Laminate of silicon substrate and thin film of compound having perovskite-type structure, and magnetic memory element having said laminate

Abstract

Provided are a magnetic memory element and a magnetic memory device which use a multiferroic material integrated on a silicon substrate, and which are capable of writing and reading information by means of an electric field at room temperature. This laminate is composed of a silicon substrate and a thin film of a compound having a perovskite-type structure and represented by formula (1). (1): BiFe1-xAxO3. (In formula (1), A is selected from the group consisting of Co and Mn, and x satisfies 0.05≤x≤0.25.)

Description

Laminate of silicon substrate and thin film of compound having perovskite structure, and magnetic memory element having said laminate

[0001] The present disclosure relates to a stack of a silicon substrate and a thin film of a compound having a perovskite structure, and a magnetic memory element including the stack.

[0002] In recent years, progress has been made in the development of multiferroic materials, which have multiple properties such as ferromagnetism, ferroelectricity, and ferroelasticity. Among these multiferroic materials, those that possess both ferroelectricity and ferromagnetism and whose magnetization can be controlled by an electric field are expected to be used in applications such as low-power magnetic memory devices that utilize the response of magnetization to an electric field.

[0003] Conventionally, elements using multiferroic materials include AFeO ３ Multiferroic elements made of orthoferrites and other materials are known (Patent Document 1). Most of the multiferroic materials known to date exhibit both ferromagnetism and ferroelectricity only at low temperatures below −200°C, or are unable to reverse their magnetization by applying an electric field, making them difficult to put into practical use as magnetic memory elements.

[0004] In response to this, Patent Document 2 proposes a magnetic memory element in which information can be written and read by an electric field at room temperature.

[0005] JP 2010-161272 A Japanese Patent No. 6902783

[0006] The proposal in Patent Document 2 is a very useful technology that brings multiferroic elements closer to practical use. On the other hand, the substrate on which the multiferroic material is integrated is made of GdScO with a 110 orientation. ３ , 110-oriented DyScO ３ , 110-oriented SrTiO ３ , 111-oriented SrTiO ３ and 001-oriented SrTiO ３ To commercialize this proposal, the challenge was to develop a technology to integrate a large number of multiferroic materials that operate at room temperature onto the more versatile silicon substrate.

[0007] The present invention has been made in view of these circumstances, and aims to provide a magnetic memory element and a magnetic memory device that use a multiferroic material integrated on a silicon substrate, and that allow writing and reading of information using an electric field at room temperature.

[0008] As a result of intensive research into solving the above problems, the inventors of the present invention succeeded in producing a laminate of a silicon substrate and a thin film of a specific compound having a perovskite structure, thereby arriving at the present invention.

[0009] [1] A laminate comprising a silicon substrate and a thin film of a compound having a perovskite structure and represented by formula (1). １－ｘ A ｘ O ３ ... (1) (In formula (1), A is selected from the group consisting of Co and Mn, and x satisfies 0.05≦x≦0.25.) [2] The laminate according to [1], wherein the surface of the silicon substrate is a (001) plane. [3] A magnetic memory element comprising the laminate according to [1] or [2]. [4] A magnetic memory device comprising the magnetic memory element according to [3]. [5] A thin film of a compound represented by formula (1) having a perovskite structure, wherein the film thickness of the thin film is 10 nm or more and less than 200 nm. BiFe １－ｘ A ｘ O ３ ... (1) (In formula (1), A is selected from the group consisting of Co and Mn, and x satisfies 0.05≦x≦0.25.)

[0010] The present disclosure makes it possible to provide a new laminate in which a thin film of a specific compound having a perovskite structure is laminated on a highly versatile silicon substrate.

[0011] 1A and 1B are schematic cross-sectional views illustrating an example of a magnetic memory element. 2A and 2B are schematic cross-sectional views illustrating a method for writing (A) and reading (B) information from a magnetic memory element. ０．９ Co ０．１ O ３ 1 is a graph showing the results of X-ray diffraction (XRD) of a laminate with a thin film (film thickness 60 nm) prepared in the example. ０．９ Co０．１ O ３ 1 is a graph showing the reciprocal lattice map (003) peak of X-ray diffraction of a laminate with a thin film (film thickness 60 nm) prepared in the example. ０．９ Co ０．１ O ３ 1 is a graph showing the reciprocal lattice map (203) peak of X-ray diffraction of a laminate with a thin film (film thickness 60 nm) prepared in the example. ０．９ Co ０．１ O ３ 1 is a graph showing the reciprocal lattice map (113) peak of X-ray diffraction of a laminate with a thin film (film thickness 60 nm) prepared in the example. ０．９ Co ０．１ O ３ BiFe thin film (film thickness 60 nm) laminate ０．９ Co ０．１ O ３ This is a piezoelectric response microscope (PFM) image of the thin film out of plane (photograph used as a drawing). ０．９ Co ０．１ O ３ Graph (a) shows the magnetic field dependence of magnetization and graph (b) shows the temperature dependence of remanent magnetization of a laminate with a thin film (film thickness 60 nm) fabricated in the example. ０．９ Co ０．１ O ３ BiFe thin film (60 nm thick) laminate before poling ０．９ Co ０．１ O ３ PFM image (a) and MFM image (b) of the thin film, and BiFe after poling ０．９ Co ０．１ O ３ The PFM image (c) and MFM image (d) of the thin film are shown (photographs used as drawing substitutes). ０．９ Co ０．１ O ３ This is a PFM image of a thin film (photograph used as a drawing).

[0012] (Laminate) One embodiment of the present disclosure is a laminate of a silicon substrate and a thin film of a compound represented by formula (1) having a perovskite structure, the laminate being formed by depositing a thin film made of a multiferroic material on a practical silicon substrate. The laminate may include films other than the silicon substrate and the thin film, and specifically may include an electrode, a buffer layer, etc.

[0013] (Thin Film) The thin film is a thin film of a compound having a perovskite structure and is represented by the following formula (1): BiFe １－ｘ A ｘ O ３ ... (1) In formula (1), A is selected from the group consisting of Co and Mn, and x satisfies 0.05≦x≦0.25. When x is 0.05 or more, ferromagnetism and ferroelectricity can be exhibited at room temperature. On the other hand, when x is 0.25 or less, changes in the crystal structure of the thin film can be suppressed. The magnitude of spontaneous magnetization of the thin film at room temperature is 1 emu / cm ３ ~10 emu / cm ３ The magnitude of spontaneous polarization is about 50 to 150 μC / cm ２ The magnetization direction of the thin film can be reversed by applying a voltage to the electrodes and generating an electric field. This allows information to be written to the thin film, and the written information can be read by detecting the reversed magnetization.

[0014] A is an element capable of substituting Fe, preferably Co or Mn, and more preferably Co. x represents the amount of Fe substituted, preferably 0.075≦x≦0.20, and more preferably 0.10≦x≦0.15. The thickness of the thin film is not particularly limited, but a thinner film tends to enable magnetization reversal at a lower voltage. Therefore, the thickness of the thin film is preferably 10 nm or more and less than 200 nm, more preferably 10 nm or more and less than 70 nm, and the upper limit may be 150 nm or less, 100 nm or less, or even 60 nm or less. Therefore, a preferred embodiment of the present disclosure is a laminate comprising a silicon substrate and a thin film of a compound represented by the above formula (1) having a perovskite structure, wherein the thickness of the thin film is 10 nm or more and less than 200 nm. Another aspect of the present disclosure is a laminate of a thin film of a compound having a perovskite structure and represented by the above formula (1), wherein the film thickness of the thin film is 10 nm or more and less than 200 nm. The thin film of the present disclosure is a novel thin film that is even thinner than the thin films of the compound having a perovskite structure and represented by the above formula (1) that have been disclosed previously.

[0015] The method for forming the thin film is not particularly limited, and the thin film can be formed by methods known to those skilled in the art, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and spin coating. Specific examples of PVD methods include pulsed laser deposition (PLD), sputtering, and electron beam evaporation. Specific examples of CVD methods include metal organic (MO) CVD and mist CVD. Sputtering methods include RF sputtering and DC sputtering, depending on the power source used.

[0016] (Silicon Substrate) The silicon substrate is a Si substrate, and any commonly used silicon substrate can be used. It may be a single crystal substrate or a polycrystalline substrate, but is typically a single crystal substrate. Examples of silicon substrates include substrates oriented in the (001) plane, (110) plane, and (111) plane, but a silicon substrate oriented in the (001) plane is preferred. The thickness of the silicon substrate is not particularly limited, but from the viewpoints of thin film formation and ease of handling, it is preferably 300 μm or more and 1000 μm or less, and more preferably 400 μm or more and 600 μm or less.

[0017] (Magnetic Memory Element) The laminate of the present disclosure can constitute a magnetic memory element. The magnetic memory element will be described below with reference to the drawings. FIG. 1 is a schematic cross-sectional view of a magnetic memory element 10 according to one embodiment of the present disclosure. The magnetic memory element 10 is formed by laminating buffer layers 5 and 6, a lower electrode 2, a thin film 3, and an upper electrode 4 in this order on a silicon substrate 1. The lower electrode 2 and the upper electrode 4 may each be connected to a power source (not shown) for applying a voltage to the magnetic memory element.

[0018] The material for the lower electrode 2 is not particularly limited, and known electrode materials can be used. Examples of electrode materials include SrRuO ３ , LaNiO ３ , La ０．５ Sr ０．５ CoO ３ , La ０．７ Sr ０．３ MnO ３ The thickness of the lower electrode 2 is not particularly limited, but is preferably 1 nm or more and 500 nm or less, more preferably 3 nm or more and 100 nm or less, from the viewpoint of ease of handling.

[0019] The material for the upper electrode 4 is not particularly limited, as is the case with the lower electrode 2, and known electrode materials can be used. The materials for the lower electrode 2 and the upper electrode 4 may be the same or different. The lower electrode 2 and the upper electrode 4 may be formed by, for example, the above-mentioned vapor phase deposition method, as with the thin film. Alternatively, a known electrode pad may be attached to the thin film 3 as the upper electrode 4.

[0020] It is preferable to provide a buffer layer between the lower electrode 2 and the silicon substrate 1. The buffer layer may be composed of multiple layers, and for example, the buffer layer 5 may be made of a noble metal material such as Pt or Au, and the buffer layer 6 may be made of a metal oxide such as zirconium oxide or hafnium oxide.

[0021] 2(A) and 2(B) are cross-sectional schematic diagrams illustrating a method for writing and reading information in a magnetic memory element. The method for writing and reading information in a magnetic memory element includes a step of applying a voltage to the lower electrode 2 and the upper electrode 4 of the magnetic memory element 10 to reverse the magnetization of the thin film and write information, and a step of detecting the reversal of the magnetization of the thin film and reading the written information. According to this method, information is written to the magnetic memory element by applying an electric field, which allows for reduced power consumption compared to conventional magnetic memory elements that write information using a magnetic field generated by a current.

[0022] Specifically, Fig. 2A shows the magnetic memory element 10 before application of an electric field and the reading unit 20 disposed above the magnetic memory element. In Fig. 2A, the magnetization direction M of the thin film 3 before application of an electric field is downward as indicated by the black arrow.

[0023] Next, as shown in FIG. 2B , a voltage E is applied to the lower electrode 2 and upper electrode 4 of the magnetic memory element 10 in the direction indicated by the white arrows, thereby applying an electric field to the thin film 3. This reverses the magnetization direction of the thin film 3 to an upward direction as indicated by the black arrows, and information is written to the thin film 3. In FIGS. 2A and 2B , the magnetization direction M of the thin film is indicated by a black arrow for ease of understanding. However, in reality, the compound represented by formula (1) constituting the thin film 3 forms a plane of easy magnetization perpendicular to the eight electric polarizations oriented in the 111 direction. In this embodiment, the magnetization of the perpendicular component of the thin film 3 is reversed by applying a voltage to the lower electrode 2 and upper electrode 4.

[0024] The information written in the thin film 3 is read by the reading unit 20 by detecting the reversal of magnetization in the thin film 3. The reading unit 20 includes a sensor that is processed to a size equal to or smaller than the magnetic domain and can detect the reversal of magnetization. Examples of such sensors include a magnetoresistive element.

[0025] Examples of the present invention will be described below, but these examples are merely illustrative examples for suitably explaining the present invention and do not limit the present invention in any way.

[0026] BiFe １ － ｘ Co ｘ O ３ To fabricate the thin film, a silicon substrate with a bottom electrode (SrRuO) was used as the substrate, which is expected to stabilize the rhombohedral crystal structure. ３ (001) / Pt / ZrO ２ The substrate was ultrasonically cleaned with acetone, ethanol, and deionized water for 10 minutes each, and then subjected to pulsed laser deposition (PLD) at an oxygen partial pressure of 15 Pa, a substrate temperature of 660°C, and a fluence of 1.1 J / cm. ２ , laser repetition frequency 4Hz, laser pulse number 28800 conditions, BiFe ０．９ Co ０．１ O ３ A thin film (film thickness 60 nm) was prepared. The crystallinity was evaluated using X-ray diffraction (XRD) (SmartLab manufactured by Rigaku Corporation). ０．９ Co ０．１ O ３ The growth of a (001) thin film was confirmed (Fig. 3). Furthermore, the splitting pattern of the 003, 203, and 113 peaks in the reciprocal lattice map confirmed that the crystal structure was a monoclinic phase with spontaneous polarization in the

[111] direction (Figs. 4A to 4C). Observation and writing of the ferroelectric domains and observation of the ferromagnetic domains were carried out using a piezoelectric microscope (PFM) and a magnetic force microscope (MFM) (Cypher, manufactured by Oxford Instruments). Note that BiFe ０．９ Co ０．１ O ３ The Miller indices are written in pseudocubic notation.

[0027] The magnetic properties were evaluated using a superconducting quantum interference device (SQUID) magnetometer. Specifically, measurements were made using a magnetic measurement system (MPMS) manufactured by Quantum Design. The measurement range of the magnetization magnetic field dependence was ±5 T. The temperature dependence of the remanent magnetization was measured by magnetizing the sample at room temperature at 5 T, returning it to 0 T, and cooling the temperature of the device from 300 K to 10 K.

[0028] The detailed results are shown below. First, BiFe with a single-phase rhombohedral symmetry structure ０．９ Co ０．１ O ３ The formation of a thin film was confirmed by XRD 2θ-ω scan and φ scan of the 121 peak. ０．９ Co ０．１ O ３ The thin films were checked for ferroelectricity at room temperature. Next, writing was performed using PFM (Figure 5). It can be seen that clear ferroelectric domains were written. From the above, it was confirmed that all thin films were ferroelectric at room temperature.

[0029] We also measured the magnetic field dependence of magnetization (Fig. 6(a)) and the temperature dependence of remanent magnetization (Fig. 6(b)). From these results, we confirmed that not only ferroelectricity but also ferromagnetism was exhibited at room temperature. Furthermore, from the observation of the temperature dependence of remanent magnetization, the magnetization decreased due to the change to a cycloid structure at low temperatures, and this ferromagnetism was confirmed to be due to BiFe ０．９ Co ０．１ O ３ It was also confirmed that this is essential for thin films.

[0030] Next, to investigate whether there is a correlation between ferroelectricity and ferromagnetism, we attempted to observe the ferroelectric and ferromagnetic domains using PFM and MFM. ０．９ Co ０．１ O ３ Since the polarization of the thin film is oriented in eight 111 directions, it is necessary to map the out-of-plane and in-plane piezoelectric responses from two orthogonal directions. Therefore, the out-of-plane and in-plane polarizations of the film surface from two orthogonal directions were measured using PFM, and the three images obtained were combined into one image by image processing to create a total polarization mapping image. Figure 7(a) shows the polarization mapping of BiFe on a silicon substrate.０．９ Co ０．１ O ３ This is a PFM image of the thin film. Contrast on the islands can be seen. This domain structure is a (001)-oriented BiFeO ３ This has often been reported in thin films and is thought to be a 71° domain. Figure 7(b) shows an MFM image of the same region. It can be seen that a contrast similar to that in the PFM image is present. The contrast was confirmed to be reversed by reversing the magnetization direction of the magnetic cantilever, and these contrasts corresponded to magnetic domains. These results clearly demonstrate the existence of a correlation between the ferroelectric and ferromagnetic domains. Next, we applied an electric field using PFM to reverse the out-of-plane polarization, and then performed similar PFM and MFM measurements to verify whether magnetization reversal due to the application of the electric field occurred.

[0031] BiFe after -8V poling ０．９ Co ０．１ O ３ The PFM and MFM images of the thin film are shown in Figures 7(c) and 7(d), respectively. From Figures 7(c) and 7(d), it can be seen that the application of an electric field reversed only the polarization perpendicular to the plane of the ferroelectric domain while maintaining the domain shape, and consequently reversed the perpendicular magnetization. This confirmed that local magnetization reversal occurs when an electric field is applied. Furthermore, from the state shown in Figure 7(c), the BiFe film after +8V poling ０．９ Co ０．１ O ３ A PFM image of the thin film is shown in Figure 8. It was confirmed that the ferroelectric domains returned to the state before the -8V poling, and that repeated polarization reversal (writing) was possible.

[0032] Although the present invention has been described above with reference to the above-mentioned embodiments, the present invention is not limited to the above-mentioned embodiments, and suitable combinations and substitutions of the configurations of the embodiments are also included in the present invention. Furthermore, it is possible to suitably rearrange the combinations and order of steps in the embodiments based on the knowledge of a person skilled in the art, and to make modifications to the embodiments such as various design changes, and such modified embodiments are also included in the scope of the present invention.

[0033] [Additional remarks] The film, laminate, element, and magnetic memory device 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 infrastructure for industry, innovation and sustainability."

[0034] 10 magnetic memory element 1 silicon substrate 2 lower electrode 3 thin film 4 upper electrode 5, 6 buffer layer 20 reading section

Claims

1. A laminate comprising a silicon substrate and a thin film of a compound having a perovskite structure and represented by formula (1). BiFe １－ｘ A ｘ O ３ ... (1) (In formula (1), A is selected from the group consisting of Co and Mn, and x satisfies 0.05≦x≦0.25.) 2. The stack according to claim 1, wherein the surface of the silicon substrate is a (001) surface.

3. A magnetic memory element comprising the laminate according to claim 1 or 2.

4. A magnetic memory device comprising the magnetic memory element according to claim 3.

5. A thin film of a compound represented by formula (1) having a perovskite structure, the film thickness of which is 10 nm or more and less than 200 nm. BiFe １－ｘ A ｘ O ３ ... (1) (In formula (1), A is selected from the group consisting of Co and Mn, and x satisfies 0.05≦x≦0.25.)

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