A multi-layer thin film structure for realizing vertical exchange bias and full-current reversal without external field assistance, its preparation method and application

Abstract

The present invention discloses a multi-layer thin film structure for realizing vertical exchange bias and all-current switching without external field assistance, and its preparation method and application. The multi-layer thin film structure sequentially includes a heavy metal underlayer, an atomic-level interlayer, a ferromagnetic layer, and an antiferromagnetic layer from bottom to top, and the multi-layer thin film structure is prepared by molecular beam epitaxy. The present invention realizes a significant vertical exchange bias effect at room temperature by introducing anisotropic epitaxial strain in the antiferromagnetic layer. At the same time, the anisotropic epitaxial strain induces a global uncompensated magnetic moment in the antiferromagnetic layer, reduces the energy barrier for vertical exchange bias switching, and further realizes current-driven exchange bias switching without an external field through the spin-orbit torque effect, overcoming the dependence on an external magnetic field in the prior art and significantly improving the integrability and application potential of spintronic devices. This technology lays a foundation for the design and development of high-efficiency and high-density magnetic storage and logic devices.

Description

Technical Field

[0001] The present invention relates to the field of spintronic storage and logic devices, and particularly to a multi-layer thin film structure that realizes all-current switching of perpendicular exchange bias without external field assistance, and a preparation method and application thereof. Background Art

[0002] In recent years, the field of spintronics has developed rapidly, and the research focus has gradually shifted to the development of a new generation of high-performance storage and logic devices using antiferromagnetic materials. Antiferromagnetic materials have attracted much attention due to their unique properties, including zero net magnetic moment, high robustness to external perturbations, and spin dynamics on the terahertz time scale. There is an exchange bias effect in ferromagnetic / antiferromagnetic material heterojunctions, which is manifested as unidirectional exchange anisotropy, characterized by the shift of the hysteresis loop along the external magnetic field direction, and it has important application value in the field of spintronics, especially in high-capacity and low-power magnetic storage and logic devices. By regulating this effect, the detection and writing of information can be realized in spintronic devices. However, there are certain technical bottlenecks in the two main methods for regulating exchange bias at present: the field cooling method requires an external magnetic field to re-determine the pinning direction at high temperature, and this process is complex and difficult to integrate into actual devices; the exchange bias can also be flipped by current-induced, but in an exchange bias system with perpendicular magnetic anisotropy, an external magnetic field is still required for assistance, which limits its feasibility in practical applications. Therefore, it is of great significance to develop a structure that can realize the flipping of perpendicular exchange bias without the participation of an external magnetic field. Summary of the Invention

[0003] Based on this, the purpose of the present invention is to design a multi-layer thin film structure that realizes all-current switching of perpendicular exchange bias without external field assistance, and a preparation method and application thereof, so as to provide a feasible solution for realizing the flipping of perpendicular exchange bias without an external field at room temperature.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0005] A multi-layer thin film structure that realizes all-current switching of perpendicular exchange bias without external field assistance, comprising:

[0006] A heavy metal buffer layer; the material of the heavy metal buffer layer is a metal such as Pt, Pd, Ir or W.

[0007] An atomic-level interlayer, disposed on the heavy metal buffer layer, for generating spatial inversion symmetry breaking to realize field-free magnetization switching driven by spin-orbit torque; the material of the atomic-level interlayer is a metal such as Cr, Mn, Au or Cu.

[0008] A ferromagnetic layer, disposed on the atomic-level interlayer, having perpendicular magnetic anisotropy; the material of the ferromagnetic layer is Co, CoNi, CoPt or FePt, etc.

[0009] The antiferromagnetic layer is arranged on the ferromagnetic layer, used to form an antiferromagnetic / ferromagnetic interface and introduce a vertical exchange bias effect, and has anisotropic epitaxial stress; the material of the antiferromagnetic layer is IrMn3, IrMn, PtMn3 or Mn3Sn, and the thickness of the antiferromagnetic layer cannot be too large because the epitaxial stress is gradually released as the thickness increases, and the epitaxial stress is the key to the exchange bias flipping; and because the thickness is too small, the exchange bias effect cannot be formed at room temperature, because there is a thickness threshold for the formation of the exchange bias effect. In order to ensure that the exchange bias can be formed at room temperature (thick thickness is required), the epitaxial stress is maintained as much as possible (thin thickness is required) so that the exchange bias flips, and the thickness takes a balance between the two, so the thickness of the antiferromagnetic layer is preferably 1-3 nm.

[0010] In a further solution, in order to prevent the multi-layer thin film structure from being oxidized when exposed to air, the multi-layer thin film structure further includes a protective layer; the protective layer includes a MgO layer and a Cr layer.

[0011] The present invention also provides a method for preparing the above-mentioned multilayer film structure, comprising the following steps: using molecular beam epitaxy technology to sequentially deposit a heavy metal base layer, an atomic level intercalation layer, a ferromagnetic layer and an antiferromagnetic layer on a single crystal substrate, that is, to prepare a multilayer film structure, wherein the prepared multilayer film must have a flat interface and a good epitaxial structure to maintain anisotropic epitaxial stress in the antiferromagnetic layer. The single crystal substrate is required to have a lattice mismatch of less than 7% with the single crystal film, a close-packed crystal plane, a high surface flatness, and high temperature resistance (above 800°C). Preferably, the material of the single crystal substrate is Al2O3(0001), MgO(111), AlN(0001), SrTiO3, etc.

[0012] The multilayer film structure provided by the present invention has a stable exchange bias effect at room temperature and excellent all-electrical control characteristics, and is suitable for high-density magnetic storage and logic devices.

[0013] The present invention has the following beneficial effects:

[0014] In the multilayer film structure prepared by molecular beam epitaxy in the present invention, the thickness of the antiferromagnetic layer is 1 - 3 nm, belonging to an ultrathin antiferromagnetic layer. By introducing anisotropic epitaxial strain into the antiferromagnetic layer, the anisotropy energy is increased, thereby reducing the thickness threshold of the antiferromagnetic layer, and a significant perpendicular exchange bias effect at room temperature is achieved at a relatively thin thickness. At the same time, the anisotropic epitaxial strain induces a global uncompensated magnetic moment in the antiferromagnetic layer, reducing the energy barrier for the reversal of the perpendicular exchange bias, and further realizing the current-driven exchange bias reversal without an external magnetic field through the spin-orbit torque effect, overcoming the dependence on an external magnetic field in the prior art, and significantly enhancing the integrability and application potential of spintronic devices. This technology lays a foundation for the design and development of high-efficiency and high-density magnetic storage and logic devices.

[0015] The atomic-level intercalation in the multilayer film structure provided by the present invention has an atomic-level thickness. By introducing spatial inversion symmetry breaking through this intercalation, the magnetization reversal of the magnetic moment in the ferromagnetic layer of the multilayer film structure is realized by spin-orbit torque driving without an externally applied auxiliary field. In the multilayer film structure of the present invention, without an externally applied auxiliary magnetic field, the exchange bias field can be reversed only by applying a pulsed current. Description of the Drawings

[0016] Figure 1 Schematic diagram of a multilayer film structure without atomic-level intercalation;

[0017] Figure 2 is Figure 1 the high-angle annular dark-field image and energy spectrum diagrams of different components of the multilayer film structure in

[0018] Figure 3 Schematic diagram of a multilayer film structure containing atomic-level intercalation;

[0019] Figure 4 is the room-temperature perpendicular exchange bias effect of the device;

[0020] Figure 5 is the reflection high-energy electron diffraction pattern and line scan curve of each layer in the device;

[0021] Figure 6 is the X-ray diffraction curve of the IrMn3 layer in the device;

[0022] Figure 7 is the local high-resolution transmission electron microscope image of the device;

[0023] Figure 8 is the schematic diagram of the anisotropic epitaxial strain in the epitaxial IrMn3 layer;

[0024] Figure 9 is the global uncompensated magnetic moment in the epitaxial IrMn3 layer;

[0025] Figure 10 Flow chart of the operation for current-induced perpendicular exchange bias reversal

[0026] Figure 11 Perpendicular magnetic anisotropy of a device with Cr interlayer

[0027] Figure 12 Spin-orbit torque-driven field-free magnetization reversal behavior of a device with Cr interlayer

[0028] Figure 13 Flow chart of the operation for full current-induced perpendicular exchange bias reversal

[0029] Figure 14 Stability detection and statistics of the perpendicular exchange bias of the device

[0030] Reference numerals: 1 - heavy metal underlayer, 2 - atomic-level interlayer, 3 - ferromagnetic layer, 4 - antiferromagnetic layer Detailed implementation manners

[0031] The present invention will be further described below in conjunction with embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the exemplified embodiments are not intended to limit the present invention

[0032] In addition, in the preparation processes of the following embodiments, unless otherwise specified, they are all conventional means in the prior art in this field, and therefore, will not be described in detail; the raw materials used in the following implementation manners are all commercially available products and can be obtained by purchase

[0033] In order to verify whether the epitaxial stress in the multilayer film plays a role, the present invention first prepared a multilayer film structure without atomic-level interlayer, and its structural schematic diagram is as Figure 1 shown, which successively includes a heavy metal underlayer 1, a ferromagnetic layer 3, and an antiferromagnetic layer 4 from bottom to top. The materials and thicknesses of each layer are Pt (3 nm) / Co (1 nm) / IrMn3(2 nm) respectively. The preparation method of this multilayer film structure includes the following steps: using molecular beam epitaxy (MBE) to deposit a heavy metal underlayer (Pt layer), a ferromagnetic layer (Co layer), and an antiferromagnetic layer (IrMn3 layer) successively on a single crystal substrate Al2O3(0001), that is, the multilayer film structure is prepared. The prepared multilayer film must have a flat interface and a good epitaxial structure to maintain the epitaxial stress of the anisotropy in the antiferromagnetic layer

[0034] Figure 2 are the high-angle annular dark field image and energy spectrum diagrams of different components of this multilayer film structure. Among them, Figure A is the high-angle annular dark field image (HAADF) of the multilayer film structure, Figure B is the energy spectrum diagram of Pt in the heavy metal underlayer, Figure C is the energy spectrum diagram of Co in the ferromagnetic layer, Figure D is the energy spectrum diagram of Mn in the antiferromagnetic layer, and Figure E is the energy spectrum diagram of Ir in the antiferromagnetic layer. FromFigure 2 It can be seen that the interface between the multi-layers is flat and the epitaxial quality is good, and the epitaxial stress in the IrMn layer can be effectively maintained.

[0035] As an embodiment of the present invention, the schematic structural diagram of the multi-layer film is as Figure 3 shown. Compared with the multi-layer film structure in Figure 1 , the multi-layer film structure in this embodiment has one more atomic-level interlayer 2. The material of the atomic-level interlayer 2 is Cr, and the thickness is 0.2 nm. Other structures are the same as those in Figure 1 . The multi-layer film structure prepared in this embodiment includes a heavy metal underlayer 1, an atomic-level interlayer 2, a ferromagnetic layer 3, and an antiferromagnetic layer 4 from bottom to top in sequence. The materials and thicknesses of each layer are Pt (3 nm) / Cr(0.2 nm) / Co (1 nm) / IrMn3(2 nm). The preparation method of the multi-layer thin film structure includes the following steps: using molecular beam epitaxy (MBE) to sequentially deposit a heavy metal underlayer, an atomic-level interlayer, a ferromagnetic layer, and an antiferromagnetic layer on a single-crystal substrate Al2O3(0001), that is, a multi-layer thin film structure is prepared. The prepared multi-layer film must have a flat interface and a good epitaxial structure to maintain the anisotropic epitaxial stress in the antiferromagnetic layer.

[0036] Performance testing and analysis

[0037] Device preparation: Through ultraviolet exposure and argon ion etching technology, the multi-layer film structures in the above Figure 1 and Figure 3 are respectively etched into Hall bar devices, which are respectively denoted as device 1 and device 2. Device 1 and device 2 have current arms (feature size 15 μm) and voltage arms (feature size 4 μm) for pulsed current to pass through and detect the reversal of perpendicular exchange bias through the anomalous Hall effect.

[0038] Verify the room-temperature perpendicular exchange bias effect of device 1: Verify the perpendicular exchange bias effect of the device at room temperature through the anomalous Hall curve and the hysteresis loop, as Figure 4 shown. Figure 4 The figure in the lower right corner of Figure 4 is a schematic diagram of the Hall bar device configuration of device 1. The device in the figure includes a pair of current arms and two pairs of voltage arms. During the test, current I is passed through the current arms, and the Hall voltage V is measured at the voltage arms, and then the Hall resistance of the device can be obtained. It can be seen that on the premise that the thickness of the IrMn3 layer is only 2 nm, this structure has good perpendicular magnetic anisotropy and exhibits an observable exchange bias effect at room temperature.

[0039] Verify the anisotropic epitaxial stress of the IrMn3 layer in device 1: Perform intensity line scanning through the reflection high-energy electron diffraction pattern, Figure 5are the reflection high energy electron diffraction patterns and line scan curves of each layer. According to the distances between the first-order diffraction beams and the reflected beams of the Pt layer and the IrMn3 layer (393 pixels and 397 pixels respectively), it can be deduced that the IrMn3 layer has an average in-plane tensile strain of 2.7%, as Figure 5 shown; Figure 6 is the X-ray diffraction pattern of the multilayer film. The peak position of the 2-nm-thick IrMn3 layer is 40.68°, as Figure 6 shown in the local enlarged view of the IrMn3 peak position in the upper left corner. According to its standard peak position (41.36°), it can be deduced that this layer has an average tensile strain of 1.35% in the direction normal to the film plane, as Figure 6 shown; Through the atomic images taken by high-resolution transmission electron microscopy, it can be roughly measured that the local IrMn3 layer has a tensile strain of 4.8% in the in-plane direction and 2.3% in the direction normal to the film plane, as Figure 7 shown. The ultra-thin IrMn3 layer has anisotropic epitaxial strain in the in-plane direction and the direction normal to the film plane, as Figure 8 shown.

[0040] Verify the uncompensated magnetic moment of the IrMn3 layer in Device 1: Detect the magnetic-depth curve of the device by polarized neutron reflection technology, as Figure 9 shown. There is an uncompensated magnetic moment throughout the 2-nm IrMn3 layer in the device. Figure 9 The MgO and Cr layers in x are protective layers to protect the underlying core structure from being exposed to air oxidation. CrO

[0041] is the result of the Cr layer being oxidized by air. Since these two protective layers are not core functional layers but only used to protect the core structure, they were not mentioned earlier. Figure 10 Verify the feasibility of current-induced perpendicular exchange bias flipping in the structure of Device 1 with the assistance of an external magnetic field, as

[0042] a. Use the anomalous Hall curve to test the initial exchange bias field of the thin film H PEB = -454 Oe, as Figure 10 shown in Figure G of Figure 10 . At this time, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interfacial IrMn3 layer is downward, as

[0043] b. As Figure 10 shown in Figure A of Figure 10 , apply an external magnetic field of -5000 Oe along the normal direction of Device 1 for 5 s and then remove it. At this time, the magnetic moment of the Co layer is downward, and the uncompensated magnetic moment of the interfacial IrMn3 layer is downward, as

[0044] c. Pass a pulsed current along the device current arm direction, with an amplitude of 2.7×10 7 A / cm 2 , a pulse width of 80 μs. At this time, the magnetic moment of the Co layer is downward, and the uncompensated magnetic moment of the interfacial IrMn3 layer flips upward, as shown in Diagram D of Figure 10 ; meanwhile, the perpendicular exchange bias field of the thin film flips H PEB = +558 Oe, as shown in Diagram J of Figure 10 ;

[0045] d. Apply an external magnetic field of +5000 Oe along the normal direction of the thin film for 5 s and then remove it. At this time, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interfacial IrMn3 layer is upward, as shown in Diagram E of Figure 10 ;

[0046] e. Pass a pulsed current along the device current arm direction, with an amplitude of 2.7×10 7 A / cm 2 , a pulse width of 80 μs. At this time, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interfacial IrMn3 layer flips downward, as shown in Diagram F of Figure 10 , and meanwhile, the perpendicular exchange bias field of the thin film flips H PEB = -437 Oe, as shown in Diagram K of Figure 10 .

[0047] Through the above tests, it is verified that the epitaxial stress in the multilayer film structure plays a role. With the assistance of an external magnetic field, the exchange bias can be flipped by current.

[0048] Based on Device 1, an atomic-level Cr interlayer with a thickness of 0.2 nm is introduced to optimize the multilayer film structure, namely Device 2 mentioned above. The perpendicular magnetic anisotropy of Device 2 is verified by the anomalous Hall curve, as shown in Figure 11 . Figure 11 In Cr , t Figure 11 = 0.2 nm indicates that Device 2 has an atomic-level Cr interlayer with a thickness of 0.2 nm. It can be seen from

[0049] Verify the field-free magnetization reversal induced by the spin-orbit moment of device 2: By modifying the interface with a 0.2 nm thick Cr intercalation layer, the orbital hybridization between the 3d orbital and the 5d orbital between the Pt layer and the Co layer is adjusted without affecting the upper single crystal epitaxial structure, thereby regulating the perpendicular magnetic anisotropy of the Co layer. At the same time, by utilizing the exchange interaction between Co and IrMn, a slight in-plane exchange bias component can be introduced into the IrMn3 layer, breaking the spatial inversion symmetry and realizing the field-free magnetization reversal induced by the spin-orbit moment (the external auxiliary magnetic field is 0 Oe), as shown in Figure 2. Figure 12 shown.

[0050] Without the assistance of an external magnetic field, the full current regulation of the vertical exchange bias of device 2 is verified. The operation process is as follows: Figure 13 As shown:

[0051] (1) If Figure 13 As shown in Figure A, a pulse current is introduced along the current arm direction of device 2 to initialize the vertical exchange bias of the film, with a magnitude of +3.0×10 7 A / cm 2 , the pulse width is 80 μs, and the anomalous Hall curve is used to verify that the initial exchange bias field of the film is negative, such as Figure 13 As shown in Figure B, at this time, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interface IrMn3 layer is downward;

[0052] (2) A pulse current of -3.0×10 7 A / cm 2 , the pulse width is 80 μs, at this time, the magnetic moment of the Co layer is downward, the uncompensated magnetic moment of the interface IrMn3 layer is flipped to upward, and at the same time, the vertical exchange bias field of the film is flipped to positive, such as Figure 13 As shown in Figure C;

[0053] (3) A pulse current of +3.0×10 7 A / cm 2 , the pulse width is 80 μs, at this time, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interface IrMn3 layer is flipped to downward. At the same time, the vertical exchange bias field of the film is flipped to negative, such as Figure 13 As shown in Figure D.

[0054] The multilayer film structure prepared by the present invention still has a stable and considerable vertical exchange bias effect at room temperature under the premise that the thickness of the antiferromagnetic layer IrMn3 is only 2 nm. Figure 14For the stability detection and statistics of the perpendicular exchange bias of device 2, where Figure A shows the perpendicular exchange bias fields of 10 devices randomly extracted from five groups of repeatedly prepared multilayer film samples (Sample 1, Sample 2, Sample 3, Sample 4, Sample 5) with the same structure, and Figure B shows the statistical histogram of the exchange bias fields ( H PEB ) of a total of 50 devices. It can be seen from Figure 14 that in the five groups of repeatedly prepared multilayer film samples with the same structure, a perpendicular exchange bias field of about 400 Oe at room temperature can be stably achieved, meeting the requirements of large-scale integrated circuits.

[0055] The flipping process of the perpendicular exchange bias of the multilayer film structure provided by the present invention is fully current-controlled, without the need for an auxiliary magnetic field, improving the feasibility of application, and is suitable for new magnetic storage and logic devices; moreover, the preparation process is mature and can be extended to various magnetic multilayer film systems according to requirements.

[0056] It should be noted that in other embodiments, when the experimental process meets the following conditions, the purpose of the present invention can be achieved:

[0057] For the material of the antiferromagnetic layer, in addition to the above-mentioned IrMn3, IrMn, PtMn3, Mn3Sn, etc. can also be selected; for the thickness of the antiferromagnetic layer, it is preferably 1 - 3 nm, and specifically can also be 1 nm, 1.5 nm, 3 nm, etc.

[0058] For the material of the ferromagnetic layer, CoNi, CoPt, FePt, etc. can also be selected; for the thickness of the ferromagnetic layer, it depends on different material requirements, as long as it has good perpendicular magnetic anisotropy at the corresponding thickness;

[0059] For the material of atomic-level intercalation, Mn, Au, Cu can also be selected; for the thickness of atomic-level intercalation, it is preferably 0.5 to 3 angstroms;

[0060] For the material of the heavy metal underlayer, Pd, Ir, W, etc. can also be selected; for the thickness of the heavy metal underlayer, it is preferably 2 to 6 nm.

[0061] For the above processes, those skilled in the art can make appropriate selections according to actual needs, and they can all achieve the purpose of the present invention.

[0062] Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

Claims

1. A multi-layer thin film structure for realizing vertical exchange bias and full-current switching without external field assistance, characterized in that, Comprising: Heavy metal bottom layer; The material of the heavy metal bottom layer is Pt; Atomic-level intercalation layer, disposed on the heavy metal bottom layer, for generating spatial inversion symmetry breaking to achieve spin-orbit torque-driven field-free magnetization reversal; the material of the atomic-level intercalation layer is Au or Cu; the thickness of the atomic-level intercalation layer is 0.1 - 0.3 nm; Ferromagnetic layer, disposed on the atomic-level intercalation layer, having perpendicular magnetic anisotropy; the material of the ferromagnetic layer is Co; Antiferromagnetic layer, disposed on the ferromagnetic layer, for forming an antiferromagnetic / ferromagnetic interface and introducing a perpendicular exchange bias effect, and having anisotropic epitaxial stress; the material of the antiferromagnetic layer is IrMn3.

2. The multi-layer thin film structure for realizing vertical exchange bias and all-current switching without external field assistance according to claim 1, characterized in that: The thickness of the antiferromagnetic layer is 1 - 3 nm.

3. The multi-layer thin film structure for realizing vertical exchange bias and full-current switching without external field assistance according to claim 1 or 2, characterized in that: The multi-layer thin film structure further includes a protective layer; the protective layer includes a MgO layer and a Cr layer.

4. A method for preparing a multi-layer thin film structure, characterized in that: The multi-layer thin film structure is the multi-layer thin film structure according to claim 1 or 2, and the preparation method of the multi-layer thin film structure includes the following steps: sequentially depositing a heavy metal bottom layer, an atomic-level intercalation layer, a ferromagnetic layer, and an antiferromagnetic layer on a single crystal substrate using molecular beam epitaxy technology to obtain the multi-layer thin film structure.

5. The manufacturing method of the multi-layer thin film structure according to claim 4, characterized in that, The material of the single crystal substrate is Al2O3, MgO, AlN or SrTiO3.

6. Use of a multi-layer thin film structure in the preparation of a magnetic memory or logic device, characterized in that: The multi-layer thin film structure is the multi-layer thin film structure according to claim 1 or 2.

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