JPWO2013140677A1 - Manganese oxide thin film and oxide laminate - Google Patents

Abstract

Provided is a thin film or a laminate that realizes a switching function by phase transition by Mott transition at room temperature. In one embodiment of the present invention, there is provided a perovskite-type manganese oxide thin film 2 formed on the surface of a substrate 1 having a (110) plane orientation or a (210) plane orientation. The manganese oxide thin film has a composition formula Ln1-xAexMnO3 (where Ln is at least one trivalent rare earth element selected from lanthanoids, and Ae is at least one selected from the group consisting of Ca, Sr, and Ba). A composition of a certain alkaline earth element), and in the composition formula, 0 <x≰1 / 18.

Description

  The present invention relates to a manganese oxide thin film and an oxide laminate. More particularly, the present invention relates to a perovskite-type manganese oxide thin film and an oxide laminate that undergo Mott transition at room temperature and switch electrical, magnetic, or optical properties.

  In recent years, there is a concern that the scaling law, which has been a guideline for improving the performance of semiconductor devices, will eventually face limitations. Accordingly, materials that enable a new principle of operation other than silicon that has been used for a long time have attracted attention. For example, in the field of spintronics that incorporates the degree of freedom of spin, development aimed at high-density nonvolatile memory capable of high-speed operation similar to DRAM (Dynamic Random Access Memory) is underway.

  On the other hand, research on strongly correlated electron materials that cannot apply the band theory that supports the basics of semiconductor device design has also progressed. Among them, a substance showing a huge and high-speed property change due to a phase transition of an electronic system has been found. In a strongly correlated electron system material, not only spin but also the degree of freedom of electron orbital is involved in the phase state of the electron system, and various electronic phases consisting of various orders formed by spin, charge, and orbital appear. To do. A representative example of strongly correlated electron materials is perovskite-type manganese oxide, in which the charge-ordered phase in which 3d electrons of manganese (Mn) are aligned by the first-order phase transition or the electron orbital. It is known that an orbital-ordered phase that aligns

  In the charge alignment phase or the orbital alignment phase, carriers are localized, so that the electric resistance is high, and the electronic phase is an insulator phase. Further, the magnetic property of this electronic phase is an antiferromagnetic phase due to superexchange interaction and double exchange interaction. In many cases, the electronic state of the charge alignment phase or the orbital alignment phase should be regarded as a semiconductor. This is because, in the charge alignment phase or the orbital alignment phase, although the carriers are localized, the electric resistance is lower than that of a so-called band insulator. However, by convention, the electronic phase of the charge alignment phase or the orbital alignment phase is expressed as an insulator phase. On the contrary, when the electric resistance is low and the metal behavior is exhibited, the spin is aligned and the electronic phase is a ferromagnetic phase. There are various definitions of the metal phase, but here, “the sign of the temperature differential coefficient of resistivity is positive” is expressed as the metal phase. Corresponding to this expression, the insulator phase is redefined as “the sign of the temperature differential coefficient of resistivity is negative”.

  In addition to the charge alignment phase and orbital alignment phase described above, any one of electronic phases such as a phase in which both charge alignment and orbital alignment are established (charge and orbital ordered phase) It has been disclosed that a phenomenon in which various switching functions are expressed in a single crystal bulk material of the substance is observed in the possible substances (Patent Documents 1 to 3). These phenomena are typically observed as a large change in electrical resistance or a transition between antiferromagnetic and ferromagnetic phases. For example, resistance changes of several orders of magnitude due to application of a magnetic field are well known as the giant magnetoresistance effect.

In order to manufacture any device (device) such as a practical electronic device, magnetic device, or optical device that uses these phenomena as a switching function, the phenomenon that brings about the switching function is a temperature range above room temperature (for example, 300 K). Needs to be realized. However, all of the switching functions disclosed in Patent Documents 1 to 3 have been confirmed at a low temperature such as a liquid nitrogen temperature (77K) or lower. Perovskite type manganese oxide in these disclosures, when denoted the chemical composition as ABO _3, atomic stacking surface AO layers, BO ₂ layers, AO layer, a laminated structure which is ... repeatedly stacked. Hereinafter, the crystal structure of such a stacked structure is referred to as AO-BO ₂ -AO. In the unit cell of the perovskite structure, the A site occupies the apex, the B site occupies the body center, and O (oxygen) occupies the face center. Manganese is arranged at the B site.

In the perovskite-type manganese oxide disclosed in each of the above documents 1 to 3, it is considered that this is related to a decrease in the temperature at which the switching phenomenon is observed, that is, the temperature at which the charge orbital order develops (hereinafter referred to as “expression temperature”). What is the type of element or ion occupying the A site of the perovskite crystal structure. In short, the A site of the perovskite crystal structure is randomly occupied by trivalent rare earth cations (hereinafter referred to as “Ln”) and divalent alkaline earth (“Ae”). The onset temperature has decreased. Conversely, if the element or ion at the A site can be ordered as AeO—BO ₂ —LnO—BO ₂ —AeO—BO ₂ —LnO—BO ₂ —..., The transition temperature to the charge alignment phase It is also known that can be raised to around 500K. Hereinafter, the regular arrangement of ions occupying the A site, as exemplified here, is referred to as “A site ordering”, and the perovskite type manganese oxide in which such A site ordering is realized. Is called A-site ordered perovskite manganese oxide. A group of substances exhibiting such a high transition temperature is characterized by containing Ba (barium) as the alkaline earth Ae. For example, Ba is contained as the alkaline earth Ae, and Y (yttrium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd (gadolinium), Eu (europium) having a small ionic radius as the rare earth element Ln, It has been reported that when Sm (samarium) is used, the transition temperature exceeds room temperature.

  In order to realize any device (electronic device) such as a magnetic device or an optical device using these phenomena, it is necessary to realize the above switching phenomenon after forming a perovskite-type manganese oxide in a thin film form. Become. However, there is a problem that even if the thin film is formed on the (100) plane orientation substrate, it is difficult to realize the switching function. This is because the lattice deformation called Jahn-Teller mode necessary for the phase transition to the charge alignment phase or the orbital alignment phase is suppressed due to the in-plane four-fold symmetry. .

On the other hand, Patent Document 4 discloses that a perovskite oxide thin film is formed using a (110) plane orientation substrate. According to this disclosure, when the in-plane four-fold symmetry is broken in the (110) plane orientation substrate, shear deformation of the crystal lattice is allowed when the formed thin film is switched. When this shear deformation occurs, the crystal lattice is oriented parallel to the substrate surface, and the charge alignment surface and the orbital alignment surface are not parallel to the substrate surface. Further, Patent Document 5 discloses an example in which the A-site ordered perovskite-type manganese oxide is thinned. This disclosure reports a coating light irradiation method in which an amorphous thin film is once deposited and then crystallized and A-site ordering is performed by laser annealing. In fact, it has been confirmed by electron beam diffraction that the A site is ordered in the SmBaMn ₂ O ₆ thin film formed on the (100) plane orientation SrTiO ₃ (lattice constant 0.3905 nm) substrate.

JP-A-8-133894 Japanese Patent Laid-Open No. 10-255481 JP-A-10-261291 Japanese Patent Laid-Open No. 2005-213078 JP 2008-156188 A

  However, a substance that exhibits charge orbital order at room temperature and higher by ordering ions at the A site has a degree of order in the ions at the A site that is the temperature at which the switching phenomenon is realized, that is, the expression of charge orbital order. There is a problem that the temperature is greatly affected. In particular, in a thin film made of an A-site ordered perovskite-type manganese oxide, the degree of order of A-site ions is reduced even if defects are introduced into the formed thin film or a slight shift occurs in the composition of the thin film. There are concerns. The thin film on the (110) plane orientation substrate reported in Patent Document 4 has a problem that it does not contribute to the solution of any of the problems of the reduction in the degree of order and the reduction in the expression temperature. As described above, in the conventional perovskite type oxide thin film, there is a problem that the temperature at which the charge orbital order appears is low, and the temperature at which the charge orbital order rises above room temperature depending on the degree of order at the A site and the anxiety. The qualitative problem is still unresolved.

  The present invention has been made in view of the above problems. The present invention creates a novel device by providing a manganese oxide thin film or an oxide laminate that realizes a switching function by controlling phase transition by some external stimulus (external field) at room temperature. It contributes to.

  As a result of examining the above-mentioned problems, the inventors of the present application have found that the problem of the function expression temperature remaining at a low temperature and the function expression temperature above room temperature depending on the order degree of the A site and hence the instability are the perovskite type Mn oxidation. It has been considered that the composition ratio of two kinds of cations occupying the A site of the product, that is, trivalent rare earth cation (Ln) and divalent alkaline earth (Ae), has found a means for solving this problem.

  In other words, each of the above-mentioned problems is first manifested in a situation where a considerable amount of divalent alkaline earth (Ae, for example, Sr or Ba) is present for two types of cations, that is, trivalent rare earth elements (Ln). The inventor of the present application thought that it was caused by utilizing the first order transition. A conventionally attempted ratio of Ae is 0.3 to 0.5 with respect to the sum of Ae and Ln. In that case, the goal is to develop switching at room temperature by raising the transition temperature, which is lower than room temperature. However, the inventors of the present application have conceived that switching should be allowed to occur near room temperature even by a completely reverse approach, and have found a specific means. In this application, an approach that lowers the transition temperature from a composition having a transition temperature higher than room temperature is adopted.

In one embodiment of the present invention, as a specific means for solving at least one of the above problems, a perovskite-type manganese oxide thin film formed on a (110) or (210) oriented substrate surface And Ln _1-x Ae _x MnO ₃ (where Ln is at least one trivalent rare earth element selected from lanthanoids, and Ae is selected from the group consisting of Ca, Sr, and Ba) A manganese oxide thin film having a composition of at least one alkaline earth element), wherein 0 <x ≦ 1/18 is provided.

The manganese oxide of the perovskite-type manganese oxide thin film has a crystal lattice having a composition expressed as ABO ₃ . And the crystal | crystallization of the manganese oxide thin film in this aspect has the oxygen octahedron which has Mn (manganese) in the B site similarly to the normal perovskite type crystal, and surrounds the Mn. In particular, the element Ln and, for example, divalent alkaline earth (Ae) are arranged at the A site in this embodiment. Holes are introduced corresponding to the composition ratio x of alkaline earth ions (Ae). At this time, by setting the above range of x, a two-dimensional region in which two or more Mn ³⁺ ions are continuously adjacent to each other is surely secured regardless of the distribution of holes, that is, the spatial distribution of Mn ⁴⁺ ions. Is possible. Therefore, since the region of the orbital alignment phase in the Mott insulator where x = 0, that is, the transition temperature is much higher than room temperature, decreases, the orbital alignment phase itself is reduced while lowering the transition temperature of the orbital alignment phase itself. Expected to be maintained. In addition, since the in-plane four-fold symmetry is broken when the plane orientation of the substrate is the (110) or (210) plane orientation, a misfit or the like in which the insulator-metal transition, which is the primary transition, grows coherently on the substrate, etc. It can be used in an epitaxial thin film having no defects.

When x ≦ ^1/27 , it is possible to always ensure a three-dimensional region in which two or more Mn ³⁺ ions that are ^orbitally aligned are adjacent to each other regardless of the distribution state of Mn ⁴⁺ ions. In this case, it is easy to maintain the orbital alignment phase in the Mott insulator, but the threshold value of the external field required to collapse and metallize the Mott insulator is increased.

  Accordingly, when it is desired to reduce the threshold value of the external field necessary for collapsing and metallizing the Mott insulator while taking a relatively large resistance change ratio due to the insulator-metal transition, 1/27 <x ≦ 1/18. The range is preferable.

In the above-described manganese oxide thin film according to the present invention, the composition of the manganese oxide thin film has a composition formula Ln _1-x Ae _x MnO ₃ (where Ln is La, Ce, Pr, Nd, Pm, Sm, It is preferable that it is represented by at least one trivalent rare earth element selected from the group consisting of Eu, Gd, Tb, and Dy.

  The group of trivalent rare earth elements selected as the element Ln is an element group obtained by arranging lanthanoids in the order of atomic numbers and excluding Ho. When a trivalent rare earth element is selected from the above group, there is an advantage that the degree of rotation of the oxygen octahedron can be controlled and the ease of occurrence of orbital alignment can be adjusted.

  In addition, the present invention also provides an oxide stack with additional layers added. That is, an aspect of the present invention includes the manganese oxide thin film according to any one of the above aspects, and a strongly correlated oxide thin film in contact with any surface of the manganese oxide thin film. The thickness t of the entire body, the thickness tm of the manganese oxide thin film, and the thickness t1 of the strongly correlated oxide thin film are compared with the critical film thickness tc for the strongly correlated oxide thin film to become a metal phase. T = tm + t1> tc and t1 <tc, an oxide stack satisfying the relationship is provided.

  In the oxide laminated body of this aspect, the detection of the switching function of the manganese oxide thin film by an insulator metal transition (Mott transition) becomes easy compared with the thing of the manganese oxide thin film single-piece | unit mentioned above. This is because the switching function of the manganese oxide thin film, that is, the change in the electronic state due to the insulator-metal transition (Mott transition) can be easily detected from the outside, for example, as a resistance change of the oxide laminate sample. This mechanism for facilitating detection is called dimensional crossover, and details thereof will be described in detail in the section “1-4 Improvement of detectability by stacking (dimensional crossover)”. Furthermore, as a result of easy detection, it becomes possible to reduce the threshold value of the external field necessary for causing a secondary transfer. This is because the thickness of the entire manganese oxide thin film that undergoes Mott transition can be made thinner than that of a single manganese oxide thin film.

  Furthermore, in the oxide laminate provided in the present invention, the manganese oxide thin film according to any one of the above aspects, a first strongly correlated oxide thin film in contact with one surface of the manganese oxide thin film, A second strongly correlated oxide thin film in contact with the other surface of the manganese oxide thin film, the thickness t of the whole oxide stack, the thickness tm of the manganese oxide thin film, the first And the thickness t1 and t2 of each of the second strongly correlated oxide thin films are t = tm + t1 + t2> tc and max (t1, t2) <tc, where max () includes a function that returns the maximum value of variables, and an oxide stack that satisfies the relationship.

  According to the above aspect, the thickness of the perovskite-type manganese oxide thin film can be further reduced, so that the external field strength necessary for switching can be further reduced. In the oxide laminate of this embodiment, the strongly correlated oxide thin film is disposed on both sides of the manganese oxide thin film. This is because the effect of bringing the strongly correlated oxide thin film into contact is more remarkably obtained as compared with the case of only one side.

  The electronic phase in the composition of the manganese oxide thin film of the above aspect of the present invention exhibits the property of phase transition between the insulator and the metal due to Mott transition. Here, such a manganese oxide is also a kind of material group generally called a Mott insulator. However, the manganese oxide thin film in the present application is a thin film exhibiting a property of causing a metal-insulator transition and is not always an insulator phase. Such a material is hereinafter referred to as “manganese oxide”. Mott transition generally involves not only temperature but also external stimuli (hereinafter referred to as “external field”). In the Mott transition caused only by the temperature when no external field is applied, the insulator phase appears on the low temperature side, and the metal phase or low resistance phase appears on the high temperature side. On the other hand, in the Mott transition caused by changing the external field at a certain temperature, the insulator phase appears on the side where the external field is weak, and the metal phase appears on the side where the external field is strong. In the manganese oxide thin film according to the above-described aspect of the present invention, the Mott transition can be caused by an external field at room temperature (for example, 300 K). This is because, in the above embodiment, the transition temperature of the Mott transition that usually appears at a temperature higher than room temperature is lower than the conventional one, and the threshold of the external field for the Mott transition is smaller than the conventional one. Means both or either. The external field here typically includes a magnetic field, an electric field, a current, light, and pressure, and any combination thereof.

  It should be noted that in the present application, the element Ln is at least one element and may include a combination of a plurality of elements.

Incidentally, supplementary case where trivalent rare earth element Ln in the formula _{Ln 1-x} Ae _x MnO ₃ manganese oxide thin film of the present invention is the combination of a plurality of elements. For example, when the chemical composition when two kinds of trivalent rare earth elements are used as the element Ln is expressed in another form, it is expressed as Ln _1-x Ae _x MnO ₃ which is the manganese oxide of this embodiment. In the composition, Ln ¹ and Ln ² are used as the different rare earth elements that can be trivalent cations, and (Ln ¹ _1-x Ae _x MnO ₃ ) _y (Ln ² _1-x Ae _x MnO ₃ ) _1-y However, a composition expressed as 0 <y <1 is included. The composition expressed in this way is typically a manganese oxide Ln ¹ _1-x Ae _x MnO ₃ containing a rare earth element Ln ¹ and a manganese oxide Ln ² _1-x Ae containing a rare earth element Ln ^2. _x A solid solution having an arbitrary ratio y: 1-y with MnO ₃ .

  The perovskite-type manganese oxide thin film and oxide laminate of the present invention have a composition ratio by keeping the composition ratio of the trivalent rare earth cation (Ln) and the divalent alkaline earth (Ae) in the A site within a certain range. The problem of dispersion due to distribution is solved, and the Mott transition controlled by the external field is realized at room temperature by dramatically reducing the external field threshold necessary for the insulator-metal transition at room temperature.

It is a schematic sectional drawing of an example of the manganese oxide thin film in one embodiment of this invention. FIG. 1 (a) is an overall view showing the structure of a manganese oxide thin film formed on a substrate 1, and FIGS. 1 (b) and 1 (c) show the (110) plane orientation of the substrate 1, respectively. It is an enlarged view which shows the atomic lamination surface (sectional drawing) of a manganese oxide thin film when it is a board | substrate and a (210) surface orientation board | substrate. In the manganese oxide thin film of an embodiment of the present invention, in the case where electrons have orbital order, the arrangement of Mn ^{4+ in} the case where one Mn ⁴⁺ is arranged on an average in a 3 × 3 unit cell on the XY plane. It is explanatory drawing which shows a condition. FIG. 2 (a) is a diagram illustrating a 2 × 2 Mn ³⁺ arrangement state in the case where one Mn ⁴⁺ is ^averagely arranged in a 3 × 3 unit cell on the XY plane, and FIG. b) shows orbital alignment in a 2 × 2 Mn ³⁺ configuration. It is explanatory drawing which shows the arrangement ^| positioning condition of ^{Mn4 +} by which one Mn4 ⁺ is arrange ^| positioned at 3x3 unit cell in the XY plane in the manganese oxide thin film of one embodiment of this invention. FIG. 3 (a) shows a case where they are arranged on average, and FIG. 3 (b) shows a case where they are arranged unevenly. In the manganese oxide thin film of an embodiment of the present invention, a situation is shown in which one Mn ⁴⁺ is arranged in a region occupied by 3 × 3 unit cells in the XY plane and a plurality of unit cells in the Z direction. It is explanatory drawing. FIGS. 4 (a) shows a state that Mn ⁴⁺ two unit cell in the Z direction is averaged arranged, FIG. 4 (b), Mn ⁴⁺ are arranged unevenly on two unit cell in the Z-direction Fig. 4 (c) shows an average arrangement of Mn ^{4+ in} three unit cells in the Z direction, and Fig. 4 (d) shows an Mn ⁴⁺ unevenly arranged in three unit cells in the Z direction. Each of them is shown. It is explanatory drawing explaining the mode of an assembly of the unit cell in the manganese oxide thin film formed in the (110) surface orientation board | substrate of embodiment with this invention. 5A is a cross-sectional view taken from the [001] axis direction, and FIG. 5B is a cross-sectional view taken from the [1-10] axis direction. It is explanatory drawing explaining the mode of the assembly of the unit cell in the manganese oxide thin film formed in the (210) surface orientation board | substrate of embodiment with this invention. 6A is a cross-sectional view taken from the [001] axis direction, and FIG. 6B is a cross-sectional view taken from the [1-20] axis direction. It is a schematic sectional drawing which shows the structure of an example of the oxide laminated body containing the manganese oxide thin film produced in contact with the strongly correlated oxide thin film in one embodiment of this invention. FIG. 7A shows an example in which a strongly correlated oxide thin film is formed on the substrate side of the manganese oxide thin film, and FIG. 7B shows an example in which a strongly correlated oxide thin film is formed on the surface of the manganese oxide thin film. is there. 1 is a schematic cross-sectional view of an example of an oxide laminate formed by bringing a strongly correlated oxide thin film into contact with both surfaces of a manganese oxide thin film in an embodiment of the present invention.

  Hereinafter, embodiments of a manganese oxide thin film and an oxide laminate according to the present invention will be described with reference to the drawings. In the description, unless otherwise specified, common parts or elements are denoted by common reference numerals throughout the drawings. In the drawings, each element of each embodiment is not necessarily shown in a scale ratio.

<First Embodiment>
[1 Basic principle]
[1-1 Structure]
Hereinafter, embodiments of a perovskite-type manganese oxide thin film and an oxide laminate according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic cross-sectional view of an example of the manganese oxide thin film 2 in the present embodiment. FIG. 1A is an overall view showing a configuration of a manganese oxide thin film 2 formed on a substrate 1, and FIGS. 1B and 1C show the (110) plane of the substrate 1, respectively. It is an enlarged view which shows the atomic lamination surface (sectional drawing) of the manganese oxide thin film 2 when it is an orientation board | substrate and a (210) plane orientation board | substrate. The crystal structures of FIGS. 1B and 1C are those of the manganese oxide thin film 2 cut along a plane perpendicular to the substrate surface.

The manganese oxide thin film 2 of the present embodiment is a generic term for the manganese oxide thin film 20 formed on the (110) plane-oriented substrate and the manganese oxide thin film 21 formed on the (210) plane-oriented substrate. . As shown in FIG. 1B, the manganese oxide thin film 20 has a crystal structure of an atomic stacked surface in which (Ln, Ae) MnO layers and O ₂ layers are alternately stacked in the direction perpendicular to the substrate surface, , (Ln, Ae) MnO—O ₂ — (Ln, Ae) MnO. With this crystal structure, the in-plane four-fold symmetry is broken and becomes two-fold symmetry. As a result, the insulator-metal transition due to the Mott transition can be realized. On the other hand, as shown in FIG. 1C, the manganese oxide thin film 21 has an atomic stacked surface in which (Ln, Ae) O layers and MnO ₂ layers are alternately stacked in the direction perpendicular to the substrate surface. A material having a crystal structure, that is, a crystal structure aligned with (Ln, Ae) O—MnO ₂ — (Ln, Ae) O. With this crystal structure, in both the manganese oxide thin film 20 and the manganese oxide thin film 21, the in-plane four-fold symmetry is broken and becomes two-fold symmetry. As a result, insulator-metal transition due to the Mott transition can be realized.

FIGS. 1B and 1C illustrate a case where the crystal structure of the perovskite structure of the manganese oxide thin films 20 and 21 of the present embodiment is a cubic crystal. However, the composition of the manganese oxide thin films 20 and 21 of the present embodiment, that is, the perovskite structure and the manganese oxide represented by the composition formula Ln _1-x Ae _x MnO ₃ is a crystal lattice other than a cubic crystal, that is, a tetragonal crystal. Perovskites in crystal structures with lower order symmetry such as (tetragonal), orthorhombic, monoclinic, triclinic, trigonal, hexagonal It can be a structure. This is because, regardless of the crystal lattice, the manganese oxide thin films 20 and 21 of the present embodiment are allowed to be deformed due to the broken symmetry. Note that the perovskite structure of the present embodiment includes a substance having a crystal structure in which, for example, a basic unit cell of a crystal lattice can be obtained only by connecting a plurality of the unit cells described above.

In the present embodiment, the manganese oxide thin film 2 has the composition formula Ln _1-x Ae _x MnO ₃ (where Ln is at least one trivalent rare earth element selected from lanthanoids, and Ae is Ca, At least one alkaline earth element selected from the group consisting of Sr and Ba), and 0 <x ≦ 1/18 in the composition formula. Here, in the manganese oxide thin film 2, holes are introduced corresponding to the composition ratio x of alkaline earth ions (Ae). For this reason, x may be described as a hole doping amount.

[1-2 Facilitating Mott transition in manganese oxide thin film]
Next, in the manganese oxide thin film 2 having the above-described structure, a mechanism that facilitates Mott transition and allows switching to occur at room temperature will be described. First, a divalent alkaline earth (Ae) with respect to a trivalent rare earth cation (Ln) at the A site, that is, a hole doping amount has been conventionally attempted so that x ≦ 1/18 or x ≦ 1/27. The reason why the number is significantly reduced as compared with x ≦ 0.3 or 0.5 will be described.

The amount of hole doping that has been attempted in the past is as high as several tens of percent as described above. And Mn ⁴⁺ corresponding to such a large amount so introduced holes, electrons and the jammed Mn ³⁺ in e _g orbitals to form a charge ordering phase and charge trajectory aligned phases also say electronic self-organizing. Most of the conventional approaches pay attention to this point. From the viewpoint of electrical properties such as an electronic phase that is an insulator and a metal phase that is a good conductor, a huge physical property change (electrically) Was aimed at lowering the threshold of resistance change. However, in exchange, the above phase boundary exists in a low temperature region below room temperature, and is not suitable for room temperature operation.

  On the other hand, the inventors of the present application have focused on a diluted doped region with a small amount of hole doping. Conventionally, a Mott insulator with x = 0 is known as having a very high transition temperature of 700 to 1000 K or higher, and therefore, the insulating phase is too “robust” and the insulator due to the external field Metal transition was difficult. Then, to what extent can the orbital alignment phase be maintained when holes are gradually doped into the orbital alignment phase of the Mott insulator that exhibits this very high transition temperature? This question is the starting point of the present invention. That is, if the phase boundary of the Mott insulator at the high temperature orbital alignment phase is realized rather than aiming at the well-known phase boundary of the charge alignment phase or charge orbital alignment phase at the low temperature, the phase boundary The idea is reached that it is possible to achieve both low threshold and room temperature operation. Of course, since carriers (holes) are slightly doped, it is considered that the carriers move more easily than the Mott insulator and the resistance change ratio in the insulator-metal transition becomes small. However, it can be said that such an approach is very useful in view of the fact that there is actually an application that can sufficiently cope with multi-values if the resistance change ratio is half digits to one digit as in a nonvolatile memory, for example.

  From this point of view, there have been no reports on perovskite-type manganese oxide thin films, so a physical search for boundary conditions at high temperatures (above room temperature) relative to the amount of hole doping in orbital alignment phases in Mott insulators Consideration in various aspects is useful.

  The question the inventor of this application has is what is the mechanism that governs the robustness of the order of the electronic system in a Mott insulator such as manganese oxide. By addressing this question, the present inventor has come to the hypothesis that the robustness in orbital alignment is due to cooperative phenomena and depends on the number of electron orbitals. If this hypothesis is correct, even if the manganese oxide has a “strong” electron system that is difficult to deal with in the form of bulk crystals, it only has to be formed in a thin film and the number of orbits reduced sufficiently. There is a high possibility that it will be possible to deal with the above trade-off. That is, it is thought that the above-mentioned “strength” can be reduced to a degree that can be controlled by an external field by making the manganese oxide into a thin film form. This is the concept behind the idea of the present invention.

Phenomena observed in strongly correlated electron systems, such as charge alignment and orbital alignment, are not only cooperative but also one aspect of the many-body effect in substances with a large electron correlation effect. In other words, as long as only one unit cell containing only one Mn ³⁺ ion having a 3d orbital is targeted, the definition itself that charges and electron orbits are aligned does not apply. Therefore, consider the electronic state in a system in which two unit cells are connected. In this case, the electron orbital state (orbital state) of one unit cell and the orbital state of the other unit cell are mutually competitive. For this reason, if the system is more stable when both electron orbits are aligned with each other, the orbital alignment state is realized. If the system is more stable when they are not aligned, the orbital alignment state is collapsed. Actually, the energy difference between these two states, that is, the state where the orbital alignment state is realized and the state where it is collapsed, makes the system of two unit cells stable in either state. It may be too small.

  However, consider a system in which the size of the system is increased and there are N unit cells (N is an integer sufficiently larger than 2). In that case, all the orbitals contained in the N unit cells are aligned as compared with the state in which the trajectory of only one unit cell out of the N unit cells is different from the orbits of the other N-1 unit cells. It can be said that it is more stable. That is, an interaction works from the surrounding N−1 unit cells to the orbit of the one unit cell so as to align different orbits. Furthermore, even if the orbits of a couple of unit cells out of the N unit cells are different from the orbits of other unit cells, the orbits included in the N unit cells It is more stable if everything is aligned. In this way, in a system in which a sufficiently larger number of N unit cells than two exists, an interaction acts between the orbits of the unit cells so that the entire orbits are aligned, and the entire system is stabilized.

  What can be seen from the above properties of each of the two unit cells and the N unit cell is that the stability of the electron system depends on the size of the electron system itself, that is, the number of unit cells. Here, again, since both charge alignment and orbital alignment are brought about by a cooperative phenomenon in a material having a strong electron correlation, these alignments can be said to be the product of self-assembly of electrons. Therefore, the number of unit cells themselves plays the most essential role for charge alignment and orbital alignment. Thus, the above hypothesis that the robustness in orbital alignment, which is a cooperative phenomenon, depends on the number of electron orbitals has sufficient rationality theoretically.

The present inventor has further advanced the idea, and has focused on the following facts relating to the unit cell that maintains the order of the orbital alignment phase. Which in orbit aligned phases Mott insulator is ^{to 3x} 2 -r ² and ^3y 2 -r ² is _{e g} orbitals in ^{Mn 3+} ions are alternately arranged. FIG. 2 shows an arrangement state when one Mn ⁴⁺ is arranged on an average in a 3 × 3 unit cell on the XY plane when electrons have orbital order in the manganese oxide thin film of this embodiment. It is explanatory drawing shown. When having orbital order, as shown in FIG. 2, and _{e g} orbitals ^(3x 2 ^-r 2) 11 and _{e g} orbitals ^(3y 2 ^-r 2) 12 in the ^{Mn 3+} ions are alternately arranged. The XY plane is one plane that specifies the crystal axis direction of the manganese oxide thin film 2 as the X, Y, and Z directions. When one Mn ⁴⁺ is arranged on an average in a set of 3 × 3 unit cells on the XY plane, as shown in FIG. 2A, in a set of 2 × 2 unit cells, Mn ³⁺ Only placed.

At this time, as shown in FIG. 2B, in the set of 2 × 2 unit cells, the electron orbit of Mn ³⁺ is arranged in an orbital alignment. The inventors of the present application focused on the smallest unit that maintains the orbital alignment phase in two dimensions. This minimum unit is a set of unit cells that can obtain the arrangement of electron orbits as shown in FIG. 2B, that is, 2 × 2 = 4 Mn ³⁺ ions are arranged. In compositions where it is always possible to find such a minimal unit in a crystal in which Mn ⁴⁺ is arranged, the orbital alignment phase is maintained.

Next, the upper limit value of the hole doping amount x that realizes such an arrangement will be described. FIG. 3 is an explanatory diagram showing an arrangement state of Mn ⁴⁺ when one Mn ⁴⁺ is arranged in a 3 × 3 unit cell on the XY plane in the manganese oxide thin film of the present embodiment. (A) is a case where it arranges on average, FIG.3 (b) is a case where it arranges unevenly. FIGS. 3A and 3B both show conditions for x = 1/9 in the two-dimensional XY plane. A region surrounded by a dotted line in the figure corresponds to 3 × 3 = 9 unit cells. In the orbital alignment phase, since the electronic phase is an insulating phase, the arrangement of Mn ⁴⁺ ions (squares having black dots in the figure) can take various patterns. FIG. 3A shows the most averaged case, and FIG. 3B shows the most unevenly distributed case. In any case, 2 × 2 = 4 unit cells can be arranged as an assembly of unit cells containing Mn ³⁺ ions ( ^open squares in the figure) and not containing Mn ⁴⁺ ions. That is, when x = 1/9 in the two-dimensional XY plane, it does not depend on the degree of variation of the divalent alkaline earth (Ae) with respect to the trivalent rare earth cation (Ln) at the A site. The minimum unit of orbital alignment phase is realized. Such an idea corresponds to the fact that the mean free path in the perovskite-type manganese oxide thin film is about the unit cell. In other words, the fact that the itinerant nature of carriers is extremely small due to strong electron correlation is utilized.

Considering the actual three-dimensional arrangement, the description will be made taking the two-dimensional case as an example, and correction is necessary for the situation where x = 1/9. FIG. 4 shows a situation in which one Mn ⁴⁺ is arranged in a region occupied by 3 × 3 unit cells in the XY plane and a plurality of unit cells in the Z direction in the manganese oxide thin film of this embodiment. It is explanatory drawing shown. FIGS. 4 (a) shows a state that Mn ⁴⁺ two unit cell in the Z direction is averaged arranged, FIG. 4 (b), Mn ⁴⁺ are arranged unevenly on two unit cell in the Z-direction FIG. 4 (c) shows that Mn ⁴⁺ is ^averagely arranged in three unit cells in the Z direction, and FIG. 4 (d) shows that Mn ⁴⁺ is unevenly distributed in three unit cells in the Z direction. It shows how they are arranged.

FIGS. 4 (a) and 4 (b) show a situation where two unit cells are assumed in the Z direction corresponding to FIGS. 3 (a) and 3 (b), respectively (region surrounded by a dotted line in the figure). In this case, Mn ⁴⁺ ions and Mn ³⁺ ions are alternately arranged on a line in which Mn ⁴⁺ ions are arranged along the Z direction. For this reason, the condition that 2 × 2 = 4 Mn ³⁺ ions are arranged on this line is not satisfied, and the orbital alignment phase becomes unstable. However, since there is a Mott insulator in which 2 × 2 = 4 Mn ³⁺ ions are arranged at positions surrounding the alternate arrangement, the alternate arrangement of Mn ⁴⁺ ions and Mn ³⁺ ions is one-dimensional. Eventually, in the situation shown in FIGS. 4A and 4B, the manganese oxide thin film is not metallized as a whole. This is a situation where there is only one Mn ⁴⁺ ion in the range of 18 unit cells that are 3 × 3 of the XY plane multiplied by 2 times in the Z direction, that is, a hole doping concentration of x = 1/18. It corresponds to. Therefore, at the hole doping concentration of x = 1/18, although the resistance in the orbital alignment phase is low, the external field threshold required for the insulator-metal transition is considered to be reduced.

Further, FIGS. 4C and 4D show a situation in which three unit cells are assumed in the Z direction corresponding to FIGS. 3A and 3B (region surrounded by a dotted line in the figure). This time, Mn ³⁺ ions are always adjacent to each other even in the Z direction. For this reason, the one-dimensional Mn ⁴⁺ ions and Mn ³⁺ ions are not alternately arranged as seen at the hole doping concentration of x = ^1/18 . This situation is that there is only one Mn ⁴⁺ ion in a set of 27 unit cells obtained by multiplying 3 × 3 of the XY plane by 3 times the Z direction, that is, a hole of x = 1/27. Corresponds to the dope concentration. In this case, since the condition that 2 × 2 = 4 Mn ³⁺ ions are always arranged in a three-dimensional arrangement is satisfied, the orbital alignment phase is maintained more stably. As a result, at the hole doping concentration of x = 1/27, it is considered that the resistance value of the orbital alignment phase is higher than that of x = 1/18, and the threshold value of the external field required for the insulator-metal transition is increased. It is done.

[1-3 (110) plane orientation, (210) plane orientation]
The idea of maintaining the minimum unit of the orbital alignment phase with respect to the amount of hole doping described above is valid regardless of the type of ions at the A site, that is, the bandwidth affecting the electrical conductivity. Moreover, this is also true for the thin film or laminated body on the (110) or (210) plane orientation substrate shown in FIG.

FIG. 5 is an explanatory diagram for explaining the state of unit cells in the manganese oxide thin film 20 formed on the (110) plane orientation substrate of the present embodiment. 5A is a cross-sectional view taken from the [001] axis direction, and FIG. 5B is a cross-sectional view taken from the [1-10] axis direction. As shown in FIG. 5A, in the manganese oxide thin film 20 formed on the surface of the substrate 1 having the (110) plane, that is, the plane extending in the left-right direction on the paper surface of FIG. When growing in the film thickness direction (vertical direction on the paper surface of FIG. 5A), epitaxial growth occurs. Therefore, the crystal orientation described based on FIG. 3 is also inclined, and the assembly of unit cells is described based on the inclined crystal. As shown in FIG. 5A, the arrangement of unit cells in FIG. 3A is established on the paper surface of FIG. 5A, which is a plane perpendicular to the substrate 1 and including the [1-10] axis. To do. Since the arrangement of the unit cells in FIG. 4C is the arrangement in the [001] axis direction, as shown in FIG. 5B, the unit cells of Mn ⁴⁺ are distributed in the direction of the [001] axis. Incidentally, the white circle indicates the ^{Mn 4+} in a position different from the ^{Mn 4+} indicated by the black circle as viewed in [1-10] axis.

FIG. 6 is an explanatory diagram for explaining a state of unit cell aggregation in the manganese oxide thin film 21 formed on the (210) plane orientation substrate of the present embodiment. 6A is a cross-sectional view taken from the [001] axis direction, and FIG. 6B is a cross-sectional view taken from the [1-20] axis direction. Also in this case, since the crystal orientation described based on FIG. 3 is inclined, the unit cell assembly is described based on the inclined crystal. As shown in FIG. 6A, the arrangement of unit cells in FIG. 3A is established on the paper surface of FIG. 6A, which is a plane perpendicular to the substrate 1 and including the [1-20] axis. To do. Since the arrangement of unit cells in FIG. 4C is an arrangement in the [001] axis direction, Mn ⁴⁺ unit cells are distributed in the [001] axis direction as shown in FIG. 6B.

  It should be noted that the unit cell is a unit cell as a pseudo-cubic crystal, and the lattice constant of the substrate is similarly a pseudo-cubic crystal.

[1-4 Improvement of detectability by stacking (dimensional crossover)]
When a Mott insulator such as manganese oxide is formed on a thin film as described above, it may be difficult to detect a change in characteristics from the outside. This problem does not always occur. If the problem can arise in relation to the Drude component, which is the DC resistance component, which appears in the conductivity of the electrons reflecting the electrical properties, it is the low-dimensional nature of the system (thin film In this case, it can be said that carriers (electrons) are localized due to two-dimensionality. For example, since the manganese oxide thin film 2 is a thin film, it is possible that electrons are localized in a part of the two-dimensional region and conductivity is lowered. As a countermeasure against this problem, in the present embodiment, it is preferable to use a mechanism called dimension crossover.

  Here, the thin film formed in contact with the manganese oxide thin film 2, that is, the strongly correlated oxide thin film 3 (FIG. 7) formed so as to be continuous with the manganese oxide thin film 2 will be described. The strongly correlated oxide thin film 3 is a layer that is a material different from the manganese oxide that undergoes Mott transition and is added in order to use dimensional crossover. In general, in the strongly correlated oxide thin film 3, in order to stabilize the metal phase or to realize the metal-insulator transition, it is desirable that the thin film is formed to be thicker to some extent. This is because if the thickness of the strongly correlated oxide thin film 3 is too thin, it is difficult to realize a stable metal phase or metal-insulator transition. That is, when the strongly correlated oxide thin film 3 is formed thicker than a certain critical thickness (hereinafter referred to as “critical thickness”), a metal phase is realized in the strongly correlated oxide. Or metal-insulator transition. In that sense, the critical film thickness can be said to be a lower limit value of the film thickness of a strongly correlated oxide that is preferable because the above-described metal phase is stably present or metal-insulator transition occurs. Consider the structure of an oxide laminate in which the strongly correlated oxide thin film 3 and the manganese oxide thin film 2 are in contact with each other, that is, continuously formed on the substrate.

FIG. 7 is a schematic cross-sectional view showing a configuration of an example of an oxide laminate including the manganese oxide thin film 2 produced by contacting the strongly correlated oxide thin film 3 in the present embodiment. FIG. 7A shows an example in which the strongly correlated oxide thin film 3 is formed on the substrate side of the manganese oxide thin film 2, and FIG. 7B shows the formation of the strongly correlated oxide thin film 3 on the surface of the manganese oxide thin film 2. This is an example. In this oxide laminate, the strongly correlated thin film 3 is first formed on the surface of the substrate 1 and then the manganese oxide thin film 2 is formed (FIG. 7A). Even if the manganese oxide thin film 2 is formed first and then the strong correlation thin film 3 is formed (FIG. 7B), either configuration can be employed. Further, here, the total thickness t of the oxide stack, the thickness tm of the manganese oxide thin film 2, and the thickness t1 of the strongly correlated oxide thin film 3 are the critical film thickness tc of the metal phase of the strongly correlated oxide thin film 3. Against
t = tm + t1> tc and t1 <tc
Shall be satisfied. For example, as the critical film thickness tc of the strongly correlated oxide thin film 3 (assuming that it becomes a metal phase at room temperature), when a La _0.7 Sr _0.3 MnO ₃ thin film that is a ferromagnetic metal is used, 8 unit cells (About 3 nm). In addition, when the perovskite-type manganese oxide thin film is transferred to the metal, it takes into account that one electron per unit cell that has been localized propagates to the entire sample. _{A 0.7} Sr _0.3 MnO ₃ thin film having a higher hole concentration is desirable. For example, a La _0.5 Sr _0.5 MnO ₃ thin film is preferably used. More specifically, it can be estimated from the ratio between the thickness of the perovskite-type manganese oxide thin film and the thickness of the oxide laminate.

  When each layer is formed so as to satisfy such a thickness relationship, the manganese oxide thin film 2 is localized in the strongly correlated oxide thin film 3 when the Mott transition which is an insulator metal transition is caused by external field application. The carrier that had been felt will feel t = tm + t1 instead of the film thickness t1 that has been felt so far. Here, t1 is less than tc, whereas t exceeds tc. That is, when the manganese oxide thin film 2 is transferred from the insulator to the metal by the Mott transition, the effect is reflected in the transition of the strongly correlated oxide thin film 3 and the detectability is enhanced. This is the principle of dimensional crossover. When this dimensional crossover is used, it becomes easy to take out a change in electrical resistance due to Mott transition as a current. In other words, in order to realize switching due to a weaker external field, even if the thickness of the manganese oxide thin film 2 is made to be less than the lower limit required to ensure the detectability by current, strong correlation oxidation is performed. It is possible to detect the current with the help of the physical thin film 3. Of course, in that case, the carrier of the Mott insulator also has an effect of increasing the current and enhancing the detectability. Note that the dimensional crossover effect using the strongly correlated oxide thin film 3 is arranged in contact with either the first manganese oxide thin film 20 or the second manganese oxide thin film 21 of the manganese oxide thin film 2, There is no difference in its action.

More preferably, in order to make this dimensional crossover work more effectively, the strongly correlated oxide thin film is formed in contact with both sides of the manganese oxide thin film 2 instead of one side. FIG. 8 is a schematic cross-sectional view of an example of an oxide laminate formed by bringing the strongly correlated oxide thin films 31 and 32 into contact with both surfaces of the manganese oxide thin film 2 in the present embodiment. That is, the total thickness t of the oxide stack, the thickness tm of the manganese oxide thin film 2, the thickness t1 of the first strongly correlated oxide thin film 31, and the thickness t2 of the second strongly correlated oxide thin film 32 are: T = tm + t1 + t2> tc and max (t1, t2) <tc with respect to the critical film thickness tc of the metal phase of the strongly correlated oxide thin films 31 and 32
Satisfy the relationship. Here, max () is a function that returns the maximum value of variables. Thus, when the strongly correlated oxide thin film is formed in contact with both sides of the manganese oxide thin film 2, the effect of the dimension crossover is more effectively exhibited. That is, the thickness tm of the manganese oxide thin film 2 may be thinner than that in the case where the strongly correlated oxide thin film is disposed only on one side. As a result, switching with a weaker external field is realized.

[2 Examples]
Next, the present embodiment will be described based on more specific examples. The materials, amounts used, ratios, processing contents, processing procedures, orientations and specific arrangements of elements or members, and external fields adopted for measurement shown in the following examples are appropriately changed without departing from the spirit of the present invention. I can do it. Therefore, the scope of the present invention is not limited to the following specific examples. Further, description will be continued with reference to each drawing as appropriate.

[2-1 Example 1]
Now, in the following, Pr _1-x Ca _x MnO ₃ (x = 1/27 and 1/18 as perovskite-type manganese oxide thin film 20 formed on the (110) substrate, respectively, PCMO (27) and PCMO ( 18) and _{denoted), La 0.5 Sr 0.5 MnO 3} ( hereinafter as strongly correlated oxide thin film 31 and 32, referred to as LSMO), as the substrate _{1 (LaAlO 3) 0.3 - (} SrAl 0. ₅ Ta _0.5 O ₃ ) _0.7, that is, an example (FIG. 8) in which a perovskite-type manganese oxide thin film 2 is sandwiched between strongly correlated oxide thin films using a (110) plane orientation substrate of LSAT Explained.

First, a method for manufacturing a thin film will be described. The perovskite-type manganese oxide thin film 2 and the strongly correlated oxide thin films 31 and 32 were produced by a laser ablation method. As the target, a polycrystalline material produced by a solid phase reaction method and formed into a cylindrical shape of φ20 mm × 5 mm was used. First, the substrate 1 which is an LSAT (110) substrate was mounted in a vacuum chamber and then evacuated to 3 × 10 ⁻⁹ Torr (4 × 10 ⁻⁷ Pa) or less. Thereafter, high purity oxygen gas was introduced at 1 mTorr, and the substrate was heated to reach an ultimate temperature of 900 ° C.

  Next, an LSMO thin film of only eight atomic layers was formed as the first strongly correlated oxide thin film 31 by irradiating the LSMO target with a KrF excimer laser having a wavelength of 248 nm through the laser beam introduction port of the chamber. Here, the atomic layer is one in which one atomic layer has a (110) spacing d (110). Further, the control of the film thickness, that is, the number of atomic layers, is determined based on the relationship between the number of shots of the laser pulse and the number of atomic layers studied in advance. Subsequently, 10 atomic layers of PCMO (27) or PCMO (18) thin film were produced as the perovskite-type manganese oxide thin film 2 in the same atmosphere. Then, as the second strongly correlated oxide thin film 32, an LSMO thin film was further formed in an 8 atomic layer. The strongly correlated oxide thin film LSMO has a thickness t1 of 4 unit cells (about 2.2 nm), and the perovskite-type manganese oxide thin film PCMO (27) or PCMO (18) has a thickness tm of 5 unit cells (about 2.7 nm). The thickness t2 of the oxide thin film LSMO is 5 unit cells (about 2.2 nm), and the total thickness of the thin film sample is 7.1 nm. That is, the relationship of t = tm + t1 + t2> tc and max (t1, t2) <tc is satisfied.

  Subsequently, a four-terminal electrode was formed on the oxide laminate including the produced manganese oxide thin film 2, and a magnetoresistance measurement was performed at room temperature (300K). The reason for adopting a magnetic field as the external field is that measurement is easy. The resistance value of the sample in this measurement is as follows: the resistance is reduced by applying a magnetic field of 2.0 T in the oxide laminate including PCMO (27) and the resistance ratio is 11 times, and that in the oxide laminate including PCMO (18) is 1. Resistance was reduced by applying a magnetic field of 0 T, and a resistance ratio of 7 times was obtained. As described above, as the doping amount x increases, the resistance change ratio due to the insulator-metal transition decreases, but switching at room temperature is possible.

[2-2 Example 2]
Next, as Example 2, Pr _1-x Ca _x MnO ₃ was produced as a perovskite-type manganese oxide thin film 21 using a (210) plane orientation substrate, and similarly x = 1/27 and 1/18. did. These are subsequently denoted as PCMO (27) and PCMO (18). The difference from Example 1 is that a LSAT (210) plane orientation substrate is used as the substrate 1 and the film thickness of each layer. The LSAT (210) plane orientation substrate was preliminarily heat-treated in the atmosphere at 1100 ° C. in an electric furnace to form a flat substrate surface at the atomic layer level. The strongly correlated oxide thin film 31 has a 15 atomic layer (about 2.6 nm), the manganese oxide thin film 21 has a 6 atomic layer (about 1.1 nm), and the strongly correlated oxide thin film 32 has a 15 atomic layer (about 2.6 nm). ). Here, the atomic layer is one in which one atomic layer has a (210) spacing d (210).

  Subsequently, the magnetoresistance was measured at room temperature (300 K) in the same manner as in Example 1. The resistance value of the sample in this measurement is as follows. In the oxide laminate including PCMO (27), the resistance is reduced by applying a magnetic field of 2.2 T, the resistance ratio is 9 times, and in the oxide laminate including PCMO (18), 1. The resistance was reduced by applying a 1T magnetic field, and a resistance ratio of 5 times was obtained.

  As described above, a perovskite-type manganese oxide thin film and oxide stack that can be switched at room temperature can be realized. In addition, the material of the thin film and the substrate exemplified in the present embodiment, the composition, the film thickness, the formation method, the type of the external field, the application method, and the like are not limited to the above embodiment.

[Modification of this embodiment]
This embodiment can also be carried out with a structure of a manganese oxide thin film or an oxide laminate other than those explicitly shown in the examples.

  Each of the manganese oxides to be the manganese oxide thin films 20 and 21 can be made of a material containing a plurality of types of lanthanoid elements Ln. Even in this case, the present invention is not limited to only two types of solid solutions that are solid solutions in the entire region. In order to employ a plurality of lanthanoid manganese oxides, the composition ratio of the manganese oxide thin film is obtained by preparing the target in a desired ratio in laser ablation similar to each example described above as an example. Can be adjusted.

  Further, there is no particular limitation on the combination of the thickness of the strongly correlated oxide thin film 3 and the thickness of the first strongly correlated oxide thin film 31 and the second strongly correlated oxide thin film 32. About these film thicknesses, in order to facilitate detection by dimensional crossover as described above, it is preferable to satisfy the above-described relationship specified by the inequality. In addition to these, from the practical aspect, for example, for adjusting the action of the additional strain generated by the strongly correlated oxide thin film 3, the first strongly correlated oxide thin film 31, and the second strongly correlated oxide thin film 32. The film thickness of the film can be adjusted.

  In addition, as described above, in the embodiment, a strongly correlated oxide thin film is added to facilitate the operation of detecting the Mott transition from the outside by dimensional crossover. However, even when implemented with only the manganese oxide thin film 2, the switching function itself of controlling the Mott transition by an external field at room temperature is realized. This is because the dimensional crossover is only a means for facilitating electrical detection, and the Mott transition realized in the manganese oxide thin film 2 occurs even without a strongly correlated oxide thin film.

  The substrate 1 may have a configuration other than that exemplified in the present embodiment. For example, it is not excluded to adopt a substrate in which an appropriate buffer layer is formed on a crystal body that does not have a perovskite structure, for example, a silicon single crystal substrate. This is because an effect similar to that of the above-described embodiment can also be achieved by forming a manganese oxide thin film having a perovskite structure on the surface of such a substrate.

  The embodiment of the present invention has been specifically described above. Each of the above-described embodiments and examples are described for explaining the invention. For example, the material and composition of the thin film and the substrate exemplified in the present embodiment, the film thickness, the forming method, and the type of external field The application method and the like are not limited to the above embodiment. Rather, the scope of the invention of the present application should be determined based on the description of the claims. In addition, modifications that exist within the scope of the present invention including other combinations of the embodiments are also included in the scope of the claims.

  The present invention provides a manganese oxide thin film or oxide laminate that realizes a switching function by realizing a Mott transition controlled by an external field at room temperature, thereby applying an external field such as a magnetic field, light, electricity, and pressure. It is used as a device that utilizes the switching phenomenon caused by

_{E g} orbitals in one substrate 11 Mn ³⁺ ions ^(3x 2 ^-r 2)
_{E g} orbitals in 12 Mn ³⁺ ions ^(3y 2 ^-r 2)
2 Manganese oxide thin film 20 Manganese oxide thin film (when formed on (110) plane orientation substrate)
21 Manganese oxide thin film (when formed on (210) plane substrate)
3 strongly correlated oxide thin film 31 first strongly correlated oxide thin film 32 second strongly correlated oxide thin film

Claims (5)

A perovskite-type manganese oxide thin film formed on a (110) or (210) orientation substrate surface,
Composition formula Ln _1-x Ae _x MnO ₃ (where Ln is at least one trivalent rare earth element selected from lanthanoids, and Ae is at least one selected from the group consisting of Ca, Sr, Ba) Alkaline earth element)
A manganese oxide thin film wherein 0 <x ≦ 1/18 in the composition formula. The manganese oxide thin film according to claim 1, wherein in the composition formula, 1/27 <x ≦ 1/18. The composition of the manganese oxide thin film is selected from the group consisting of the composition formula Ln _1-x Ae _x MnO ₃ (where Ln is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy). The manganese oxide thin film according to claim 1, wherein the manganese oxide thin film is represented by at least one trivalent rare earth element. The manganese oxide thin film according to any one of claims 1 to 3,
A strongly correlated oxide thin film in contact with any surface of the manganese oxide thin film,
The total thickness t of the oxide stack, the thickness tm of the manganese oxide thin film, and the thickness t1 of the strongly correlated oxide thin film are the critical film thickness tc for the strongly correlated oxide thin film to become a metal phase. Against
t = tm + t1> tc and t1 <tc,
An oxide laminate that satisfies the relationship of The manganese oxide thin film according to any one of claims 1 to 3,
A first strongly correlated oxide thin film in contact with one surface of the manganese oxide thin film;
A second strongly correlated oxide thin film in contact with the other surface of the manganese oxide thin film,
The total thickness t of the oxide stack, the thickness tm of the manganese oxide thin film, the thicknesses t1 and t2 of the first and second strongly correlated oxide thin films, For the critical film thickness tc for
t = tm + t1 + t2> tc and max (t1, t2) <tc,
Where max () is a function that returns the maximum value of the variables,
An oxide laminate that satisfies the relationship of

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