Film, laminate, element, device, and structure

Abstract

This film contains ferrite represented by the general formula (Ba1−xSrx)2Co2Fe12−y−δAlyXδO22. X is at least one element selected from the group consisting of Cr, Sb, In, and V. The relationship 0 ≤ x ≤ 1 is satisfied. The relationships 0 < y ≤ 2 and 0 ≤ δ ≤ 1, or 0 ≤ y ≤ 2 and 0 < δ ≤ 1, are satisfied. The film has a film thickness of at least 1.0 nm and less than 70 μm.

Description

Films, laminates, elements, devices, and structures

[0001] The present disclosure relates to films, laminates, elements, devices, and structures.

[0002] Since the 1990s, IT devices such as personal computers and mobile phones have rapidly become widespread. Representative memory LSIs (Large Scale Integration) that have supported this widespread use are dynamic random access memories (DRAMs) and flash memories.

[0003] Memory LSIs are required to have low power consumption in addition to large capacity and high-speed response. Non-volatility is essential from the viewpoint of low power consumption. Universal memory that meets all of these conditions is being actively developed. Flash memory is one of the currently popular non-volatile memories. Because flash memory is based on the principle of charge injection, the write speed is slow, on the order of microseconds, and the number of rewrites is limited to 10 ６ Therefore, flash memory has many problems to overcome as a universal memory.

[0004] Magnetoresistive Random Access Memory (MRAM) utilizing the magnetoresistive effect is known (see, for example, Patent Documents 1 and 2). In MRAM, a stacked structure including magnetic elements is arranged in a matrix. Information recorded in each magnetic element is read using the magnetoresistive effect of the magnetic element. Information is written by controlling the magnetization direction of the magnetic element. Since the magnetization state is replaced with the recorded state, MRAM is nonvolatile in principle. For this reason, MRAM is expected to be a nonvolatile memory that meets all requirements, including low power consumption, high-speed response, large capacity, write endurance, and compatibility with semiconductors.

[0005] JP 2001-196661 A JP 2001-93273 A JP 2016-199411 A

[0006] J. C. Slonczewski, “Current-driven excitation of magnetic multilayers”, June 1996, Journal of Magnetism and Magnetic Materials, volume 159, Issues 1-2, pages L1-L7J. E. Mattson et al., “Photoinduced antiferromagnetic interlayer coupling in Fe / (Fe-Si) superlattices”, 5 July 1993, Physical Review Letters, 71, 185H. Ohno et al., “Electric-field control of ferromagnetism”, 21 December 2000, Nature 408, 944-946

[0007] When considering MRAM with a large capacity exceeding the gigabit level, there is a high possibility that the power consumption for writing will increase, and therefore there is a demand for reducing the power consumption.

[0008] An object of the present disclosure is to provide a film, a laminate, an element, a device, and a structure that can control the magnetization direction with low power consumption.

[0009] The present inventors have conducted extensive research to solve the above problems and have found that the magnetization direction of a specific ferrite can be controlled with low power consumption, leading to the invention.

[0010] [1] General formula (Ba １－ｘ Sr ｘ ) ２ Co ２ Fe １２－ｙ－δ Al ｙ X δ O ２２A film containing a ferrite represented by the formula (1), wherein X is at least one element selected from the group consisting of Cr, Sb, In, and V, and 0≦x≦1, and 0<y≦2 and 0≦δ≦1, or 0≦y≦2 and 0<δ≦1, and having a film thickness of 1.0 nm or more and less than 70 μm. [2] The film according to [1], wherein X is Cr. [3] A laminate comprising: a first ferromagnetic layer which is a magnetization fixed layer; a second ferromagnetic layer which is a magnetization free layer; and the film according to [1] or [2]. [4] An element comprising the laminate according to [3]. [5] A device comprising the element according to [4]. [6] A structure comprising: a film containing ferrite having a hexagonal crystal structure; and a pair of conductive layers provided in the film so as to be spaced apart from each other in a first direction parallel to the easy axis of magnetization of the ferrite crystal, wherein the distance between the pair of conductive layers in the first direction is 1 nm or more and less than 18 μm. [7] The ferrite is a compound represented by the general formula (Ba １－ｘ Sr ｘ ) ２ Co ２ Fe １２－ｙ－δ Al ｙ X δ O ２２ wherein X is at least one element selected from the group consisting of Cr, Sb, In, and V, and 0≦x≦1, and 0<y≦2 and 0≦δ≦1, or 0≦y≦2 and 0<δ≦1. [8] A stacked body comprising: a first ferromagnetic layer that is a magnetization fixed layer; a second ferromagnetic layer that is a magnetization free layer; and the structure according to [6] or [7], wherein the structure, the first ferromagnetic layer, and the second ferromagnetic layer are stacked in a second direction perpendicular to the easy axis of magnetization. [9] An element comprising the stacked body according to [8].

[10] A device comprising the element according to [9].

[0011] According to one aspect of the present disclosure, there are provided a film, a laminate, an element, a device, and a structure that can control the magnetization direction with low power consumption.

[0012] FIG. 1 is a cross-sectional view of an element according to the first embodiment. FIG. 2 is a cross-sectional view of an element according to a first modification. FIG. 3 is a cross-sectional view of an element according to a second modification. FIG. 4 is a cross-sectional view of an element according to the second embodiment. FIG. 5 is an SEM image showing the arrangement of an insulating substrate, a thin film sample, and a pair of conductive layers. FIG. 6 is a schematic diagram showing the electromagnetic state of a Y-type hexaferrite thin film. FIG. 7(a) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Example 1 after poling treatment. FIG. 7(b) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Example 1 after application of a pulsed electric field. FIG. 8 is a graph comparing changes in signal intensity of the Y-type hexaferrite thin film according to Example 1. FIG. 9(a) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Comparative Example 1 after poling treatment. FIG. 9(b) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Comparative Example 1 after application of a pulsed electric field. Fig. 10 is a graph comparing the change in signal intensity of the Y-type hexaferrite thin film according to Comparative Example 1. Fig. 11 is a graph showing the measurement results of the magnetic moment of the Y-type hexaferrite thin film according to Comparative Example 2.

[0013] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or equivalent elements are designated by the same reference numerals, and redundant description will be omitted.

[0014] First Embodiment FIG. 1 is a cross-sectional view showing an element according to the first embodiment. The element 1 according to the first embodiment is, for example, an information storage element or a magnetic functional element. The element 1 is a GMR (giant magnetoresistance) element. Examples of devices according to the first embodiment that include the element 1 include magnetic heads mounted in hard disk drives (HDDs), magnetic memories such as MRAMs, magnetic sensors such as highly sensitive magnetic sensors, microwave generating elements, and electronic devices such as spin transistors. The element 1 is, for example, a structure that has one function in a circuit, and a device is a product that is made by combining the elements 1.

[0015] 1 , the element 1 includes a ferromagnetic layer 11 (first ferromagnetic layer), a non-magnetic metal layer 12, a ferromagnetic layer 13 (second ferromagnetic layer), a film 14, and a pair of conductive layers 15. The element 1 includes a stack 2 in which the ferromagnetic layer 11, the non-magnetic metal layer 12, the ferromagnetic layer 13, and the film 14 are stacked in this order. The element 1 also includes a structure 3 made up of the film 14 and a pair of conductive layers 15. It can also be said that the element 1 includes a stack 4 in which the ferromagnetic layer 11, the non-magnetic metal layer 12, the ferromagnetic layer 13, and the structure 3 are stacked in this order.

[0016] Hereinafter, the direction parallel to the top surface 14a of the film 14 is referred to as the first direction D1, and the direction perpendicular to the top surface 14a of the film 14 is referred to as the second direction D2. The top surface 14a is the surface of the film 14 opposite the ferromagnetic layer 13. The first direction D1 and the second direction D2 are perpendicular to each other. The stacking directions of the stack 2 and the stack 4 are both the second direction D2.

[0017] The ferromagnetic layer 11 and the ferromagnetic layer 13 are arranged with the nonmagnetic metal layer 12 sandwiched between them in the second direction D2. The ferromagnetic layer 11 and the ferromagnetic layer 13 are provided adjacent to the nonmagnetic metal layer 12. The ferromagnetic layer 11 is a magnetization fixed layer (reference layer). The magnetization direction of the ferromagnetic layer 11 is fixed in a predetermined direction. The ferromagnetic layer 13 is a magnetization free layer (storage layer). The magnetization direction of the ferromagnetic layer 13 can easily be changed by magnetization reversal.

[0018] The ferromagnetic layers 11 and 13 are, for example, horizontally magnetized films magnetized parallel to the film surface or in-plane magnetized films. The ferromagnetic layers 11 and 13 are magnetized so that the magnetization direction is parallel to the first direction D1. The ferromagnetic layers 11 and 13 include a ferromagnetic material. Examples of the ferromagnetic material include ferromagnetic metals such as iron (Fe), cobalt (Co), and FeCo alloys, as well as ferromagnetic half-metals. The ferromagnetic layers 11 and 13 may include the same ferromagnetic material or different ferromagnetic materials.

[0019] The nonmagnetic metal layer 12 is a spacer layer interposed between the ferromagnetic layer 11 and the ferromagnetic layer 13. The nonmagnetic metal layer 12 magnetically separates the ferromagnetic layer 11 and the ferromagnetic layer 13. The nonmagnetic metal layer 12 is made of a nonmagnetic metal such as Cu or Ag. The film thickness of the nonmagnetic metal layer 12 is, for example, 4 nm. In the element 1, when the magnetization direction of the ferromagnetic layer 13 is the same as that of the ferromagnetic layer 11 (parallel state), the electrical resistance of the nonmagnetic metal layer 12 decreases due to the GMR effect, and when the magnetization direction is opposite to that of the ferromagnetic layer 11 (antiparallel state), the electrical resistance of the nonmagnetic metal layer 12 increases.

[0020] The ferromagnetic layer 11, the non-magnetic metal layer 12, and the ferromagnetic layer 13 are formed by, for example, sputtering, atomic layer deposition, or ion plating, with sputtering being preferred.

[0021] The film 14 is provided on the ferromagnetic layer 13. The film 14 is provided adjacent to the ferromagnetic layer 13. The film 14 is, for example, a compound represented by the general formula (Ba １－ｘ Sr ｘ ) ２ Co ２ Fe １２－ｙ－δ Al ｙ X δ O ２２ The film 14 includes, for example, a ferrite having a hexagonal crystal structure. The film 14 is, for example, a Y-type hexaferrite thin film whose magnetic moment has a helical structure. This Y-type hexaferrite thin film is preferably a single crystal.

[0022] X is at least one element selected from the group consisting of Cr, Sb, In, and V. X is, for example, Cr. x satisfies 0≦x≦1. That is, the film 14 contains at least one of Ba and Sr. y and δ satisfy 0<y≦2 and 0≦δ≦1, or 0≦y≦2 and 0<δ≦1. That is, the film 14 contains at least one of Al and X. δ may be 0.01≦δ≦0.1.

[0023] The film 14 has a thickness of, for example, 1.0 nm or more and less than 70 μm. A film thickness of 1.0 nm or more makes the film 14 easy to manufacture. A film thickness of less than 70 μm makes magnetization reversal by pulse voltage possible. The smaller the film thickness of the film 14, the easier the magnetization reversal. The film thickness may be 10 μm or less, or 1000 nm or less. The film thickness may be 1.0 nm or more and 10 μm or less, or 1.0 nm or more and 1000 nm or less. The film 14 is formed by, for example, a polishing method using a single crystal, a processing method using a focused ion beam (FIB), a sputtering method, a spray deposition method, a spin coating method, or a plasma laser deposition method.

[0024] The film 14 is provided so that the easy axis of magnetization of the ferrite crystal is oriented horizontally (first direction D1). Hereinafter, the easy axis of magnetization of the ferrite crystal contained in the film 14 will also be simply referred to as the easy axis of magnetization. The stacking direction (second direction D2) of the laminate 2 and the laminate 4 is perpendicular to the easy axis of magnetization.

[0025] The pair of conductive layers 15 are provided on the film 14 so as to be spaced apart from each other in a direction parallel to the easy axis of magnetization (first direction D1). In the element 1, the pair of conductive layers 15 are both provided on the side surfaces 14b of the film 14. The pair of conductive layers 15 function as electrodes for applying a voltage to the film 14. The distance (shortest distance) between the pair of conductive layers 15 in the first direction D1 is 1 nm or more and less than 18 μm. When the distance between the pair of conductive layers 15 is 1 nm or more, the film 14 is easily manufactured. When the distance between the pair of conductive layers 15 is less than 18 μm, magnetization reversal by a pulse voltage is possible. The shorter the distance between the pair of conductive layers 15, the easier the magnetization reversal. The distance between the pair of conductive layers 15 may be 2 μm or less. The distance between the pair of conductive layers 15 may be 1 nm or more and 2 μm or less.

[0026] The pair of conductive layers 15 includes a conductive material. Examples of the conductive material include titanium nitride (TiN), titanium (Ti), tungsten nitride (WN), tungsten (W), tantalum nitride (TaN), and tantalum (Ta). The pair of conductive layers 15 may be formed of the same conductive material or different conductive materials.

[0027] The inventors have discovered that the magnetization direction of the film 14 can be controlled by applying an electric field to the film 14. That is, in the element 1, the film 14 can be magnetized in the horizontal direction by applying a voltage between the pair of conductive layers 15 and applying a horizontal (first direction D1) electric field to the film 14. If the magnetization direction of the film 14 differs from the magnetization direction of the ferromagnetic layer 13, the magnetization direction of the ferromagnetic layer 13 is reversed. Furthermore, the magnetization direction of the film 14 can be reversed by controlling the voltage applied between the pair of conductive layers 15. This reverses the magnetization direction of the ferromagnetic layer 13, changing the electrical resistance of the nonmagnetic metal layer 12. The change in the electrical resistance of the nonmagnetic metal layer 12 can be detected by measuring the current Id flowing through the nonmagnetic metal layer 12 in the horizontal direction (first direction D1).

[0028] Two main methods have been proposed for controlling the magnetization direction of MRAM. The first method uses a classical current magnetic field, utilizing wiring placed in close proximity to the target magnetic element. However, this method has the following problems: (1) the smaller the size of the magnetic element, the larger the current required; and (2) the leakage magnetic field from the wiring can cause write errors in magnetic elements close to the target magnetic element. For this reason, the first method is considered inapplicable to gigabit-class large-capacity MRAM.

[0029] The second method is expected to solve the problems of the first method. The second method uses spin-transfer magnetization reversal (see Non-Patent Document 1). Spin-transfer magnetization reversal is a phenomenon that occurs when the spin angular momentum of conduction electrons is directly transferred to the localized spins of a magnetic element. The current density required for reversal does not depend on the element size. Therefore, theoretically, the smaller the element size, the smaller the required write current. Therefore, the second method has attracted attention as a promising writing method for next-generation MRAM. However, because the critical current density required for writing has not yet been sufficiently reduced, it has not yet achieved a low enough power consumption to surpass competing memories.

[0030] Several techniques for inducing magnetic responses through the electric field effect on devices have been proposed. For example, Non-Patent Document 2 proposes a method for controlling the magnetic interlayer exchange coupling generated through a sandwich structure consisting of a very thin semiconductor layer sandwiched between two ferromagnetic layers by varying the carrier concentration in the semiconductor layer. The carrier concentration in the semiconductor layer is controlled by an applied electric field. Patent Document 1 proposes a method for growing a sandwich structure consisting of a ferromagnetic layer / nonmagnetic layer / ferromagnetic layer on a semiconductor (or insulator) and controlling the interlayer exchange coupling between the ferromagnetic layers through the nonmagnetic layer by utilizing modulation of the surface barrier height at the semiconductor (or insulator) / ferromagnetic layer interface caused by the application of a voltage. Such magnetization reversal control using electric field stimulation has the potential to achieve data writing with lower power consumption than methods using a current-induced magnetic field (external magnetic field) or the spin transfer effect. Therefore, it is expected to be applied to small, nonvolatile solid-state magnetic memories.

[0031] Patent Document 2 proposes a method of controlling the magnetization state by inducing magnetostriction by configuring a piezoelectric / electrostrictive material such as PZT, which can be induced by an electrical stimulus, in contact with a magnetic body. However, the current problem is that the fatigue resistance of piezoelectric / electrostrictive materials is insufficient. Non-Patent Document 3 proposes a method of controlling ferromagnetism by changing the carrier density by applying an electric field to a ferromagnetic semiconductor InMnAs. However, because a ferromagnetic semiconductor with a Curie temperature above room temperature has not yet been realized, this method has not yet been put to practical use.

[0032] There have been no reported examples of field-effect-induced magnetic response, not only in applications but also at the basic research level. Furthermore, when considering modulating the interlayer exchange coupling strength of ferromagnetic / nonmagnetic / ferromagnetic layers by controlling the surface barrier height of a semiconductor or insulator to control magnetization reversal using an electric field, quantum interference, which is the origin of interlayer exchange coupling, must occur across multiple layers. Therefore, a very high-quality stacked structure is required. For example, the only examples to date in which oscillations of interlayer exchange coupling, including ferromagnetic layers, have been observed are epitaxially grown Fe / Au and Fe / Cr systems. This poses severe limitations when considering applications. For the reasons already mentioned, the methods described in the patent and non-patent literature mentioned above are unable to realize randomly accessible solid-state magnetic memories at room temperature.

[0033] Patent Document 3 discloses an oxide compound having a Y-type ferrite structure. However, Patent Document 3 does not disclose the ferrite contained in the film according to the present embodiment, that is, the oxide compound having the general formula (Ba １－ｘ Sr ｘ ) ２ Co ２ Fe １２－ｙ－δ Al ｙ X δ O ２２ The ferrite represented by the formula (I) is not disclosed.

[0034] As described above, the magnetization direction of the film 14 can be controlled by applying a pulse electric field without using a current magnetic field or spin transfer torque. Therefore, in the device 1 using the film 14, the generation of Joule heat is suppressed, thereby enabling low power consumption. Furthermore, since the magnetization direction of the film 14 can be controlled at room temperature, the device 1 can be operated at room temperature. Therefore, the device 1 has high practicality.

[0035] (Modifications) The pair of conductive layers 15 may be provided on a portion other than the side surface 14b of the film 14, as long as they are provided on the film 14 so as to be spaced apart from each other in the first direction D1. FIG. 2 is a cross-sectional view showing an element according to a first modification. As shown in FIG. 2, in an element 1A according to the first modification, one conductive layer 15 is provided on the side surface 14b of the film 14, and the other conductive layer 15 is provided on the top surface 14a. FIG. 3 is a cross-sectional view showing an element according to a second modification. As shown in FIG. 3, in an element 1B according to the second modification, both of the pair of conductive layers 15 are provided on the top surface 14a of the film 14.

[0036] In the elements 1A and 1B, similarly to the element 1, a horizontal electric field (first direction D1) can be applied to the film 14 by applying a voltage between the pair of conductive layers 15. The pair of conductive layers 15 are also arranged such that the distance (shortest distance) between the pair of conductive layers 15 in the first direction D1 is 1 nm or more and less than 18 μm.

[0037] Second Embodiment FIG. 4 is a cross-sectional view showing an element according to a second embodiment. The second embodiment will be described below, focusing on the differences from the first embodiment. The element 5 according to the second embodiment is a TMR (tunnel magnetoresistance) element. An example of a device according to the second embodiment that includes the element 5 is an electronic device similar to the device according to the first embodiment. As shown in FIG. 4, the element 5 differs from the element 1 in that it includes a tunnel insulating layer 16 instead of the nonmagnetic metal layer 12 (see FIG. 1).

[0038] The tunnel insulating layer 16 is a spacer layer interposed between the ferromagnetic layer 11 and the ferromagnetic layer 13. The tunnel insulating layer 16 electrically and magnetically separates the ferromagnetic layer 11 and the ferromagnetic layer 13. The tunnel insulating layer 16 includes an insulator. Examples of the insulator include magnesium oxide (MgO) and aluminum oxide (Al ２ O ３), or a mixture of these materials. The thickness of the tunnel insulating layer 16 is such that carriers can tunnel in the thickness direction, and is, for example, 1 nm or more and 2 nm or less. The tunnel insulating layer 16 is formed by, for example, sputtering, atomic layer deposition, or ion plating. Sputtering is preferably used.

[0039] In the element 5, the stack 2 is formed by stacking a ferromagnetic layer 11, a tunnel insulating layer 16, a ferromagnetic layer 13, and a film 14 in this order. The stack 4 is formed by stacking a ferromagnetic layer 11, a tunnel insulating layer 16, a ferromagnetic layer 13, and a structure 3 in this order.

[0040] The ferromagnetic layer 11, the tunnel insulating layer 16, and the ferromagnetic layer 13 form a magnetic tunnel junction (MTJ). That is, when the magnetization direction of the ferromagnetic layer 13, which is the magnetization free layer, is the same as (parallel to) the magnetization direction of the ferromagnetic layer 11, which is the magnetization fixed layer, the electrical resistance of the MTJ is low, and when they are opposite (antiparallel to) the magnetization direction of the MTJ, the electrical resistance of the MTJ is high.

[0041] In the element 5, the applied voltage between the pair of conductive layers 15 is controlled to reverse the magnetization direction of the film 14, thereby reversing the magnetization direction of the ferromagnetic layer 13, and as a result, the electrical resistance of the MTJ changes. The change in the electrical resistance of the MTJ can be detected by measuring the current Id flowing through the element 5 in the perpendicular direction (second direction D2).

[0042] In element 5, the pair of conductive layers 15 may also be provided on a portion other than side surface 14b of film 14, as long as they are provided on film 14 so as to be spaced apart from each other in first direction D1. For example, in element 5, the pair of conductive layers 15 may also be arranged in the same manner as in element 1A or element 1B.

[0043] The present invention is not necessarily limited to the above-described embodiment and modifications, and various modifications are possible without departing from the spirit of the present invention. The above-described embodiment and modifications may be combined as appropriate.

[0044] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0045] (Example 1) <Preparation of Sample> In Example 1, Ba ０．８ Sr １．２ Co ２ Fe １１．０５ Al ０．９ Cr ０．０５ O ２２ A thin film sample of Y-type hexaferrite represented by the formula: ３ (2.2g), BaCO ３ (3.0 g), Co ３ O ４ (2.4 g), Fe ２ O ３ (13.4g), Al ２ O ３ (0.7 g), and Cr ２ O ３ (0.06 g) were mixed and reacted in air at 1150° C. for 24 hours to obtain a precursor, which was then heated for 14 hours to obtain a rod.

[0046] Subsequently, a Y-type hexaferrite single crystal was grown from the rod using a laser floating zone method in an oxygen atmosphere at 10 atmospheres. The growth direction of the single crystal was parallel to the a-axis of the crystal. The obtained Y-type hexaferrite rod-shaped single crystal ingot (7 mm diameter, 10 cm length) was cut into a disk. The disk-shaped sample was then sealed in a vacuum quartz tube and then filled with Ag as an oxygen source. ２ O was introduced into the quartz tube, and the sample was annealed in an oxygen atmosphere at 10 atmospheres for 100 hours, resulting in a Y-type hexaferrite disk.

[0047] Next, using an FIB device (Thermo Fisher Scientific, product number: Helios 5 UX), a beam of ionized gallium (Ga) was irradiated onto the Y-type hexaferrite disk, and a thin film sample was obtained by microfabrication so that the c-axis of the crystal was oriented in-plane and the size was 2 μm in length, 2 μm in width, and 1 μm in thickness.

[0048] Next, an insulating substrate was prepared, consisting of a silicon substrate with an insulating film (300 nm) formed on it. A thin film sample was placed on the insulating substrate, and a pair of conductive layers was evaporated onto both ends of the thin film sample in the b-axis direction of the crystal. The pair of conductive layers consisted of platinum two-terminal electrodes. The distance between the electrodes was 2 μm. Next, the microfabricated sample surface was annealed at 400°C for 1 hour under a flow of ultraviolet-excited ozone gas to compensate for oxygen deficiencies caused by Ga ion irradiation.

[0049] Figure 5 is an SEM image showing the arrangement of an insulating substrate, a thin film sample, and a pair of conductive layers. As shown in Figure 5, a groove 22 is provided on the surface of an insulating substrate 21. A thin film sample 23 is placed on the insulating substrate 21 so that the b-axis direction of the crystal is the horizontal direction in Figure 5 and the c-axis direction of the crystal is the vertical direction in Figure 5. The thin film sample 23 is bridged over the groove 22, which extends parallel to the c-axis direction of the crystal. A pair of conductive layers 24 is provided on both ends of the thin film sample 23 in the b-axis direction of the crystal. Since the pair of conductive layers 24 are arranged on either side of the groove 22, the occurrence of short circuits and leakage currents is suppressed.

[0050] <Confirmation of electric field induced magnetization reversal> An MFM (magnetic force microscope) was used to confirm the electric field induced magnetization reversal of the Y-type hexaferrite thin film according to Example 1. To perform the electric field induced magnetization reversal, a spin arrangement in which the magnetization vector M and the polarization vector P are orthogonal to each other is utilized in an ordered phase called the FE3 phase.

[0051] Figure 6 is a schematic diagram showing the magnetoelectric state of a Y-type hexaferrite thin film. The dashed-line frame in Figure 6 shows two magnetoelectric states that are realized under a high magnetic field of 5 T (tesla) or more. In a forced ferromagnetic state of 5 T or more, the magnetization vectors M are all oriented in one direction. In contrast, the polarization vectors P are oriented at ±90 degrees relative to the magnetization vector M. The magnetoelectric state at +90 degrees is defined as the "+1" state, and the magnetoelectric state at -90 degrees is defined as the "-1" state.

[0052] To form a single magnetoelectric state, a 5 T magnetic field was applied in the perpendicular direction (the a-axis direction of the crystal) at 300 K, and a voltage of -10 V (-5 MV / m in terms of electric field) was applied to the voltage terminals (i.e., between the pair of conductive layers) for several seconds. This created only the "+1" state within the sample (magnetoelectric poling operation). The magnetic field was then returned from 5 T to 0 T. In a zero magnetic field, the state in which the magnetization vectors are aligned is energetically unstable. Therefore, while the magnetization vector M and polarization vector P maintain their relative relationship, some of the "+1" states rotate 180 degrees to become "+1, π" states. The solid-line frame in Figure 6 shows the magnetoelectric state realized in zero magnetic field after the magnetoelectric poling operation under a high magnetic field.

[0053] After returning to zero magnetic field as described above, the magnetic texture was observed at room temperature and zero magnetic field using MFM (see Figure 7(a)). Next, to perform magnetization reversal, a rectangular pulse voltage of 10 V (-5 MV / m electric field) with a duration of 0.1 ms was applied five times at room temperature and zero magnetic field. After that, the magnetic texture was observed again at room temperature and zero magnetic field using MFM (see Figure 7(b)).

[0054] Figure 7(a) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film of Example 1 after poling. The MFM signal intensity is represented by the phase angle (deg) of the resonant motion of the cantilever on which the magnetic thin film is deposited. In MFM, the resonant characteristics of the cantilever change depending on the magnetization state of the sample, so the phase angle of the resonant motion of the cantilever reflects the magnetization state of the sample. As shown in Figure 7(a), after poling, a striped magnetic texture was observed due to the hybridization of the "+1" state and the "+1, π" state. In this magnetic texture, multiple lines parallel to the b-axis direction are aligned along the c-axis direction of the crystal.

[0055] Figure 7(b) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Example 1 after application of a pulsed electric field. As shown in Figure 7(b), a striped magnetic texture was again observed after application of a pulsed voltage. However, it was found that the position of the magnetic texture was different before and after application of the pulsed voltage. In the magnetic texture shown in Figure 7(b), multiple lines parallel to the b-axis direction are aligned along the c-axis direction of the crystal, and multiple lines parallel to the c-axis direction are aligned along the b-axis direction of the crystal. Therefore, this magnetic texture can also be described as lattice-like.

[0056] FIG. 8 is a graph comparing the change in signal intensity of the Y-type hexaferrite thin film according to Example 1. The same portion of the magnetic texture image was extracted before and after the application of a pulse voltage, and the change in signal intensity was compared in detail. Here, the change in signal intensity in the rectangular framed areas shown in FIGS. 7( a) and 7(b) was compared. The upper graph in FIG. 8 shows the change in signal intensity before the application of a pulse voltage. The lower graph in FIG. 8 shows the change in signal intensity after the application of a pulse voltage. The horizontal axis in FIG. 8 indicates the position X (μm) in the c-axis direction within the rectangular frame in FIGS. 7( a) and 7(b). Although the origin of the position X is set outside the rectangular frame, the origin of the position X can be set to any position. In FIG. 8, the "+1" state is indicated by an upward arrow, and the "+1, π" state is indicated by a downward arrow.

[0057] As shown in Figure 8, the boundary between the "+1" state and the "+1, π" state changes before and after the application of the pulse voltage, and localized electric field-induced magnetization reversal was successfully demonstrated. When a voltage of 10 V was applied, the leakage current flowing through the Y-type hexaferrite thin film sample was approximately 1 nA. Therefore, the electrical energy required for localized magnetization reversal was approximately 10 × 10 －９ x10 －４ It is estimated to be ×5 = 5 pJ.

[0058] As described above, electric field induced magnetization reversal was demonstrated using MFM for the Y-type hexaferrite thin film according to Example 1. Therefore, it was confirmed that the film according to the above embodiment can be used adjacent to the magnetization free layer of a voltage-driven information storage element and a magnetic function element.

[0059] (Comparative Example 1) <Preparation of Sample> As Comparative Example 1, Ba ０．８ Sr １．２ Co ２ Fe １１．０５ Al ０．９ Cr ０．０５ O ２２ A thin film sample of Y-type hexaferrite represented by the formula (I) was prepared, measuring 18 μm in length, 13 μm in width, and 1 μm in thickness. The preparation procedure for the thin film sample of Comparative Example 1 was the same as that for Example 1, except that the sample was processed so that its dimensions in the length and width directions were larger. The distance between the electrodes was 18 μm.

[0060] <Confirmation of Electric Field-Induced Magnetization Reversal> The electric field-induced magnetization reversal was confirmed for the thin film sample of Comparative Example 1 using MFM. As described above, to form a single magnetoelectric state, it is necessary to apply a 5 T magnetic field in the perpendicular direction (the a-axis direction of the crystal) at 300 K and to apply a -5 MV / m electric field to the voltage terminals (i.e., between a pair of conductive layers). Because the thin film sample of Comparative Example 1 is larger than the thin film sample of Example 1, an electric field of -5 MV / m corresponds to a high voltage of 90 V. Therefore, after applying a 5 T magnetic field, no electric field was applied and the magnetic field was returned to 0 T. This zero magnetic field state was used as the initial state for MFM observation.

[0061] After returning to zero magnetic field as described above, the magnetic texture was observed at room temperature and zero magnetic field using MFM (see Figure 9(a)). Next, to confirm that magnetization reversal did not occur with the same pulse voltage as in Example 1, a rectangular pulse voltage of 10 V (0.56 MV / m in electric field equivalent) with a time width of 0.1 ms was applied five times at room temperature and zero magnetic field. Thereafter, the magnetic texture was again observed at room temperature and zero magnetic field using MFM (see Figure 9(b)).

[0062] FIG. 9(a) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Comparative Example 1 after poling. As shown in FIG. 9(a), in the initial state, a striped magnetic texture was observed due to the mixing of the "+1" state and the "+1, π" state, and the "-1" state and the "-1, π" state. FIG. 9(b) is an MFM image showing the magnetic texture of the Y-type hexaferrite thin film according to Comparative Example 1 after the application of a pulsed electric field. As shown in FIG. 9(b), the striped magnetic texture was again observed even after the application of the pulsed voltage. It was found that the position of the magnetic texture was almost the same before and after the application of the pulsed voltage.

[0063] FIG. 10 is a graph comparing the change in signal intensity of the Y-type hexaferrite thin film according to Comparative Example 1. The same portion of the magnetic texture image was extracted before and after the application of a pulse voltage, and the change in signal intensity was compared in detail. Here, the change in signal intensity in the rectangular framed areas shown in FIGS. 9( a) and 9(b) was compared. The top graph in FIG. 10 shows the change in signal intensity before the application of a pulse voltage. The bottom graph in FIG. 10 shows the change in signal intensity after the application of a pulse voltage. The horizontal axis in FIG. 10 indicates the position X (μm) in the c-axis direction within the rectangular frame in FIGS. 9( a) and 9(b). Although the origin of the position X is set outside the rectangular frame, the origin of the position X can be set to any position. In FIG. 10, the "+1" state or the "-1" state is indicated by an upward arrow, and the "+1, π" state or the "-1, π" state is indicated by a downward arrow.

[0064] 10, it was confirmed that the boundary line between the "+1" state or "-1" state and the "+1, π" state or "-1, π" state hardly changed, demonstrating that local electric field induced magnetization reversal does not occur in Comparative Example 1.

[0065] (Example 2 and Comparative Example 2) <Sample Preparation> For Example 2 and Comparative Example 2, thin film samples of Y-type hexaferrite with thicknesses of 69 μm and 147 μm were prepared, respectively. Similar to the procedure for preparing the thin film sample in Example 1, a rod-shaped single crystal ingot of Y-type hexaferrite (7 mm in diameter, 10 cm in length) was formed, and then the rod-shaped single crystal ingot of Y-type hexaferrite was cut into disks and annealed in an oxygen atmosphere. The obtained disk-shaped samples were polished to obtain Y-type hexaferrite thin films with thicknesses of 69 μm and 147 μm, in which the c-axes of the crystals were oriented in-plane.

[0066] To perform electric-field-induced magnetization reversal, a 5 T magnetic field was applied in the perpendicular direction (the a-axis direction of the crystal) at 240 K, and a voltage of 350 V (5 MV / m for a 69 μm-thick thin film and 2.4 MV / m for a 147 μm-thick thin film in terms of electric field) was applied to the voltage terminals (i.e., between the pair of conductive layers) for several seconds (magnetic-electric poling operation).The magnetic field was then returned from 5 T to 0 T.Next, the magnetic moment was measured using a SQUID while changing the applied voltage at 240 K.

[0067] 11 is a graph showing the electric field dependence of the magnetization of the Y-type hexaferrite thin films according to Example 2 and Comparative Example 2. The vertical axis of FIG. 11 represents the value of the magnetic moment (μ Ｂ / f.u.). The horizontal axis of FIG. 11 indicates the value of the voltage applied to the sample during measurement. As shown in FIG. 11, in the 69 μm-thick thin film, the polarity of the magnetic moment is reversed with a positive / negative reversal of the applied voltage, whereas in the 147 μm-thick thin film, no reversal of the polarity of the magnetic moment is observed. This confirmed that in the Y-type hexaferrite thin film (thickness 147 μm) according to Comparative Example 2, no electric-field-induced magnetization reversal occurs when a voltage is applied under the same conditions as in the Y-type hexaferrite thin film (thickness 69 μm) according to Example 2.

[0068] [Additional remarks] The film, laminate, element, device, and structure disclosed herein enable low power consumption, which can contribute to the achievement of Goal 9 of the Sustainable Development Goals (SDGs) led by the United Nations. Goal 9: "Build resilient infrastructure, promote inclusive and sustainable industrialization, innovate and foster innovation."

[0069] 1, 1A, 1B…element, 2…laminated body, 3…structure, 4…laminated body, 5…element, 11…ferromagnetic layer (first ferromagnetic layer), 13…ferromagnetic layer (second ferromagnetic layer), 14…film, 15…conductive layer, D1…first direction, D2…second direction.

Claims

1. General formula (Ba １－ｘ Sr ｘ ) ２ Co ２ Fe １２－ｙ－δ Al ｙ X δ O ２２ wherein X is at least one element selected from the group consisting of Cr, Sb, In, and V, 0≦x≦1, 0<y≦2 and 0≦δ≦1, or 0≦y≦2 and 0<δ≦1, and the film has a thickness of 1.0 nm or more and less than 70 μm.

2. The film of claim 1, wherein X is Cr.

3. A stack comprising: a first ferromagnetic layer serving as a magnetization fixed layer; a second ferromagnetic layer serving as a magnetization free layer; and the film according to claim 1 or 2.

4. A device comprising the laminate according to claim 3.

5. A device comprising the element of claim 4.

6. A structure comprising: a film containing ferrite having a hexagonal crystal structure; and a pair of conductive layers provided on the film so as to be spaced apart from each other in a first direction parallel to the easy axis of magnetization of the ferrite crystal, wherein the distance between the pair of conductive layers in the first direction is 1 nm or more and less than 18 μm.

7. The ferrite has the general formula (Ba １－ｘ Sr ｘ ) ２ Co ２ Fe １２－ｙ－δ Al ｙ X δ O ２２ 7. The structure according to claim 6, wherein X is at least one element selected from the group consisting of Cr, Sb, In, and V, and 0≦x≦1, and 0<y≦2 and 0≦δ≦1, or 0≦y≦2 and 0<δ≦1.

8. A stack comprising: a first ferromagnetic layer that is a magnetization fixed layer; a second ferromagnetic layer that is a magnetization free layer; and the structure according to claim 6 or 7, wherein the structure, the first ferromagnetic layer, and the second ferromagnetic layer are stacked in a second direction perpendicular to the axis of easy magnetization.

9. A device comprising the laminate according to claim 8.

10. A device comprising the element of claim 9.

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