WO2013121660A1

WO2013121660A1 - Manganese oxide thin film laminate and oxide laminate

Info

Publication number: WO2013121660A1
Application number: PCT/JP2012/081761
Authority: WO
Inventors: 荻本　泰史
Original assignee: 富士電機株式会社
Priority date: 2012-02-15
Filing date: 2012-12-07
Publication date: 2013-08-22

Abstract

Provided are a thin film or a laminate which undergoes phase transitions through a Mott transition at room temperature to achieve a switching function. Provided is a manganese oxide thin film laminate formed on the surface of a substrate (1) and provided with a first manganese oxide thin film (20) and a second manganese oxide thin film (21) which are in contact with each other. In the material of the first manganese oxide thin film, the cubic root of the unit cell volume in the bulk is greater than the lattice constants of the substrate, but is smaller in the material of the second manganese oxide thin film. The materials of the first and second manganese oxide thin films have the compositional formulae RMnO₃ and LMnO₃ (wherein R and L are each at least one mutually different trivalent rare earth element selected from lanthanoids). In both manganese oxide thin films, an atomic layer containing R (L) and not containing Mn, and an atomic layer containing Mn and not containing R are laminated alternately in the vertical direction of the substrate surface, and there are two non-equivalent crystal axes in the in-plane direction of the substrate surface.

Description

Manganese oxide thin film laminate and oxide laminate

The present invention relates to a manganese oxide thin film laminate and an oxide laminate. More specifically, the present invention relates to a manganese oxide thin film laminate and an oxide laminate that undergo Mott transition by controlling temperature, electric field, magnetic field, or light irradiation, and switch electrical, magnetic, or optical properties. .

In recent years, there are concerns that the scaling law, which has been a guideline for improving the performance of semiconductor devices, will soon 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 strongly correlated electron materials, 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, It is known that an orbital-ordered phase that aligns is expressed.

In the charge alignment phase and orbital alignment phase, carriers are localized, so that the electrical 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 alignment phase: charge and orbital ordered phase) It has been disclosed that a phenomenon in which various switching functions are manifested in a single crystal bulk material of the substance can be observed as a possible substance (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 (77 K) or lower. When the perovskite-type manganese oxide in these disclosures is expressed as ABO ₃ , the atomic stacking surface is a stacked body in which an AO layer, a BO ₂ layer, an AO layer,. Hereinafter, the crystal structure of such a laminate 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 oxides disclosed in each of the above documents 1 to 3, it is considered to be 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 “R”) 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 —RO—BO ₂ —AeO—BO ₂ —RO—BO ₂ —..., The transition temperature to the charge aligned 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 ion radius as the rare earth element R, 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, the above switching phenomenon must be realized after forming a perovskite-type manganese oxide in a thin film form. Necessary. 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, the A-site ordered perovskite-type manganese oxide has a problem that the degree of order in the A-site ions greatly affects the temperature at which the switching phenomenon is realized, that is, the temperature at which the charge orbital order is developed. In particular, in a thin film made of 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. In addition, 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 lowering the degree of order and lowering 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 provides a novel device by providing a manganese oxide thin film stack or an oxide stack that realizes a switching function by controlling phase transition by some external stimulus (external field) at room temperature. It contributes to the creation of

As a result of examining the above problems, each of the above problems is caused by the fact that two types of cations, that is, both cations of trivalent rare earth element (R) and divalent alkaline earth (Ae, for example, Sr or Ba) are perovskite type Mn. The inventor of the present application thought that it was caused by occupying the A site of the oxide. The approach using the perovskite type Mn oxide in which two types of cations occupy the A site has led to the idea that the above problem cannot be solved, and a method different from that is searched to find a specific means for solving the above problem. It was.

The present invention solves at least one of the above problems based on a completely new principle. As a specific solution thereof, in one aspect of the present invention, a manganese oxide thin film laminate formed on the surface of a substrate, the first and second manganese oxide thin films formed in contact with each other is provided. The first manganese oxide thin film has a composition formula RMnO ₃ (wherein R is at least one trivalent selected from lanthanoids), wherein the cubic root of the unit cell volume in bulk is larger than the lattice constant of the substrate. Of the rare earth element), the atomic layer containing the element R and not containing Mn, and the atomic layer containing Mn and not containing the element R are alternately stacked in the direction perpendicular to the substrate surface. The manganese oxide thin film has a composition formula LMnO _{3 in which the} cube root of the unit cell volume in the bulk is smaller than the lattice constant of the substrate (where L is selected as R, at least one selected from lanthanoids) And an atomic layer containing element L and not containing Mn and an atomic layer containing Mn and not containing element L are alternately stacked in the direction perpendicular to the substrate surface. A manganese oxide thin film stack in which each of the first manganese oxide thin film and the second manganese oxide thin film has two crystal axes that are not equivalent to each other in the in-plane direction of the substrate surface. Provided. The unit cell volume in the bulk means a unit cell volume in a pseudo cubic crystal. This is because the unit cell of RMnO ₃ or LMnO ₃ is usually defined as a GdFeO ₃ type orthorhombic crystal, so that it is four times as many as the unit cell in a cubic crystal (= (2) ^1/2 × (2) ^{This is because 1/2} × 2). Therefore, in the following description, it is noted that the unit cell volume means a value in a pseudo cubic crystal and corresponds to 1/4 of the unit cell volume in the orthorhombic crystal. Also, two crystal axes that are not equivalent to each other mean two crystal axes that are asymmetric with respect to the in-plane four-fold symmetry operation.

The manganese oxide thin film laminate in this embodiment is a thin film laminate made of perovskite manganese oxide. Each manganese oxide thin film contained in this manganese oxide laminate has a crystal lattice having a composition expressed as ABO ₃ . And the crystal | crystallization of the 1st and 2nd manganese oxide thin film in this aspect is equipped with the oxygen octahedron which has Mn (manganese) in B site, and surrounds the Mn like a normal perovskite type crystal. . In particular, in the crystals of the first and second manganese oxide thin films in this embodiment, the A sites are occupied only by the trivalent rare earth element cations, elements R and L, respectively. That is, unlike the conventional one, no divalent alkaline earth (Ae) is disposed at the A site. The rare earth elements R and L of this embodiment are typically lanthanoid trivalent rare earth elements, that is, La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium). , Sm, Eu, Gd, Tb, Dy, Ho, Er (erbium), Tm (thulium), Yb (ytterbium), and at least one element selected from the group consisting of Lu (lutetium).

In addition, the substance disclosed by the said patent document 4 and the patent document 5 is a substance containing Sr (patent document 4) and Ba (patent document 5) which are bivalent alkaline-earth (Ae) in A site.

Furthermore, in the first manganese oxide thin film in the manganese oxide thin film laminate of the above aspect of the present invention, an atomic layer containing element R and not containing Mn, and an atomic layer containing Mn and not containing element R are substrate surfaces. Are stacked alternately in the vertical direction. Also in the second manganese oxide thin film, an atomic layer containing element L and not containing Mn and an atomic layer containing Mn and not containing element R are stacked so as to be alternately arranged in a direction perpendicular to the substrate surface. . In these manganese oxides, an atomic layer containing element R (element L) and not containing Mn is typically an RO layer (LO layer), that is, a layer composed of elements R (element L) and O (oxygen). . On the other hand, the atomic layer that contains Mn and does not contain the element R (element L) is typically a MnO ₂ layer, that is, a layer composed of Mn and O. When taking these typical layer configurations, it is considered that the RO layers (LO layers) and MnO ₂ layers alternately stacked in the direction perpendicular to the substrate surface are charged to +1 and −1, respectively. In addition, in the crystal of the manganese oxide thin film of the above aspect, a voltage or electric field due to a charged polar surface is always applied. This is because the formal valence of each element is R (L) is +3 valence, O is -2 valence, and Mn is +3 valence due to charge neutral conditions of RMnO ₃ and LMnO ₃ . When such an electric field is generated, it is possible to expect a reduction in the external field threshold necessary for developing the insulator-metal transition.

In addition, each of the first and second manganese oxide thin films of the above aspect has two crystal axes that are not equivalent to each other in the in-plane direction of the substrate surface. For this reason, the symmetry of the manganese oxide crystal in the plane of the substrate surface is lower than the 4-fold symmetry, and shear deformation is allowed, and a primary phase transition is possible. In the disclosure of Patent Document 5, since a (100) plane orientation SrTiO ₃ substrate is employed, the crystal lattice of the SmBaMn ₂ O ₆ thin film formed on the substrate is four-fold symmetric that does not allow shear deformation. It will have sex. In addition, if a thin film including RO layers and MnO ₂ layers alternately stacked in the direction perpendicular to the substrate surface is formed as (100) -oriented manganese oxide, two equivalent crystal axes are in the in-plane direction of the substrate surface. It is formed. For this reason, in the (100) -oriented manganese oxide, the crystal has fourfold symmetry even when the RO layer and the MnO ₂ layer are formed. Unlike this configuration, the first and second manganese oxide thin films of the above aspect have two crystal axes that are not equivalent to each other in the in-plane direction of the substrate surface. Does not include 4-fold symmetrical manganese oxide thin film. Note that having two crystal axes that are not equivalent to each other in the in-plane direction of the substrate surface means that there are no two crystal axes that are equivalent to each other in the in-plane direction of the substrate surface. For example, in a cubic (100) substrate, the two in-plane axes are [010] and [001], but these are indistinguishable when in-plane four-fold symmetry operation, that is, by rotating 90 degrees. In such a case, it is assumed that the two crystal axes are equivalent. On the other hand, in the cubic (210) substrate, the two in-plane axes are [−120] and [001]. These do not coincide by the above four-fold symmetry operation, and in such a case, the two crystal axes will be referred to as non-equivalent.

The electronic phase in the composition of the first and second manganese oxide thin films 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 first and second manganese oxide thin films in the present application are thin films exhibiting the property of causing a metal-insulator transition, and are 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 first and second manganese oxide thin films of the laminate of the above-described aspect of the present invention, it is possible to cause a Mott transition 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.

And the 1st and 2nd manganese oxide thin film of the said aspect WHEREIN: The cube root of the unit cell volume in a bulk is larger than the lattice constant of a board | substrate (1st manganese oxide thin film), and a small thing (1st 2 manganese oxide thin film), and the manganese oxide thin film laminate including the first and second manganese oxide thin films is formed on the surface of the substrate. From these relationships, the first and second manganese oxide thin films are in a state where compressive strain and extension strain are applied. Due to the effect of this strain, the orbital alignment surfaces of the first and second manganese oxide thin films have different directions. In addition, since they are formed in contact with each other, a new disturbance, that is, a difference in the direction of the orbital alignment plane, is introduced at the boundary or interface of the first and second manganese oxide thin films or in the vicinity thereof. For this disturbance, electrons of the boundary or interface or e _g band of Mn 3d near either of these may be arranged aligned in any orbital alignment surface of the first or second manganese oxide thin film It becomes a state. That this turbulence results in a conflict (frustration) as mutual contested trajectory alignment order of both manganese oxide thin film is going Shitagaeyo electrons of the e _g band to their order. This competition has the effect of reducing the “strength” of the electronic phase in the direction of canceling the orbital alignment, and lowers the threshold necessary for the external field to transfer the insulating phase to the metal phase.

In this embodiment, it is not limited which of the first and second manganese oxide thin films is in contact with the substrate or formed on the side close to the substrate. Further, in the present application, it should be noted that since each of the element R and the element L is at least one element, at least one or both of them may include a combination of a plurality of elements. Therefore, when the element L is selected from those other than those selected as the element R, or conversely, when the element R is selected from other than those selected as the element L, the elements R and L are common. May contain elements. That is, when the element group that constitutes the element R and the element group that constitutes the element L do not completely coincide with each other including the ratio, the element R is other than the element L, and the element L is other than the element R. It is assumed that each item is selected.

Incidentally, the case where each of the trivalent rare earth elements R and L in the composition formulas RMnO ₃ and LMnO ₃ of the first and second manganese oxide thin films of the present invention is a combination of plural kinds of elements will be supplemented. For example, when the chemical composition when two kinds of trivalent rare earth elements are used as the element R is expressed in another form, the composition expressed as RMnO ₃ which is the first manganese oxide of this embodiment is A composition represented by (R ₁ MnO ₃ ) _x (R ₂ MnO ₃ ) _1-x , where 0 <x <1, where R ₁ and R ₂ are different rare earth elements that can become trivalent cations. It becomes. The composition expressed in this way typically has an arbitrary ratio x of manganese oxide R ₁ MnO ₃ containing rare earth element R ₁ and manganese oxide R ₂ MnO ₃ containing rare earth element R ₂ : 1-x solid solution. The same applies to the element L in the second manganese oxide thin film.

In one embodiment of the present invention, the composition of the first manganese oxide thin film has a composition formula RMnO ₃ (where R is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy). There is provided a manganese oxide thin film laminate according to the above aspect, which is represented by at least one trivalent rare earth element other than those selected as L) selected from the group. Furthermore, the composition of the second manganese oxide thin film has a composition formula LMnO ₃ (where L is selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy) There is also provided a manganese oxide thin film laminate according to the above aspect, which is represented by one kind of trivalent rare earth elements other than those selected as R).

The above group of trivalent rare earth elements is an element group obtained by arranging lanthanoids in order of atomic number 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, in one aspect of the present invention, there is provided a manganese oxide thin film laminate according to the above aspect, wherein the plane orientation of the substrate is a (210) plane orientation.

According to this aspect, epitaxial growth using the atomic layer surface of the substrate is possible, and a single crystal thin film having no defects such as misfit can be used. Further, in the RMnO ₃ thin film and the LMnO ₃ thin film on the (210) plane orientation substrate, polarization in the in-plane [1-20] axis direction slightly inclined in the perpendicular direction occurs due to the symmetry breaking. As a result, in this embodiment, a voltage (electric field) is inherently applied in the in-plane direction in addition to the perpendicular direction, so that an anti-electric field due to polarization is also applied in the in-plane direction. -The external field threshold for metal transition is reduced.

Also, in an embodiment of the present invention, an oxide laminate to which an additional layer is added is also provided. That is, an aspect of the present invention includes the manganese oxide thin film laminate 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 laminate. The thickness t of the entire oxide stack, the thickness tm of the manganese oxide thin film stack, and the thickness t1 of the strongly correlated oxide thin film are critical for the strongly correlated oxide thin film to become a metal phase. An oxide stack that satisfies the relationship of t = tm + t1> tc and t1 <tc with respect to the film thickness tc is provided.

In the oxide laminate of this embodiment, the strongly correlated oxide thin film is disposed in contact with the manganese oxide thin film laminate. The crystal structure of the material forming the strongly correlated oxide thin film has a perovskite structure expressed as ABO _{3 in the same} manner as the first or second manganese oxide thin film. However, unlike the first or second manganese oxide thin film laminate, in the crystal lattice of the strongly correlated oxide thin film, the A site is always occupied only by the cation of the trivalent rare earth element (R). Is not limited. In the oxide laminated body of this aspect, the detection of the switching function of the manganese oxide thin film laminated body by an insulator metal transition (Mott transition) becomes easy compared with the above-mentioned single thing of the manganese oxide thin film laminated body. This is because the switching function of the manganese oxide thin film stack due to the insulator-metal transition (Mott transition), that is, the change of the electronic state can be easily detected from the outside as, for example, the resistance change of the oxide stack sample. . This mechanism for facilitating detection is called dimensional crossover, and details thereof will be described in detail in the section “1-6 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 it is possible to make the thickness of the Mott-transferred manganese oxide thin film stack as a whole thinner than that of a single manganese oxide thin film.

In one embodiment of the present invention, the manganese oxide thin film laminate according to any one of the above embodiments, the first strongly correlated oxide thin film in contact with one surface of the manganese oxide thin film laminate, A second strongly correlated oxide thin film in contact with the other surface of the manganese oxide thin film stack, the thickness t of the entire oxide stack, the thickness tm of the manganese oxide thin film stack, The thicknesses t1 and t2 of each of the first and second strongly correlated oxide thin films are t = tm + t1 + t2> tc and max with respect to the critical film thickness tc for each strongly correlated oxide thin film to be a metal phase. An oxide stack that satisfies the relationship of (t1, t2) <tc, where max () is a function that returns the maximum value of variables is also provided.

In the oxide laminate of this embodiment, the strongly correlated oxide thin film is disposed on both sides of the manganese oxide thin film laminate. The effect of bringing the strongly correlated oxide thin film into contact is more remarkably obtained than in the case of only one side.

In the manganese oxide laminate or oxide laminate of any aspect of the present invention, the element at the A site is a rare earth element R or L having a valence of +3, and the element R or L is included and Mn is contained. In principle, an atomic layer that does not contain and an atomic layer that contains Mn and does not contain elements R and L are alternately arranged in the direction perpendicular to the substrate surface, so that it is in principle not affected by variation in the degree of order. In addition, by introducing instability due to competition to the orbital alignment surfaces in the first and second manganese oxide thin films, the Mott transition controlled by the external field is realized at room temperature.

It is a schematic sectional drawing of the manganese oxide thin film laminated body in one embodiment of this invention. FIG. 1A is an overall view showing the structure of a manganese oxide thin film stack formed on a substrate, and FIGS. 1B and 1C are atomic stack planes of the manganese oxide thin film stack ( 2 is an enlarged view of the vicinity of the interface between the first and second manganese oxide thin films. It is explanatory drawing explaining the additional electric field which arises in the manganese oxide thin film laminated body in an embodiment with this invention. 2A and 2B are cross-sectional views similar to FIGS. 1A and 1B, respectively. In a manganese oxide thin film stack according to an embodiment of the present invention, a first manganese oxide whose orbital alignment plane is a (001) plane and a second manganese oxide whose orbital alignment plane is a (100) plane. It is explanatory drawing which shows the relationship of the track | orbit alignment surface with a thing. It is a schematic sectional drawing which shows the structure of an example of the oxide laminated body containing the manganese oxide thin film laminated body produced in contact with the strongly correlated oxide thin film in one embodiment of this invention. FIG. 4A shows an example in which a strongly correlated oxide thin film is formed on the substrate side of the manganese oxide thin film laminate, and FIG. 4B shows a strongly correlated oxide thin film on the surface of the manganese oxide thin film laminate. This is an example of formation. In embodiment with this invention, it is a schematic sectional drawing of an example of the oxide laminated body formed by making a strongly correlated oxide thin film contact both surfaces of a manganese oxide thin film laminated body. FIG. 3 is an explanatory diagram showing an angle of Mn—O—Mn in a manganese oxide thin film laminate according to an embodiment of the present invention. FIG. 6 (a) shows a state where the oxygen octahedron is deformed in the direction of rotation in the crystal lattice and the angle of Mn—O—Mn is reduced from 180 degrees, and FIG. 6 (b) shows the extension from the substrate. In the crystal lattice, the oxygen octahedron is deformed in the direction of rotation and the angle of Mn—O—Mn is expanded due to strain.

Hereinafter, embodiments of a manganese oxide thin film laminate 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 Facilitating Mott transition in manganese oxide thin film laminates]
Hereinafter, embodiments of a manganese oxide thin film laminate and an oxide laminate according to the present invention will be described with reference to FIGS. First, the basic principle for realizing the switching function at room temperature, that is, the basic principle for Mott transition of the manganese oxide thin film contained in the manganese oxide thin film laminate at room temperature by an external field will be described. In general, the orbital alignment temperature of a manganese oxide thin film is much higher than that of an A-site ordered Mn oxide. For example, the orbital alignment temperature of PrMnO ₃ is 1000K or higher. That is, for example, a manganese oxide thin film at room temperature of about 300 K is in an orbital alignment state. This is the first important recognition.

In this example, the elements of PrMnO ₃ Pr site (A site) are La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. Consider the case where elements are replaced in order. In addition, the said arrangement | sequence of a lanthanoid shows the tendency of what is called a lanthanoid contraction (Lanthanide contraction) from the thing with a large ionic radius to what is small. Then, as the elements at the A site are sequentially replaced from La, the orbital alignment temperature of the manganese oxide thin film increases while the antiferromagnetic transition temperature decreases. This tendency continues to Dy, and when it becomes the next Ho, the orbital alignment temperature decreases compared to Dy, and the antiferromagnetic transition temperature starts increasing again. Here, the oxygen octahedron is deformed so as to have a GdFeO ₃ type strained structure, that is, in the crystal lattice, the oxygen octahedron surrounding Mn rotates. The degree of displacement or rotational deformation increases in this order in the above replacement of the entire range of lanthanoids from La to Ho. In the replacement of the range from Er to Lu, the ionic radius is further reduced, so that in the bulk, the crystal structure tends to be hexagonal rather than orthorhombic, but in the thin film, a cubic perovskite substrate An orthorhombic structure can be realized by epitaxial growth. Therefore, all the elements of the lanthanoid can be adopted as the A site replacement targets. As a result, in the replacement, the angle θ (FIG. 6) of Mn—O—Mn from one unit cell to the adjacent unit cell decreases in that order. FIG. 6 is an explanatory diagram showing the angle of Mn—O—Mn in the manganese oxide thin film of this embodiment. FIG. 6A shows a state where the crystal lattice is deformed in the direction in which the oxygen octahedron rotates and the angle of Mn—O—Mn is reduced from 180 degrees, and FIG. 6B shows the extension from the substrate. This shows a state where the oxygen octahedron is deformed in the direction of rotation due to strain and the angle θ of Mn—O—Mn is expanded. The decrease of the angle θ from 180 degrees also affects the bandwidth, which is an index of carrier conductivity, so as to deteriorate the conductivity. This is to the angle θ is greatly affected by the overlap between the e _g orbitals and O ^2- of 2p orbital was crystal field splitting from the 3d orbital of Mn ^3+.

Therefore, the inventor of the present application realizes orbital alignment under the control of the same physical mechanism within the range of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, and Dy excluding Ho and later from the lanthanoid. I think. In this range, the ease of occurrence of orbital alignment shows a systematic dependence on the ionic radius as well as the antiferromagnetic transition temperature.

Next, the cause of the phenomenon in which the insulator-metal transition due to the external field remains at a low temperature, which is one of the phenomena described in the background art, will be described. In short, the order of the electronic phase is “robust” in order to transfer the electronic phase of the manganese oxide to the metallic phase by applying an external field of a size that can be normally used. The reason is too much. That is, the threshold value of the external field required for reducing the order of the electronic phase in the manganese oxide to form a metal phase is too large. Reexamining the conventional approach from this perspective makes it easier to understand the principles that have been relied on. In other words, in the manganese oxide as a Mott insulator in which the substance that should be a metal in band theory is an insulator by electron-correlation, conventionally, electron correlation is intentionally performed by doping holes. The principle of decreasing the “strength” by weakening is adopted. In fact, any conventional manner in which the Mott transition is controlled by an external field relies on this operating principle. However, as long as it is based on this principle, it cannot escape from the trade-off that the orbital alignment temperature and the charge orbital alignment temperature are below room temperature. The reality is that the trade-off has not been overcome in the past.

Based on this situation, the present inventor's question is what is the mechanism that governs the robustness of the electronic system in Mott insulators such as manganese oxides? By addressing this question, the present inventor has come to the hypothesis that the robustness in orbital alignment is due to the cooperative phenomenon 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 that gave the idea to the present invention.

In addition, regarding the certainty of the above hypothesis, experimental explanation will be left to the examples described later, and here, the theoretical explanation supporting the above hypothesis will be supplemented. 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. Therefore, in an electronic state in a system in which two unit cells are connected, it cannot be said that the orbital alignment is definite.

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 orbits 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 several of the N unit cells (a couple of unit cells) are different from the orbits of other unit cells, the orbits contained 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.

It can be seen from the above properties of the 2 unit cell and the N unit cell 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 substance having a strong electron correlation, these alignments can be said to be the product of self-assembly of electrons. Therefore, the large 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.

[1-2 Selection of crystal structure]
In order to realize the switching function in the manganese oxide thin film laminate, in addition to reducing the threshold value of the external field for realizing the switching function, it is also considered that the Mott transition is a first order phase transition (first order transition). . For this reason, the symmetry of the crystal | crystallization which accept | permits a shear deformation is employ | adopted for the manganese oxide laminated body of this embodiment, and it is made not to become an obstacle of the transition of a yarn teller mode. FIG. 1 is a schematic cross-sectional view of an example of a manganese oxide thin film laminate 2 in the present embodiment, and is a cross-sectional view of a manganese oxide thin film formed on the surface of a substrate 1 that is a (210) plane orientation substrate. Show. More specifically, as the crystal structure of the first manganese oxide thin film 20 constituting the manganese oxide thin film stack of this embodiment, atoms in which RO layers and MnO ₂ layers are alternately stacked in the direction perpendicular to the substrate surface. A crystal structure of the stacked surface, that is, a crystal structure aligned with RO—MnO ₂ —RO. Similarly, as the second manganese oxide thin film 21, the crystal structure of the atomic layer plane in which LO layers and MnO ₂ layers are alternately stacked in the direction perpendicular to the substrate surface, that is, crystals aligned with LO-MnO ₂ -LO. The structure will be adopted. FIG. 1A is an overall view showing a configuration of a manganese oxide thin film laminate 2 formed on a substrate 1, and FIG. 1B is a cross-sectional view taken along a plane perpendicular to the [001] axis. FIG. 1C shows a cross-sectional view taken along a plane perpendicular to the [1-20] axis. 1 (b) and 1 (c) are both in the vicinity of the interface between the first and second manganese oxide

thin films

20 and 21 in the manganese oxide thin film laminate cut at a plane perpendicular to the substrate surface. Is.

In the above-mentioned crystal structure in which the RO layer (or LO layer) and the MnO ₂ layer are alternately stacked on the atomic layer plane, as shown in FIGS. RO (LO) atomic layers and MnO ₂ atomic layers are alternately stacked to form an atomic layer parallel to the extending substrate surface (not shown in FIGS. 1B and 1C). The In particular, FIGS. 1B and 1C show that the crystal structure of the perovskite structure represented by the composition formulas of RMnO ₃ and LMnO ₃ of the first and second manganese oxide

thin films

20 and 21 of this embodiment is cubic. The case where it takes is illustrated. First, it can be seen from the crystal structures of the first and second manganese oxide

thin films

20 and 21 in this embodiment that the two crystal axes in the substrate plane are not equivalent. In other words, the first and second manganese oxide

thin films

20 and 21 of the present embodiment can be deformed and can undergo a primary transition. In fact, since the symmetry in the (210) plane is one-time symmetric, insulator metal transition due to Mott transition can be realized.

However, the material of the first and second 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 RMnO ₃ and the composition formula LMnO ₃ is a crystal lattice other than a cubic crystal, In other words, crystals with lower order symmetry such as tetragonal, orthorhombic, monoclinic, triclinic, trigonal, hexagonal. There may be a perovskite structure in the structure. This is because, regardless of the crystal lattice, RO (LO) atomic layers and MnO ₂ atomic layers are alternately stacked as in the first and second manganese oxide

thin films

20 and 21 of this embodiment. This is because the above-described shear deformation is allowed if the two crystal axes in the substrate plane are not equivalent. 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. The fact that the crystal structures of FIGS. 1B and 1C are realized can be confirmed by identifying crystal point groups by known X-ray diffraction. In particular, it can be confirmed by direct observation of atoms by a STEM (scanning transmission electron microscope) that RO (LO) atomic layers and MnO ₂ atomic layers are alternately stacked.

Next, the electrical properties of each atomic layer in the crystal structure of the first manganese oxide thin film 20 and the second manganese oxide thin film 21 shown in FIGS. 1B and 1C will be described. As described above, a layered structure of RO—MnO ₂ —RO—MnO ₂ —... Is realized in the direction perpendicular to the substrate surface, and includes an atomic layer that includes element R and does not include Mn (RO atomic layer), and Mn. The atomic layers not containing the element R (MnO ₂ atomic layers) are alternately stacked. The same applies to the second manganese oxide thin film 21 between the LO atomic layer and the MnO ₂ atomic layer. In general, the valence of rare earth R or L is stable at +3. If the valence of O is assumed to be −2 assuming an ionic bond, the valence of Mn becomes +3 due to charge neutrality conditions of RMnO ₃ and LMnO _3. . Under these assumptions, the charge distribution of each atomic layer forming the laminate having the above crystal structure is reviewed again. RO and LO are +1, MnO ₂ is -1, and each atomic layer is alternately + and −. Notice that it is charged. In FIG. 1 (b) and (c), this symbol is clearly shown. As a result, the manganese oxide thin film laminate 2 of the present embodiment has a polar surface.

[1-3 Substrate selection]
The white arrow in the figure of Fig.1 (a) has shown the voltage (electric field) which acts intrinsically from this polar surface. When the composition of the substrate 1 is expressed as ABO ₃ , when the surface of the substrate 1 on which the manganese oxide thin film laminate 2 is formed is terminated with a BO ₂ atomic layer, that is, the surface of the substrate 1 is a BO ₂ surface. At some point, the first or second manganese oxide

thin film

20 or 21 is grown on the substrate 1. Then, the first atomic layer where the first or second manganese oxide

thin film

20 or 21 starts to grow is the RO layer or the LO layer. In this case, the direction of the voltage (electric field) is shown in FIG. The direction of the white arrow. Conversely, when the surface of the substrate 1 is terminated with an AO plane, the direction of the arrow is reversed. Note that it is not particularly difficult to make a different one to terminate the surface of the substrate 1.

In this way, an atomic plane containing R and L and an atomic plane containing Mn, such as RO—MnO ₂ —RO—MnO ₂ —... And LO—MnO ₂ —LO—MnO ₂ —. Utilizing a polar surface as described above for the purpose of alternating lamination is one typical technique that can be applied to manganese oxide thin films. One of the most typical polar surfaces is to employ a (210) orientation substrate as the substrate 1 as described above. In this case, by forming the coherent crystal structure of the second manganese oxide thin film 21 of manganous oxide film 20 and the composition formula Lmno ₃ of formula RMnO ₃ with respect to the crystal of the substrate 1, FIG. 1 ( The crystal structures shown in b) and (c) can be formed.

The inventor of the present application has also found an effect that when a substrate having a low symmetry (210) plane orientation is adopted as the substrate 1, not only an electric field perpendicular to the substrate surface but also another electric field is generated. FIG. 2 is an explanatory diagram for explaining an additional electric field in the present embodiment, and FIGS. 2A and 2B are cross-sectional views similar to FIGS. 1A and 1B, respectively. The arrows attached to the elements R, L, and Mn on the atomic layer plane in FIG. 2B indicate the relative displacement directions of the positions of R, L, and Mn in the actual crystal lattice. That is, according to the examination of the inventors of the present application, in the actual crystal lattice in which the manganese oxide thin film stack 2 is formed on the substrate 1 having a low symmetry (210) plane orientation as the substrate 1, the first and second manganese It seems that displacements of positions of R, L, and Mn as shown in FIG. 2B are generated in the crystal lattice of the manganese oxide that forms the oxide

thin films

20 and 21. This displacement is relative to the displacement of R, L, and Mn, which are positively charged and become cations, with respect to O (−2), and is accompanied by polarization. The polarization is induced in the [−120] axis direction slightly inclined in the stacking direction, that is, the [−110] axis direction. In FIG. 2A, the direction of macroscopic polarization generated in the entire manganese oxide thin film stack 2 by the polarization generated inside the manganese oxide is indicated by an arrow. Thus, by adopting the (210) plane orientation substrate as the substrate 1, this macroscopic polarization can be generated. This is, for example, an effect that cannot be obtained with a thin film on a (100) plane orientation substrate in which the in-plane symmetry is four times. In the manganese oxide thin film laminate 2 of this configuration, since an intrinsic voltage (electric field) acts both in the film thickness direction and in the in-plane direction, the external field required for the insulator-metal transition It is expected that the threshold will be further reduced. As a result of the above displacement, slight displacement occurs between R (L) or Mn and O in the RO atomic layer (LO atomic layer) and the MnO ₂ atomic layer. For this reason, in the RO atomic layer (LO atomic layer) and the MnO ₂ atomic layer, a deviation occurs between the surface formed by each cation and the surface formed by oxygen.

[1-4 Introduction of competition due to substrate strain]
In addition to the effect of the electric field described above, the manganese oxide thin film laminate 2 in the present embodiment is transferred to the metal phase by utilizing the strain that the first and second manganese oxide

thin films

20 and 21 receive from the substrate 1. It can be made easier. The inventor of the present application determines which of the (100) plane, the (010) plane, and the (001) plane is the orbital alignment plane in the manganese oxide thin film based on the plane spacing of each plane. Has gained knowledge. When the orbital alignment plane becomes the (100) plane, all the (100) planes parallel to it become the orbital alignment plane. The same applies to other aspects. The track alignment surface has such a property that it is formed in such an orientation that the distance between the track alignment surfaces becomes as small as possible. Due to this property, the plane with the smallest plane spacing among the (100) plane, (010) plane, and (001) plane is the orbital alignment plane.

The inventor of the present application considers that the manganese oxide thin film stack 2 is already in the orbital alignment phase at room temperature, and further contrivances for reducing the switching threshold and enabling switching at room temperature have been proposed. I got the idea that it would be possible by using the orientation of. Specifically, in the first and second manganese oxide

thin films

20 and 21, the orbital alignment planes are made different from each other and both the manganese oxide thin films are brought into contact with each other, and the interface is used. If competition between the orbital alignment planes can be generated at the interface between the first and second manganese oxide

thin films

20 and 21, the orbital alignment phase can be destabilized more than when a single film is used. is there.

This point can also be explained by paying attention to the above-mentioned decrease in “strength”. As an example for explanation, in each of the first and second manganese oxide

thin films

20 and 21, it is assumed that each manganese oxide thin film has inherent strength. In this case, the reduction effect on “strength” in the bulk was based only on the formation of a thin film. In this embodiment, in addition to the decrease in strength due to the formation of the thin film described above, the strength also decreases due to instability due to the discontinuity of the orbital alignment surface at the interface where both the manganese oxide thin films are in contact. is there. In other words, the order of the electronic phase on the orbital alignment plane in both manganese oxide thin films starts to decay due to the instability of the interface and the external field that does not satisfy the respective threshold values reflecting the inherent strength. That is, the interface formed by the two manganese oxide thin films in the present embodiment functions as a seed of a metal phase that is intentionally provided to promote Mott transition.

In order to control whether the orbital alignment planes of the first and second manganese oxide

thin films

20 and 21 are the (100) plane, the (010) plane, and the (001) plane, the substrate 1 The present inventor has realized that the interplanar spacing in the crystal structure of both manganese oxide thin films may be controlled by the strain received from the above. When the crystal lattice of each of the first and second manganese oxide

thin films

20 and 21 in the present embodiment is the original lattice constant when in the bulk state, it is formed on the surface of the substrate 1. Try to match the crystal lattice of the substrate 1. Specifically, the compressive stress acts on the first manganese oxide thin film 20 in which the cube root of the unit cell volume in the bulk of the material is larger than the lattice constant of the substrate 1, and compressive strain is generated. On the contrary, tensile stress acts on the second manganese oxide thin film 21 having a material in which the third root of the unit cell volume in the bulk is smaller than the lattice constant of the substrate 1, and tensile strain is generated. For example, when a (210) plane orientation substrate of SrTiO ₃ (hereinafter referred to as STO) is adopted as the substrate 1, the crystal axis arrangement at this time is as shown in FIGS. 1 (b), 1 (c) and FIG. It will be a thing. Under this condition, the (100) plane is oriented closer to the (210) plane orientation substrate surface than the (010) plane.

FIG. 3 shows the first manganese oxide 20 in which the orbital alignment plane is the (001) plane and the first manganese oxide 20 in which the orbital alignment plane is the (100) plane in the manganese oxide thin film stack 2 of the present embodiment. It is explanatory drawing which shows the relationship of the orbital alignment surface with 2 manganese oxide 21. FIG. That is, in the first manganese oxide thin film 20, the orbital alignment plane is a (001) plane parallel to the paper surface of FIG. This is because, in the first manganese oxide thin film 20, the plane spacing of the (001) plane is greatly reduced due to compressive strain from the substrate 1, whereas the plane spacing of the (100) plane and the (010) plane is not significantly affected. Because. On the other hand, in the second manganese oxide thin film 21, the orbital alignment plane is a (100) plane that is perpendicular to the paper surface of FIG. 3 and is inclined about 26.6 degrees from the substrate surface. This is because, in the second manganese oxide thin film 21, the plane spacing of the (001) plane is greatly expanded due to the extension strain from the substrate 1, whereas the plane spacing of the (100) plane and the (010) plane is not significantly affected. Because. In the second manganese oxide thin film 21, which of the (100) plane and the (010) plane becomes the orbital alignment plane is determined by the plane spacing. When comparing the (100) plane and the (010) plane, the plane spacing of the (010) plane is wider than that of the (100) plane, so the (100) plane with the smallest plane spacing is the track alignment plane. Become. In the manganese oxide thin film laminate 2 having such a configuration, the (001) plane orbit alignment surface of the first manganese oxide thin film 20 and the (100) plane orbit alignment surface of the second manganese oxide thin film 21 are formed. It becomes possible to compete at the interface between the first manganese oxide thin film 20 and the second manganese oxide thin film 21 and intentionally cause the instability. The inventor of the present application confirmed that the orbital alignment plane is not the (010) plane but the (100) plane in the same crystal as the second manganese oxide thin film 21 by electrical measurement and optical measurement. ing.

That is, the first and second manganese oxide

thin films

20 and 21 are made of a combination of two types of manganese oxides with the ionic radii of the A sites appropriately changed, thereby realizing orbital alignment surface competition at the interface. Can do. As already mentioned, the unit cell is a unit cell as a pseudo-cubic crystal, and it is added again that the lattice constant of the substrate is also a pseudo-cubic crystal. The order of the first manganese oxide thin film 20 and the second manganese oxide thin film 21 in the present embodiment is not limited to the arrangement in which the first manganese oxide thin film 20 is placed on the substrate 1 side as described above. It also includes disposing the 2 manganese oxide thin film 21 on the substrate 1 side.

[1-5 Realization of external detectability and device function]
Whether any of the manganese oxide thin film stacks provided in the present embodiment has actually undergone Mott transition can be detected by various measuring means. For example, if transmittance or reflectance is measured by optical measurement, it is possible to measure the presence or absence of transition as a change in the electronic structure with respect to the energy of the probe light for measurement. In addition, it is possible to detect the realization of Mott transition as an arbitrary physical quantity such as magnetic characteristics, deformation, and electrical resistance. And such a change as a physical quantity provides not only whether or not the Mott transition can be detected, but also provides material characteristics that are utilized as a switching function when the manganese oxide thin film stack of this embodiment is applied to a device. .

[1-6 Improved detectability by stacking (dimensional crossover)]
However, when a Mott insulator such as manganese oxide is formed on a thin film or a laminate thereof as described above, it may be difficult to detect the 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 laminate 2 is a thin film, it is possible that electrons are localized in a part of a 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 laminate 2, that is, the strongly correlated oxide thin film 3 (FIG. 4) formed so as to be continuous with the manganese oxide thin film laminate 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 laminate 2 are in contact with each other, that is, continuously formed on the substrate. FIG. 4 is a schematic cross-sectional view showing the configuration of an example of an oxide laminate including the manganese oxide thin film laminate 2 produced by contacting the strongly correlated oxide thin film 3 in the present embodiment. 4A shows an example in which a strongly correlated oxide thin film 3 is formed on the substrate side of the manganese oxide thin film laminate 2, and FIG. 4B shows a strongly correlated oxidation on the surface of the manganese oxide thin film laminate 2. In this example, the physical thin film 3 is formed. In this oxide laminate, even if the strongly correlated metal thin film 3 is first formed on the surface of the substrate 1 and then the manganese oxide thin film laminate 2 is formed (FIG. 4A), conversely, the substrate 1 Even if the manganese oxide thin film laminate 2 is first formed on the surface, and then the strongly correlated metal thin film 3 is formed (FIG. 4B), either configuration can be employed. Further, here, the total thickness t of the oxide stack, the thickness tm of the manganese oxide, and the thickness t1 of the strongly correlated oxide thin film 3 are relative to the critical film thickness tc of the metal phase of the strongly correlated oxide thin film 3. And
t = tm + t1> tc and t1 <tc
Shall be satisfied. For example, when the La _0.7 Sr _0.3 MnO ₃ thin film that is a ferromagnetic metal is used as the critical film thickness tc of the strongly correlated oxide thin film 3 (assuming that it becomes a metal phase at room temperature), (210) On the plane orientation substrate, there are 8 unit cells (about 4 nm).

When each layer is formed so as to satisfy such a thickness relationship, when the manganese oxide thin film laminate 2 causes a Mott transition, which is an insulator-metal transition, due to an external field application, the strong correlation oxide thin film 3 is localized. The existing carrier feels 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 laminate 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 increased. 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 by a weaker external field, even if the thickness of the manganese oxide thin film laminate 2 is made to be less than the lower limit required to ensure the detectability by current, it is strong. With the help of the correlation oxide thin film 3, detection by current becomes possible. Note that the dimensional crossover action 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 laminate 2. But 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 laminate 2 instead of one side. FIG. 4 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 laminate in the present embodiment. That is, the thickness t of the entire oxide stack, the thickness tm of the manganese oxide thin film stack 2, the thickness t1 of the first strongly correlated oxide thin film 31, and the thickness of the second strongly correlated oxide thin film 32 t2 is the critical film thickness tc of the metal phase of the strongly correlated oxide

thin films

31 and 32,
t = tm + t1 + t2> tc and max (t1, t2) <tc
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 surfaces of the manganese oxide thin film laminate, the effect of the dimensional crossover is more effectively exhibited. That is, the thickness tm of the manganese oxide thin film laminate 2 may be thinner than when the strongly correlated oxide thin film is disposed only on one side. In this way, switching by 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 FIGS.

[2-1 Example 1]
Example 1 of this embodiment is an oxide stack produced in the configuration shown in FIG. 5 in which the first and second strongly correlated oxide

thin films

31 and 32 are in contact with both surfaces of the manganese oxide thin film stack 2. Example of body. Manganous oxide film 20 as a PrMnO ₃ forming the manganese oxide thin film stack 2, second manganese oxide film 21 as a SmMnO _3, _{La 0.5} as the first and second correlated

oxide films

31 and 32 Sr _0.5 MnO ₃ (hereinafter referred to as LSMO) and STO (210) plane orientation substrate were adopted as the substrate 1, respectively. The substrate 1, which is this STO (210) substrate, has a surface terminated by Ti at the B site. The cube roots of the unit cell volume in the bulk materials of PrMnO ₃ and SmMnO ₃ that are the materials of the first and second manganese oxide

thin films

20 and 21 are 0.3919 nm and 0.3889 nm, respectively. Therefore, the cubic root of the unit cell volume in the bulk material is larger in the first manganese oxide thin film 20 than in the lattice constant of 0.3905 nm of STO, which is the material of the substrate 1, and the second The manganese oxide thin film 21 is small. For this reason, compressive strain acts on the first manganese oxide thin film 20 from the substrate 1, and tensile strain acts on the second manganese oxide thin film 21. These are the (001) plane and the (100) plane, respectively. As a result, it is expected that competition occurs at the orbital alignment plane at the interface between the first manganese oxide thin film 20 and the second manganese oxide thin film 21. The first and second strongly correlated oxide

thin films

31 and 32 are not composed of La _0.7 Sr _0.3 MnO _{3 having} a composition similar to that of LSMO and the Curie temperature T _c being maximum (370 K). An over-doped LSMO with a composition ratio La _0.5 Sr _0.5 MnO ₃ increased. This is due to the intention to increase the Curie temperature _Tc of the first and second strongly correlated oxide

thin films

31 and 32 in consideration of the carriers (electrons) supplied when the manganese oxide thin film 2 undergoes the insulator-metal transition. Is.

Next, a method for manufacturing the oxide stack of Example 1 will be described. The first and second manganese oxide

thin films

20, 21 and the first and second strongly correlated oxide

thin films

31, 32 were both formed by a laser ablation method. As the target material for each thin film, a polycrystalline material made by a solid phase reaction method and formed into a cylindrical shape of φ20 mm × 5 mm was used. First, in order to make the surface of the STO (210) substrate a surface terminated by a B site, a treatment of etching the surface SrO layer with buffered hydrofluoric acid was performed. Next, after mounting the STO (210) substrate in the vacuum chamber, the substrate was evacuated to 3 × 10 ⁻⁹ Torr (4 × 10 ⁻⁷ Pa) or less. Thereafter, as a pretreatment of the substrate, first, high-purity oxygen gas was introduced at 1 mTorr (0.133 Pa), and the STO (210) substrate was heated to 600 ° C. to deposit LSAT for 4 atomic layers. Here, the atomic layer is one in which one atomic layer has a (210) spacing d (210). 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. In this preliminary film formation process, an electron beam diffraction pattern was obtained as a halo pattern indicating amorphous. When the substrate was continuously heated to 850 ° C. as a pretreatment, an electron diffraction pattern from a flat (210) plane at the atomic layer level was obtained. In this way, it was confirmed that the amorphous LSAT film formed on the STO (210) substrate was single-crystallized, and that the lattice constant in the substrate surface was the same as that of the STO substrate, and the substrate was subjected to pre-processing. 1 was prepared. The reason why such an LSAT layer is formed on the STO (210) substrate is that irregularities due to facets composed of the (100) plane and the (010) plane are likely to occur on the STO substrate surface in the case of the STO (210) substrate. Therefore, the flatness at the atomic level on the surface is prevented from being impaired.

Subsequently, the substrate 1 is heated so as to reach an ultimate temperature of 900 ° C., and a target is irradiated with a KrF excimer laser having a wavelength of 248 nm through the laser beam introduction port of the chamber to form the STO substrate that is the STO substrate on which the LSAT layer is formed. On the top, as the first strongly correlated oxide thin film 31, only 15 atomic layers of LSMO were formed. Subsequently, the PrMnO ₃ target was irradiated with the laser through the port in the same atmosphere, thereby forming only three atomic layers of PrMnO ₃ thin film as the first manganese oxide thin film 20. Next, the SmMnO ₃ target was irradiated with the laser to form a SmMnO ₃ thin film, which is the second manganese oxide thin film 21, by only three atomic layers. Furthermore, only 15 atomic layers of LSMO were formed as the second strongly correlated oxide thin film 32 using the LSMO target again. As for the thickness of each layer, the thickness t1 of the first strongly correlated oxide thin film 31 is 5 unit cells (about 2.6 nm), and the manganese oxide thin film stack 2 (the total of the first and second manganese oxide

thin films

20 and 21). ) Has a thickness tm of 2 unit cells (about 1.1 nm), and the second strongly correlated oxide thin film 32 has a thickness t2 of 5 unit cells (about 2.6 nm). The total thickness t of the oxide stack is 6.3 nm. Here, the critical film thickness tc for the LSMO of the first strongly correlated oxide thin film 31 and the second strongly correlated oxide thin film 32 to become a metal phase at room temperature (300 K) is 8 unit cells (4.1 nm). It is. Therefore, in the film thickness of each layer of the oxide stack formed in this example, 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 the 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 started to decrease by applying a magnetic field having a magnetic flux density of 3.0 T or more, and decreased to 10 kΩ under a 3.5 T magnetic field. Thus, it was confirmed that a huge negative magnetoresistance effect was obtained. When the magnetic field was reduced again, the resistance again became 10 MΩ or more, and it became clear that the insulator-metal transition, which is a Mott transition, appears at room temperature in the manganese oxide 2 contained in the oxide stack. As described above, it was experimentally confirmed that the manganese oxide thin film 2 that realizes switching at room temperature can be produced.

[2-2 Example 2]
In Example 1, the example of the oxide laminated body which made the strongly correlated oxide thin film contact both surfaces of the manganese oxide thin film laminated body 2 was demonstrated. However, dimensional crossover can be used even when an oxide laminate in which a strongly correlated oxide thin film is brought into contact with only one surface side of the manganese oxide thin film laminate 2 is employed. As an example for confirming this point, Example 2 of the present embodiment in which an oxide laminate having a two-layer structure similar to that shown in FIG. In Example 2, an STO (210) substrate pretreated in the same manner as in Example 1 is used as the substrate 1, and only 21 atomic layers of LSMO are formed as the strongly correlated oxide thin film 3. Further, manganese oxide thin film stack 2 PrMnO ₃ as manganous oxide film 20 constituting the, SmMnO ₃ were employed respectively as a second manganese oxide film 21 on. Each of the first manganese oxide thin film 20 and the second manganese oxide thin film 21 was formed by three atomic layers. The pretreatment method for determining the atomic layer that terminates the surface of the substrate 1 in Example 2, and the formation method of each of the oxide laminate, that is, the manganese oxide thin film laminate 2 and the strongly correlated oxide thin film 3, All were the same as in Example 1.

When a four-terminal electrode was formed on the sample produced as Example 2 and the in-plane resistance was measured in the absence of a magnetic field, in the process of raising the temperature from a low temperature (below the liquid nitrogen temperature), first, around 200K. Insulator-metal transition was observed. This is because LSMO, which is the strongly correlated oxide thin film 3, has undergone an insulator-metal transition. When the temperature was further raised, the entire sample became an insulator in the temperature range (253 to 353 K) assumed as the operation of the electronic device including around room temperature (300 K). Thus, when the magnetic resistance was measured at room temperature in the same manner as in Example 1, the sample of Example 2 showed a behavior of 1 kΩ in a magnetic field with a magnetic flux density of 4 T and 100 kΩ in the absence of a magnetic field. That is, the magnetoresistive effect at room temperature of Example 2 is lower than that of the sample of Example 1 while the resistance under a magnetic field is low, but does not increase so much even when no magnetic field is applied, and the resistance change is two orders of magnitude. Stayed within. Although this resistance change is sufficiently detectable, it is ideally desirable to be larger. The inventor of the present application speculates that the reason why the resistance change becomes small is due to a leakage current caused by LSMO, which is the strongly correlated oxide thin film 3 formed thick.

In addition, the structure of FIG.4 (b) formed on the surface of a board | substrate by making arrangement | positioning of each thin film reverse, ie, the 1st manganese oxide thin film 20 which makes the manganese oxide thin film laminated body 2 on the board | substrate 1 side. the PrMnO ₃ and SmMnO ₃ is a second manganese oxide thin film 21 is, even in the samples to form a LSMO is strongly correlated oxide thin film 3 subsequently produced from the substrate 1 in this order, show similar temperature dependence The magnetoresistive effect was measured. Furthermore, the substrate 1, and Preparation and SmMnO ₃ is a second manganese oxide film 21 and a PrMnO ₃ is manganous oxide film 20 in this order and these manganese oxide thin film laminate 2, strong thereafter The magnetoresistive effect showing the same temperature dependence was also measured in the sample in which LSMO, which is the correlation oxide thin film 3, was formed.

[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 described including Examples 1 and 2. In Example 1 above, an example of a laminated structure of PrMnO ₃ / SmMnO ₃ on an STO (210) substrate was shown. However, La, Pr, Nd, Gd, Eu, Tb, Dy, and the like, generally, the first and second manganese oxides of the composition formula RMnO ₃ and the composition formula LMnO _{3 that} select the rare earth element of the lanthanoid as R (or L) It is also possible to adopt a thin film. This is partly because an epitaxial thin film can be produced on a (210) plane oriented substrate. Moreover, even if at least one of these materials is selected, a combination that satisfies the magnitude relationship between the cube root of the unit cell volume and the lattice constant of the substrate can be used. As an example, when an LSAT (210) substrate is adopted as the substrate 1, the lattice constant of the material of the substrate is 0.3870 nm. In this case, SmMnO ₃ can now be selected as the material of the first manganese oxide thin film 20 as a manganese oxide larger than the lattice constant of the substrate. As an example of the second manganese oxide thin film 21 at that time, TbMnO ₃ having a small cube root of unit cell deposition of 0.3853 nm can be selected.

In addition, the manganese oxide used as the 1st and 2nd manganese oxide

thin films

20 and 21 can each be made into the material containing multiple types of lanthanoid element R (L). Even in this case, the present invention is not limited to only two types of solid solutions, such as Pr _1-x Nd _x MnO ₃ (0 <x <1). The lattice constant as an average can be adjusted by selecting three or more elements at the A site. As long as the lattice constant in the substrate and bulk satisfies the above conditions, the material of the substrate and the first and first manganese oxide thin films can be appropriately selected. Employing a material containing a plurality of elements as the material of any of the first and second manganese oxide

thin films

20 and 21 is different from the above-mentioned effect from the viewpoint of reducing the switching threshold. have. That is, even if they are the same trivalent and the average lattice constant does not change, the first manganese oxide thin film is formed with one type of lanthanoid RMnO ₃ and two types of lanthanoid (R ₁ MnO ₃ ) _x (R ₂ MnO ₃ ) _1-x is considered to be a different external field threshold. This is because the rotation angle of the oxygen octahedron surrounding Mn varies locally when the A site is formed of a plurality of elements having different ionic radii. That is, in the case of a plurality of types of elements in which non-uniform randomness is introduced into the crystal structure, the long-range order of the orbital alignment is easily broken. As a result, although not very effective, an effect of reducing the external field threshold necessary for the insulator-metal transition is also expected. This is the case when a plurality of lanthanoids are used for the manganese oxide of the second manganese oxide thin film, or when a plurality of lanthanoid manganese oxides are used for both the first and second manganese oxide thin films. Is the same.

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.

In the manganese oxide thin film laminate 2 of the present embodiment, the order of the first manganese oxide thin film 20 having a larger bulk lattice constant than the substrate 1 and the second second manganese oxide thin film 21 is not particularly limited. This is because the position of the substrate 1 is not directly related to the introduction of competition for the interface between the first manganese oxide thin film and the second manganese oxide thin film. Even when the strongly correlated oxide thin film 3 and the first strongly correlated oxide

thin films

31 and 32 are formed, the positional relationship between these films or the substrate and the first and second manganese oxide

thin films

20 and 21 is maintained. There are no restrictions.

Also, there is no particular restriction on the combination of the thicknesses of the first manganese oxide thin film 20 and the second manganese oxide thin film 21. Although the film thicknesses of these films have been described in the above examples, there are no particular limitations on the film thickness values of both films and the relative ratios of the film thicknesses.

Furthermore, the number of films formed as the first and second manganese oxide

thin films

20 and 21 is not particularly limited. For example, by using a superlattice structure in which a plurality of pairs of the first manganese oxide thin film 20 and the second manganese oxide thin film 21 are stacked, it is possible to introduce more competition of orbital alignment planes. For the effect of making the manganese oxide thin film laminate described in “1-1 Facilitating Mott transition in the manganese oxide thin film laminate” and “1-4 Introduction of competition due to substrate strain” The effects described above can be made compatible and at the same time have a complementary relationship. That is, it can be said that the competition of the orbital alignment planes introduced by forming the first and second manganese oxide thin films in contact eases the conditions for forming the manganese oxide thin film stack as a thin film.

In addition, there is no particular restriction on the thickness of the strongly correlated oxide thin film 3 and the combination of 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 each of Examples 1 and 2, 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 only with the manganese oxide thin film laminate 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 stack 2 occurs even without a strongly correlated oxide thin film.

Further, the substrate 1 may have a configuration other than that exemplified in this 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 the first and second manganese oxide thin films forming the perovskite structure manganese oxide thin film stack are formed on the surface of such a substrate, and the same effect as that of the above-described embodiment can be obtained. This is because it can also be achieved.

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 is used as a device that utilizes a switching phenomenon caused by application of an external field such as a magnetic field, light, electricity, or pressure by providing a manganese oxide thin film stack or an oxide stack that realizes a switching function.

DESCRIPTION OF SYMBOLS 1 Board | substrate 2 Manganese oxide laminated body 20 1st manganese oxide thin film 21 2nd manganese oxide thin film 3 Strong correlation oxide thin film 31 1st strong correlation oxide thin film 32 2nd strong correlation oxide thin film

Claims

A manganese oxide thin film laminate formed on the surface of the substrate,
A first manganese oxide thin film and a second manganese oxide thin film formed in contact with each other;
The first manganese oxide thin film has a composition formula RMnO 3 in which the cubic root of the unit cell volume in bulk is larger than the lattice constant of the substrate (where R is at least one trivalent rare earth selected from lanthanoids). Element) and an atomic layer containing element R and not containing Mn and an atomic layer containing Mn and not containing element R are alternately stacked in the direction perpendicular to the substrate surface,
The second manganese oxide thin film has a composition formula LMnO 3 in which the cube root of the unit cell volume in bulk is smaller than the lattice constant of the substrate (where L is at least one selected from lanthanoids, R An atomic layer containing element L and not containing Mn and an atomic layer containing Mn and not containing element L alternately in the direction perpendicular to the substrate surface. Are stacked,
Each of the first manganese oxide thin film and the second manganese oxide thin film has two crystal axes that are not equivalent to each other in the in-plane direction of the substrate surface.
The composition of the first manganese oxide thin film has a composition formula RMnO 3 (where R is at least one selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy) The manganese oxide thin film laminate according to claim 1, which is represented by a trivalent rare earth element other than those selected as L).
The composition of the second manganese oxide thin film has a composition formula LMnO 3 (where L is at least one selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy) The manganese oxide thin film laminate according to claim 1, which is represented by a trivalent rare earth element other than those selected as R).
4. The manganese oxide thin film stack according to claim 1, wherein a plane orientation of the substrate is a (210) plane orientation. 5.
The manganese oxide thin film laminate 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 laminate,
The thickness t of the whole oxide stack, the thickness tm of the manganese oxide thin film stack, and the thickness t1 of the strongly correlated oxide thin film are critical films for the strongly correlated oxide thin film to become a metal phase. For thickness tc
t = tm + t1> tc and t1 <tc,
An oxide laminate that satisfies the relationship of
The manganese oxide thin film laminate 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 laminate;
A second strongly correlated oxide thin film in contact with the other surface of the manganese oxide thin film laminate,
The total thickness t of the oxide stack, the thickness tm of the manganese oxide thin film stack, the thicknesses t1 and t2 of the first and second strongly correlated oxide thin films, For the critical film thickness tc for becoming a metal phase,
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