Current-induced magnetization reversal device

Abstract

This allows for even greater spin-orbit torque to be generated in SrRuO3. [Solution] This current-induced magnetization reversal element comprises a substrate 101 and a thin film 102 made of SrRuO3 crystals epitaxially grown on the substrate 101. The substrate 101 is made of a crystal whose lattice constant differs by 0.5% or more from the lattice constant of the pseudo-cubic crystal of SrRuO3, and whose main surface on which the thin film 102 is formed is a cubic (001) or pseudo-cubic (001) plane. In the thin film 102, the oxygen octahedron is rotated with respect to the crystal axis of the pseudo-cubic crystal of SrRuO3.

Description

[Technical Field]

[0001] The present invention relates to a current-induced magnetization reversal element. [Background technology]

[0002] SrRuO3 has a perovskite structure and a ferromagnetic transition temperature (T C ) is a ferromagnetic metal with a maximum temperature of 169 K. Due to its high chemical stability, high electrical conductivity, and high compatibility with perovskite oxides such as SrTiO3, it is a very promising material for applications in oxide-based electronics, such as magnetoresistive random access memory (MRAM) and MOSFETs (Non-Patent Documents 1 and 2). In particular, it is important for the realization of highly efficient devices that the magnetization can be controlled (reversed) using spin-orbit torque generated by applying a current rather than a magnetic field (Non-Patent Document 3).

[0003] Conventionally, current-induced spin-orbit torque magnetization reversal has been achieved using a double-layer element in which a heavy metal is bonded to a ferromagnetic material. This technology has the drawbacks of high film deposition and material costs, thermal stability, and the need for a 1000-kJ / semiconductor to induce magnetization reversal. 7 Acm -2 The problem was that a large current density was required (Non-Patent Document 4).

[0004] On the other hand, if current-induced magnetization reversal could be achieved in a ferromagnetic "monolayer," it would be advantageous for device applications from the various viewpoints mentioned above. In this case, the ferromagnetic material itself must be able to generate a large spin-orbit torque.

[0005] Massless Weyl fermions with linear band dispersion in SrRuO3 have high mobility and are expected to be applied to transistors that operate at high speed and low power consumption (Non-Patent Document 5). They are also promising for highly sensitive magnetic field sensors that utilize the giant positive magnetoresistance effect and chiral anomalous magnetoresistance, which is a large negative magnetoresistance effect. Furthermore, when spatial inversion symmetry is broken, gaps are formed at the intersections of linear bands, leading to strong orbital hybridization and the generation of spin-orbit torque due to the large Berry curvature (Non-Patent Document 6). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] HY Hwang et al., "Emergent phenomena at oxide interfaces", Nature Materials, vol. 11, pp. 103-113, 2012. [Non-patent document 2] G. Koster et al., "Structure, physical properties, and applications of SrRuO3 thin films", Reviews of Modern Physics, vol. 84, no. 1, pp. 253-298, 2012. [Non-patent document 3] IM Miron et al., "Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection", Nature, vol. 476, pp. 189-193, 2011. [Non-patent document 4] L. Liu et al., "Current-Induced Switching of Perpendicularly Magnetized Magnetic Layers Using Spin Torque from the Spin Hall Effect", Physical Review Letters, vol. 109, 096602, 2012. [Non-Patent Document 5] K. Takiguchi et al., "Quantum transport evidence of Weyl fermionsin an epitaxial ferromagnetic oxide", Nature Communications, vol. 11, 4969, 2020. [Non-patent document 6] Y. Chen et al., "Weyl fermions and the anomalous Hall effect in metallic ferromagnets", Physical Review B, vol. 88, no. 12, 125110, 2013. Summary of the Invention [Problem to be solved by the invention]

[0007] There have been no successful examples of current-induced magnetization reversal due to spin-orbit torque in a single-layer film of SrRuO3 ferromagnetic material. To achieve magnetization reversal, it is necessary to break the spatial reversal symmetry, but it is difficult to introduce spatial asymmetry into SrRuO3, whose crystal is spatially symmetric. How to achieve a large spin-orbit torque in SrRuO3 is a major challenge.

[0008] The present invention has been made to solve the above problems, and aims to make it possible to generate a larger spin-orbit torque in SrRuO3. [Means for solving the problem]

[0009] The current-induced magnetization reversal element according to the present invention comprises a substrate made of a crystal whose main surface is a cubic (001) or pseudo-cubic (001) plane and whose lattice constant differs from that of a pseudo-cubic crystal of SrRuO3 by 0.5% or more, and a thin film made of SrRuO3 crystal epitaxially grown on the substrate. [Effects of the Invention]

[0010] As described above, according to the present invention, SrRuO3 crystals are epitaxially grown on a substrate made of a crystal whose main surface is a cubic (001) or pseudo-cubic (001) plane and whose lattice constant differs from that of a pseudo-cubic crystal of SrRuO3 by 0.5% or more, thereby enabling the generation of even greater spin-orbit torque in SrRuO3. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of a current-induced magnetization reversal element according to an embodiment of the present invention. [Figure 2] FIG. 2 is a perspective view showing the crystal structure of SrRuO3. [Figure 3] FIG. 3 is a plan view showing the configuration of a current-induced magnetization reversal element according to an embodiment of the present invention. [Figure 4A] FIG. 4A is a photograph showing an annular bright-field scanning transmission electron microscope (ABF-STEM) image of the interface between the substrate 101 and the thin film 102. [Figure 4B] FIG. 4B is an explanatory diagram illustrating the bond angle θ of the Ru—O—Ru atoms. [Figure 4C] FIG. 4C is a characteristic diagram showing the relationship between the rotation angle of the oxygen octahedron and the number of layers of the pseudocubic unit cell of SrRuO3 from the SrRuO3 / SrTiO3 interface. [Figure 5] FIG. 5 is a characteristic diagram showing the temperature dependence of the resistivity between the first electrode 103 and the second electrode 104 provided on the first portion 102a of the thin film 102. As shown in FIG. [Figure 6] FIG. 6 is a characteristic diagram showing the dependence of the Hall resistance of the thin film 102 on an external magnetic field applied perpendicular to the surface at 90K. [Figure 7] FIG. 7 is a characteristic diagram showing the dependency of the Hall resistance of the thin film 102 on the in-plane pulse current at 90 K when an in-plane magnetic field Hx in the <0010> direction of the thin film 102 is applied. [Figure 8] FIG. 8 is a sequence diagram showing a sequence for passing a current between the first electrode 103 and the second electrode 104. As shown in FIG. [Figure 9] FIG. 9 is a characteristic diagram showing the in-plane pulse current dependence of the Hall resistance of the thin film 102 at 90 K when various values ​​of in-plane magnetic field Hx are applied in the <0010> direction of the thin film 102. [Figure 10] FIG. 10 is a characteristic diagram showing the in-plane pulse current dependence of the Hall resistance of the thin film 102 when an in-plane magnetic field Hx is applied in the <0010> direction of the thin film 102 at various measurement temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0012] A current-induced magnetization reversal element according to an embodiment of the present invention will now be described with reference to Figure 1. This current-induced magnetization reversal element includes a substrate 101 and a thin film 102 made of SrRuO3 crystals epitaxially grown on the substrate 101.

[0013] The substrate 101 is made of a crystal whose lattice constant differs from that of the pseudo cubic crystal of SrRuO3 by 0.5% or more, and whose main surface on which the thin film 102 is formed is a cubic (001) or pseudo cubic (001) plane. The substrate 101 is made of, for example, SrTiO3, MgO, (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 It can be composed of any of the following crystals: O3, KTaO3, DyScO3, TbScO3, GdScO3, SmScO3, NdScO3, PrScO3, or Si.

[0014] In the thin film 102 thus formed, the oxygen octahedrons are rotated relative to the pseudocubic crystal axes of SrRuO3. The displacement of oxygen atoms due to the rotation of the oxygen octahedrons introduces a breaking of the spatial inversion symmetry of SrRuO3. This generates a spin-orbit torque sufficient to reverse the magnetization in the thin film 102.

[0015] Figure 2 shows the crystal structure of SrRuO3. Because SrRuO3 has a perovskite structure, it has the advantage of being easily formed in a heterojunction state on an oxide substrate. According to the embodiment, epitaxial distortion from the substrate 101 causes rotation of oxygen octahedra within the perovskite structure of SrRuO3 in the thin film 102. According to the embodiment, there is no need to go through the hassle of changing the growth conditions for each layer of the pseudocubic unit cell of SrRuO3 in order to break the spatial inversion symmetry in the thin film 102, making film formation extremely easy.

[0016] 3, the thin film 102 includes a first portion 102a and a second portion 102b. The first portion 102a extends in the

[0010] direction of the thin film 102, and the second portion 102b extends in the [-100] direction of the thin film 102. The first portion 102a and the second portion 102b intersect at right angles at their respective centers. The first portion 102a and the second portion 102b are formed in a cross shape when viewed from the normal direction to the plane of the substrate 101.

[0017] In addition, a first electrode 103 is connected to one end of the first portion 102a, a second electrode 104 is connected to the other end of the first portion 102a, a third electrode 105 is connected to one end of the second portion 102b, and a fourth electrode 106 is connected to the other end of the second portion 102b.

[0018] When a current is applied between the first electrode 103 and the second electrode 104 under an external magnetic field in a plane parallel to the current direction, magnetization reversal occurs according to the current direction. This magnetization state can be measured (detected) by the Hall resistance between the third electrode 105 and the fourth electrode 106.

[0019] For example, the thin film 102 can be epitaxially grown on a substrate 101 made of SrTiO3 by molecular beam epitaxy. In a growth chamber of a molecular beam epitaxy apparatus evacuated to an ultra-high vacuum, the temperature of the substrate 101 is set to 780°C, and the thin film 102 is heated for 10 minutes. -6 Atomic beams of alkaline earth metal Sr and 4d transition metal Ru are supplied in a predetermined composition ratio in an active oxygen atmosphere of about Torr to form a thin film 102 made of SrRuO. Note that the epitaxial growth of the thin film 102 is not limited to molecular beam epitaxy, and other methods such as sputtering and pulsed laser ablation can also be used.

[0020] The 26-nm-thick thin film 102 formed as described above can be etched by ion milling using a mask pattern formed by known photolithography techniques to form the crossbar structure of the first portion 102a and the second portion 102b described with reference to Figure 3. The first electrode 103, the second electrode 104, the third electrode 105, and the fourth electrode 106 can be made of Ag. These electrodes can be made of conductive materials such as Au, Cr, or Nb-doped SrTiO3.

[0021] Figure 4A shows an annular bright-field scanning transmission electron microscope (ABF-STEM) image of the interface (SrRuO3 / SrTiO3 interface) between the substrate 101 and thin film 102 formed as described above. It can be seen that the thin film 102 made of SrRuO3 is a single-crystal thin film with a perovskite structure, and that the single crystal has grown epitaxially from the SrTiO3 substrate 101. It can also be seen that the in-plane displacement of oxygen (O), which constitutes the perovskite structure, due to epitaxial strain from the SrTiO3 substrate is significantly large only near the SrRuO3 / SrTiO3 interface.

[0022] The bond angle θ of the Ru-O-Ru atoms along the perpendicular direction shown in Figure 4B can be estimated from Figure 4A, and the rotation angle α of the oxygen octahedron can be calculated from this θ (Reference 1). In Figure 4B, the x-axis is the

[0010] direction, the y-axis is the [-100] direction, and the z-axis is the

[0001] direction.

[0023] As shown in Figure 4C, the large α near the SrRuO3 / SrTiO3 interface is evidence of large oxygen atom displacements at the SrRuO3 / SrTiO3 interface. This oxygen atom displacement is introduced due to the lattice constant of the SrTiO3 substrate being approximately 0.5% smaller than the pseudocubic lattice constant of bulk SrRuO3, 0.393 nm. Similarly, the MgO, (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 Even when using substrates such as )O3, KTaO3, DyScO3, TbScO3, GdScO3, SmScO3, NdScO3, PrScO3, and Si, which have a lattice constant difference of 0.5% or more from the pseudocubic lattice constant of bulk SrRuO3, which is 0.393 nm, displacement of oxygen atoms is introduced, making current-induced magnetization reversal, as described below, possible.

[0024] Figure 5 shows the temperature dependence of the resistivity between the first electrode 103 and the second electrode 104. A bend in the electrical resistance, indicating the transition from paramagnetism to ferromagnetism, is observed at 150 K, and the Curie temperature is 150 K. In Figure 5, the black triangles indicate the ferromagnetic transition temperature.

[0025] Figure 6 shows the dependence of the Hall resistance of the thin film 102 made of SrRuO3 on an external magnetic field applied perpendicular to the surface at 90 K. In Figure 6, near (a), the direction of magnetization M is from the thin film 102 side to the substrate 101 side (M<0). Also, near (b), the direction of magnetization M is from the substrate 101 side to the thin film 102 side (M>0).

[0026] Here, a current is passed between the first electrode 103 and the second electrode 104. The sign of the current is defined as positive when it flows from the first electrode 103 to the second electrode 104. The Hall resistance was measured between the third electrode 105 and the fourth electrode 106. The easy axis of magnetization of the thin film 102 on the substrate 101 is perpendicular to the surface (the

[0001] direction of the thin film 102). Therefore, when the external magnetic field is 0 T, the Hall resistance takes two saturated values ​​corresponding to the positive magnetization (M>0) and negative magnetization (M<0) in the

[0001] direction.

[0027] FIG. 7 shows the in-plane magnetic field H x The figure shows the in-plane pulse current dependence of the Hall resistance of the thin film 102 at 90 K when H x Before applying the current, an external magnetic field of -1 T is applied in the <0001> direction of the thin film 102 to initialize the magnetization of the thin film 102 to the negative direction (M<0). x The current is applied after the magnetic field in the

[0001] direction is set to 0 T.

[0028] After passing an in-plane pulse current J with a pulse width of 0.1 ms between the first electrode 103 and the second electrode 104, another in-plane pulse current with a pulse width of 0.1 s and 1 mA is passed between the first electrode 103 and the second electrode 104, and the Hall resistance is measured during the latter. Figure 8 shows the sequence of passing the current. The open rectangle indicates the pulse current J, the black pulse indicates the current when reading the Hall resistance, and I max indicates the maximum value of the current within the sequence. When the value of J is changed in the order 1 → 2 → 3 in Figure 8, the Hall resistance changes significantly at 1, and through 2 and 3, it traces a loop with hysteresis similar to that in Figure 6, crossing 0 Ω. This shows that the state of magnetization M>0 and M<0 changes depending on the direction of the current J, and that current-induced magnetization reversal is realized.

[0029] FIG. 9 shows various values ​​of the in-plane magnetic field H x7 shows the in-plane current dependence of the Hall resistance of the thin film 102 at 90 K when H is applied. The measurement range corresponds to 2 → 3 in FIG. x When mT = 0, no hysteresis loop is observed. On the other hand, the loop of the Hall resistance that depends on the direction of the applied current J is x >0 and H x <0, showing the opposite polarity (rotation), indicating that a typical current-induced magnetization reversal due to spin-orbit torque has been achieved (Reference 2).

[0030] FIG. 10 shows the in-plane magnetic field H in the

[0010] direction of the thin film 102 at various measurement temperatures. x 7 shows the in-plane current dependence of the Hall resistance of the thin film 102 when a current density J is applied. The measurement range corresponds to 2 → 3 in FIG. 7. As the temperature increases, the magnetic anisotropy of the thin film 102 weakens, and the current density J required for magnetization reversal decreases. c At 120K, J c ~4.0×10 6 Acm -2 The value obtained is an order of magnitude smaller than that of conventional double-layer systems using heavy metals. On the other hand, near the Curie temperature, the magnetization almost disappears, and the loop described above is not observed.

[0031] As described above, according to the embodiment of the present invention, SrRuO3 crystals are epitaxially grown on a substrate made of a crystal whose main surface is a cubic (001) or pseudo-cubic (001) plane and whose lattice constant differs from that of a pseudo-cubic crystal of SrRuO3 by 0.5% or more, thereby enabling the generation of even greater spin-orbit torque in SrRuO3.

[0032] According to the above-described embodiment, in-plane current-induced spin-orbit torque magnetization reversal can be achieved in a ferromagnetic monolayer. Furthermore, magnetization reversal can be achieved at a current density one order of magnitude lower than that of conventional heavy-metal bilayer systems. This enables applications to devices that utilize the structural changes that occur specifically in Weyl semimetals and perovskite oxides with strong spin-orbit interactions, with low fabrication costs and high operating efficiency. Because SrRuO3 can be epitaxially grown as a single crystal on an oxide substrate, spin-orbit torque devices such as magnetoresistive random access memories (MARAMs) that are highly compatible with oxide electronics can be realized.

[0033] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0034] [Reference 1] AT Zayak et al., "Structural, electronic, and magnetic properties of SrRuO3 under epitaxial strain", Physical Review B, vol. 74, no. 9, 094104, 2006. [Reference 2] M. Jiang et al., "Efficient full spin-orbit torque switching in a single layer of a perpendicularly magnetized single-crystalline ferromagnet", Nature Communications, vol. 10, 2590, 1039. [Explanation of symbols]

[0035] 101...substrate, 102...thin film.

Claims

1. SrRuO 3 a substrate made of a crystal having a lattice constant difference of 0.5% or more with respect to the lattice constant of a pseudo cubic crystal of the above-mentioned crystal, and a main surface of which is a cubic (001) or pseudo cubic (001) plane; SrRuO epitaxially grown on the substrate 3 A thin film made of crystals of A current-induced magnetization reversal element comprising:

2. 2. The current-induced magnetization reversal element according to claim 1, the thin film includes a first portion extending in a [010] direction of the thin film and a second portion extending in a [−100] direction of the thin film and perpendicular to the first portion; a first electrode connected to one end of the first portion; a second electrode connected to the other end of the first portion; a third electrode connected to one end of the second portion; a fourth electrode connected to the other end of the second portion; The current-induced magnetization reversal element further comprises:

3. 3. The current-induced magnetization reversal element according to claim 1, The substrate is SrTiO 3 , MgO, (La 0.3 Sr 0.7 ) (Al 0.65 Ta 0.35 ) O 3 , KTaO 3 , DyScO 3 , TbScO 3 , GdScO 3 , SmScO 3 , NdScO 3 , PrScO 3 A current-induced magnetization reversal element made of either silicon or silicon crystal.

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