.

As a conventionally known chemical oxidation method, the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 4-219303 discloses an intermediate product or a material which has shown superconductivity in the production process of an oxide superconductor. Base, powder, bulk, wire,
It is intended to treat a thin film or the like in an arbitrary shape with a solution containing a compound having an oxidizing property.The solvent is preferably a non-aqueous solvent such as methanol or toluene from the viewpoint of the stability of the superconductor. Since the non-aqueous solvent has a low boiling point, it is difficult to perform oxidation treatment at a high temperature, and the excess oxygen amount cannot be sufficiently increased. In addition, Japanese Patent Application Laid-Open No. 10-273 described above
No. 317, cited as an example of a chemical oxidation method.
E. Takayama-Muromachi et al. Physica C207 (1993) 97 describes a method of oxidizing a powdery sample obtained by pulverizing a sintered body with an aqueous KMnO ₄ solution. Is also not disclosed.

The present invention provides a new method for obtaining a superconducting oxide single crystal film having excellent properties and having a thickness on the order of microns, that is, 1 μm or more, which has been extremely difficult with the conventional method. The task is to develop

[0011]

SUMMARY OF THE INVENTION The present invention has made it possible to provide a high-quality single crystal film at a relatively low temperature in a short time by a simple method by using a chemical oxidation method.
That is, the present invention provides a single crystal substrate of a non-superconducting oxide by heat treating a single crystal substrate of a bulk superconducting oxide,
This is immersed in an aqueous solution of permanganate at 40 to 200 ° C. to chemically oxidize the surface of the single crystal substrate and introduce oxygen into the single crystal substrate to form an oxygen-excess superconducting oxide film. A method for producing a single crystal film of a superconducting oxide, characterized by being formed.

Further, the present invention provides a method for heat treating a single crystal substrate of a bulk superconducting oxide into a single crystal substrate of a non-superconducting oxide, and immersing the substrate in a permanganate aqueous solution at room temperature. The method for producing a single crystal film of a superconducting oxide as described above, wherein the temperature of the aqueous solution is raised to 100 to 200 ° C. in a closed container.

[0013] The present invention also provides the above superconducting oxide, wherein the single crystal substrate of the bulk superconducting oxide is an oxide whose superconductivity appears or disappears depending on the oxygen concentration. This is a method for producing a single crystal film.

Further, according to the present invention, the single crystal substrate of the bulk superconducting oxide is La ₂ CuO ₄ , and the oxygen-excess superconducting oxide film is La ₂ CuO _{4 + d} (0.06 ≤
d ≦ 0.13), wherein the superconducting oxide single crystal film is manufactured.

In the method of the present invention, a step of heat-treating a single crystal substrate of a bulk superconducting oxide into a non-superconducting oxide is required for the following reasons. Since a bulk single crystal is synthesized in an oxygen-containing gas atmosphere, it is known that the grown single crystal has an oxygen excess relative to the stoichiometric composition and exhibits superconductivity. Even when a stoichiometric single crystal is stored in the air at room temperature, it shows superconductivity after several days. However, in any case, since the excess oxygen amount d is about 0.03, the superconductivity is poor. If the chemical oxidation treatment is directly performed using such a single crystal substrate, the superconductivity of the superconducting single crystal film formed on the surface of the single crystal substrate is deteriorated because the superconductivity of the substrate is poor.

On the other hand, the superconducting oxide single crystal is heat-treated in an inert gas such as nitrogen or argon to remove the excess oxygen in the superconducting oxide single crystal once to make it non-superconducting. By subjecting this to a chemical oxidation treatment, a high-quality superconducting oxide single-crystal film having the same constituent elements, crystal structure, and orientation but different oxygen concentration from the non-superconducting oxide single-crystal substrate can be produced. it can. The heat treatment temperature for this is about 600
If the temperature is higher than 0 ° C., excess oxygen can be removed, but it takes a long time. The heat treatment time is 1 m, depending on the size of the sample.
If the thickness is about m, about 10 hours or more is enough,
It takes about 40 hours for a relatively thick sample.

The heat-treated non-superconductive oxide single crystal substrate is immersed in an aqueous solution of permanganate. The permanganate may be either a potassium or sodium salt. The single crystal substrate does not need to be particularly subjected to a surface treatment before being immersed in the aqueous solution of permanganate, but may be mirror-polished to obtain a smooth crystal surface.

The temperature of the aqueous solution of permanganate during chemical oxidation is preferably from 40 to 200 ° C. After immersing the single crystal substrate in the solution at room temperature, the temperature is raised to this temperature range. When the temperature of the aqueous solution is raised to 100 ° C. or higher, heating is performed using a closed container. For example, polytetrafluoroethylene (P
TFE), and then further sealed in a stainless steel container and then heated. The temperature of the aqueous solution is 40
If the temperature is lower than 0 ° C., the concentration of the permanganate saturated solution becomes low, so that it takes a long processing time and the excess oxygen amount d of La ₂ CuO _{4 +} d becomes small, so that a single crystal film having excellent superconductivity can be obtained. Absent. If the temperature exceeds 200 ° C., La ₂ CuO
_{4 +} oxygen tends to occur from _d, excess oxygen content of La ₂ CuO 4 + _d is lowered.

The concentration of the aqueous solution of permanganate may be between 1% and a saturated concentration. The solubility of KMnO ₄ increases with the temperature of the aqueous solution, for example, 5.96 g at 20 ° C., but 18.15 g at 60 ° C. Therefore, increasing the temperature of the aqueous solution increases the saturation concentration, so that the range of the solution concentration can be widened. The value of the excess oxygen amount d increases with the treatment time when the concentration of the aqueous solution is constant, but becomes constant after about 96 hours. If the treatment time is constant, the value of the excess oxygen amount d slightly increases as the solution concentration increases. If the processing temperature and solution concentration are constant,
As the temperature increases, the value of the excess oxygen amount d dramatically increases. Therefore, in order to maximize the value of d, it is advisable to use a saturated solution of permanganate at a temperature of 150 to 200 ° C. La ₂ CuO _{4 + d} is an antiferromagnetic insulator when the excess oxygen amount d is zero, but is known to exhibit superconductivity when the excess oxygen amount d is 0.03 or more. ≦ d ≦
Preferably, the excess oxygen amount is 0.13.

The method of the present invention is directed to a superconducting oxide whose superconductivity appears or disappears depending on the oxygen concentration, and is intended for oxygen deficiency in La ₂ CuO ₄ or 123-based oxide superconductors. Substrate, for example, YBa ₂ Cu ₃ O _7-d
Or NdBa ₂ Cu ₃ O _7-d or the like is used as a single crystal substrate.

In the case of La ₂ CuO ₄ , the oxidation treatment in the method of the present invention is represented by the following formula in a neutral solution by a chemical formula.

La ₂ CuO ₄ + (2d / 3) KMnO ₄ +
(D / 3) H ₂ O → La ₂ CuO _{4 + d} + (2d / 3) Mn
O ₂ + (2d / 3) KOH In an acidic solution, the results are as follows.

La ₂ CuO ₄ + (2d / 5) KMnO ₄ →
La ₂ CuO _{4 + d} + (2d / 5) MnO + (d / 5) K ₂
O

FIG. 1 shows that the method of the present invention can be used to obtain La ₂ C
FIG. ₄ is a cross-sectional view schematically showing a change in a single crystal surface when a uO ₄ single crystal substrate is processed. Since the method of the present invention chemically oxidizes the surface of the single crystal, as shown in FIG. 1, the surface of the single crystal is oxidized by the permanganate solution, and at the same time, the surface is partially eroded. It becomes rough and somewhat uneven. However, since the thickness of the superconducting layer is on the order of microns, a smooth surface can be obtained by appropriately polishing the irregularities by mechanical polishing or the like.

According to the method of the present invention, as shown in FIG. 2, the surface of the non-superconductive La ₂ CuO ₄ substrate is superconductive La ₂
A CuO _{4 + d} film is formed. The superconducting La ₂ CuO _{4 + d} film has exactly the same crystallographic orientation relationship as the non-superconducting La ₂ CuO ₄ substrate.
A single crystal film with a long axial orientation can be manufactured.

In the preparation of a conventional superconducting thin film, it is necessary to consider problems such as control of lattice matching, orientation, and supersaturation due to differences in lattice constant and thermal expansion coefficient between the substrate and the film. It is known that a high quality single crystal film can be obtained as much as possible, but a film formation condition is limited and a high quality single crystal film has not been obtained.

On the other hand, the method of the present invention only oxidizes the surface of the single crystal substrate, and the only difference between the superconducting single crystal film and the single crystal substrate is the difference in oxygen concentration.
No problem of lattice matching occurs. Further, in this method, since the film is not formed on the substrate, it is not necessary to consider the degree of supersaturation. Therefore, a superconductor single crystal film can be manufactured very easily at a low cost. Further, the method of the present invention can perform an oxidation treatment at a higher temperature than chemical oxidation using a non-aqueous solvent, thereby increasing the amount of excess oxygen d, and has a superconducting film excellent in superconductivity. Can be produced.

[0028]

Example 1 Using a four-ellipsoidal mirror type floating zone melting apparatus, a bulk La ₂ CuO ₄ single crystal was grown by the TSFZ method under the conditions of a growth rate of 1 mm / h, an oxygen atmosphere of 0.2 MPa, and a growth axis α-axis. The substrate was grown. The axis of the obtained crystal was determined by the X-ray back Laue method. As a result of the growth by the TSFZ method, a high-quality single crystal of La ₂ CuO ₄ having a metallic luster was obtained from the initial part of the crystal having a length of 50 mm and a diameter of 5 mm having a rod shape.

The La ₂ CuO ₄ single crystal was cut along the axis, and the cut piece of the single crystal was cut in nitrogen at 900 ° C. for 40 minutes.
Heat treatment for non-superconducting time for 5% KMnO
_{4 The} crystal surface was immersed in an aqueous solution, and the water temperature was raised to 50 ° C., and the crystal surface was oxidized for 48 hours, 72 hours, and 96 hours.

For each of these samples, SQUI
The sample was placed in a state where the magnetic field inside the device was removed by a D magnetometer.
After cooling to ~ 5K and applying a magnetic field of 10e to the sample,
The magnetic susceptibility was measured while heating the sample to around 40K (zero magnetic field cooling). Thereafter, the susceptibility was measured while cooling to around 5K (magnetic field cooling). As a result of the measurement, the shielding effect of eliminating the applied magnetic field, which is a property of the superconductor, was confirmed. This revealed that superconductivity was developed. The thickness of the superconducting film was about 10 μm as estimated from the magnetic susceptibility and the sample size. In addition, when the presence or absence of erosion of the single crystal surface by the aqueous solution of KMnO ₄ was observed with a bright and dark field microscope, a part of the surface was eroded.

FIG. 3 shows the magnetic susceptibility (M / M) of a single crystal treated at 50 ° C. for 48 hours in a 5% aqueous KMnO ₄ solution.
H) Changes in ■, magnetic susceptibility of as-grown single crystal (single crystal grown and not subjected to heat treatment) and magnetic susceptibility of N ₂ annealed single crystal (bulk single crystal substrate after heat treatment) 6 is a graph of a shielding curve showing the following. It can be seen that the single crystal substrate before chemical oxidation was non-superconductive, but became a superconductor by chemical oxidation. Further, it can be seen that the shielding fraction of the sample after the chemical oxidation is much larger than that of the bulk single crystal substrate before the heat treatment.

FIG. 4 is a graph showing magnetic susceptibility ● for 48 hours of processing.
It is a graph of a shielding curve which shows the change of magnetic susceptibility mark of processing time 72 hours, and change of magnetic susceptibility mark of processing time 96 hours. Although the Meissner fractions are all small, the shielding curves show that these samples show superconductivity. Moreover, it can be seen that the superconducting transition becomes sharper and the superconducting transition temperature rises according to the processing time. It should be noted that almost no change in magnetic susceptibility was observed after 96 hours, and it is considered that equilibrium is reached in 96 hours.

Example 2 In the same manner as in Example 1, La ₂ CuO _{4 + d} single crystal was
Heat treatment was performed at 900 ° C. for 40 hours to make the material non-superconductive. FIG.
As shown in FIG. 2, 10 ml of a 5% KMnO ₄ aqueous solution 1 is placed in a container 2 made of polytetrafluoroethylene, and the heat-treated La ₂ CuO ₄ single crystal substrate 3 is immersed therein. The container was sealed, and the polytetrafluoroethylene container 2 was placed in a stainless steel sealed container 5, a stainless steel plate 6 was placed on the polytetrafluoroethylene lid 4, and the stainless steel lid 7 was placed in a thermostat. The aqueous solution 1 was heated to 100 ° C. and allowed to stand for 96 hours so that the pressure of the polytetrafluoroethylene lid 4 could be adjusted by the pressure adjusting screw 8 via the pressure adjusting screw.

FIG. 6 is a graph showing the change of the magnetic susceptibility (M / H) in the second embodiment in comparison with the change of the magnetic susceptibility in the first embodiment for 96 hours. Superconducting transition temperature is about 2
Although it is around 6K, it can be seen that increasing the temperature of the aqueous solution to 100 ° C. dramatically reduces the transition width and increases the Meissner fraction and the shielding fraction, thereby dramatically improving the superconductivity. The thickness of the superconducting film was about 10 μm as estimated from the magnetic susceptibility and the sample size.

Example 3 The heat-treated single crystal substrate was immersed in a 5% KMnO ₄ aqueous solution and then oxidized in the same manner as in Example 2 except that the substrate was allowed to stand in a thermostat at 150 ° C. for 48 hours. Then, after taking out the single crystal substrate and washing it with water, the magnetic susceptibility was measured with a SQUID magnetometer. FIG. 7 shows the susceptibility (M / H) of Example 3.
6 is a graph showing the change of the mark 対 in comparison with the magnetic susceptibility ● mark and the magnetic susceptibility mark の of Example 2 for a treatment time of 96 hours in Example 1. In this embodiment, 96 hours at 50 ° C. and 96 hours at 100 ° C.
The superconducting transition temperature increased by about 5K and the superconducting transition became very sharp as compared with the case where the chemical oxidation treatment was performed for a long time. Further, the Meissner fraction was improved. The thickness of the superconducting film was about 10 μm as estimated from the magnetic susceptibility and the sample size. Therefore, it can be seen that by increasing the oxidation treatment temperature to 150 ° C., a high-quality micron-order superconductive single crystal film can be manufactured in a shorter time.

[Brief description of the drawings]

FIG. 1 shows a single crystal substrate of La ₂ C according to the method of the present invention.
FIG. ₄ is a cross-sectional view schematically showing a change in the surface of a single crystal substrate when uO ₄ is used.

FIG. 2 shows a non-superconducting La ₂ CuO _{4 according} to the method of the invention.
The superconducting La ₂ CuO 4 + _d film formed on the surface of the substrate is a perspective view schematically showing.

FIG. 3 is a graph showing a change in magnetic susceptibility (M / H) ■ of Example 1 in comparison with a change in magnetic susceptibility の of an as-grown single crystal and change of magnetic susceptibility ● of an N ₂ annealed single crystal.

FIG. 4 shows the magnetic susceptibility (M
/ H) (● mark 48 hours, ▲ mark 72 hours, △ mark 96 hours)
6 is a graph showing a change in the graph.

FIG. 5 is a schematic partial sectional view of a thermostat that can be used when the method of the present invention is carried out at an aqueous solution temperature of 100 ° C. or higher.

FIG. 6 is a graph showing a change in magnetic susceptibility (M / H) in Example 2 in comparison with a change in magnetic susceptibility in Example 1;

FIG. 7 is a graph showing a change in magnetic susceptibility (M / H) in Example 3 in comparison with a change in magnetic susceptibility in the first example and a change in magnetic susceptibility in the second example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Permanganate aqueous solution 2 PTFE container 3 Oxide single crystal substrate 4 PTFE lid 5 Stainless steel container 6 Stainless steel plate 7 Stainless steel lid 8 Pressure adjusting screw

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-219303 (JP, A) Takayama-Muromachi et al. , "Direct oxidation of lanthanum copper oxide (La2CuO4) in an aque eous solution of potasium permanate rate (KMnO4)," Physica. 207, no. 1-2, 1993, pp. 97-101 (58) Field surveyed (Int. Cl. ⁷ , DB name) C30B 1/00-35/00 C01G 1/00 CA (STN)

Claims (4)

(57) [Claims] 1. A single crystal substrate of a bulk superconducting oxide is heat-treated into a single crystal substrate of a non-superconducting oxide, and
The surface of the single crystal substrate is chemically oxidized by immersion in a permanganate aqueous solution at 0 to 200 ° C. to introduce oxygen into the single crystal substrate to form an oxygen-excess superconducting oxide film. A method for producing a single crystal film of a superconducting oxide, comprising: 2. A single crystal substrate of a bulk superconducting oxide is heat treated to form a single crystal substrate of a non-superconducting oxide, which is immersed in an aqueous solution of permanganate at room temperature, and the aqueous solution is sealed. The method for producing a superconducting oxide single crystal film according to claim 1, wherein the temperature is raised to 100 to 200 ° C. in the container. 3. The superconducting oxide according to claim 1, wherein the single crystal substrate of the bulk superconducting oxide is an oxide whose superconductivity appears or disappears depending on the oxygen concentration. A method for manufacturing a single crystal film. 4. A single crystal substrate of a bulk superconducting oxide is L
a ₂ CuO ₄ , and the oxygen-excess superconducting oxide film is L
_{2. The method according} to claim 1, wherein a2CuO4 _{+ d} (where 0.06.ltoreq.d.ltoreq.0.13) is satisfied.