Multi-layer thin film structure capable of realizing vertical exchange bias and free of external field auxiliary total current overturning and preparation method and application of multi-layer thin film structure

Abstract

The invention discloses a multi-layer thin film structure capable of achieving vertical exchange bias and free of external field auxiliary total current overturning and a preparation method and application thereof, the multi-layer thin film structure sequentially comprises a heavy metal base layer, an atomic level intercalation layer, a ferromagnetic layer and an antiferromagnetic layer from bottom to top, and the multi-layer thin film structure is prepared through molecular beam epitaxy. According to the invention, anisotropic epitaxial strain is introduced into the antiferromagnetic layer, so that a remarkable vertical exchange bias effect at room temperature is realized. Meanwhile, a global uncompensated magnetic moment is generated in the anisotropic epitaxial strain induction antiferromagnetic layer, an energy barrier of vertical exchange bias flipping is reduced, current-driven exchange bias flipping without an external field is further realized through a spin-orbit moment effect, the dependence on an external magnetic field in the prior art is overcome, and the performance of the device is improved. And the integration and the application potential of the spinning electronic device are obviously improved. The technology lays a foundation for design and development of efficient and high-density magnetic storage and logic devices.

Description

Technical Field

[0001] The present invention relates to the field of spin electronics storage and logic devices, and in particular to a multilayer film structure for realizing vertical exchange bias without external field assisted full current reversal, and a preparation method and application thereof. Background Art

[0002] In recent years, the field of spin electronics 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 manifests as a unidirectional exchange anisotropy characterized by the shift of the hysteresis loop along the direction of the external magnetic field. It has important application value in the field of spin electronics, especially in high-capacity and low-power magnetic storage and logic devices. By regulating this effect, information detection and writing can be achieved in spin electronics devices. However, the two main methods for regulating exchange bias currently have certain technical bottlenecks: the field cooling method requires an external magnetic field at high temperature to re-determine the pinning direction, which is a complex process and difficult to integrate into actual devices; the exchange bias can also be flipped by current induction, but in the exchange bias system with perpendicular magnetic anisotropy, external magnetic field assistance is still required, which limits its feasibility in practical applications. Therefore, it is of great significance to develop a structure that can achieve vertical exchange bias reversal without the involvement of an external magnetic field. Summary of the invention

[0003] Based on this, the purpose of the present invention is to design a multilayer thin film structure for realizing full current reversal of vertical exchange bias without external field assistance and its preparation method and application, so as to provide a feasible solution for realizing vertical exchange bias reversal without external field at room temperature.

[0004] To achieve the above object, the technical solution adopted by the present invention is: A multilayer thin film structure for realizing vertical exchange bias without external field assisted full current reversal, comprising: Heavy metal primer; the material of the heavy metal primer is metal such as Pt, Pd, Ir or W; Atomic-level intercalation, placed on a heavy metal base layer, is used to produce spatial inversion symmetry breaking and achieve field-free magnetization reversal driven by spin-orbit moment; the material of atomic-level intercalation is metal such as Cr, Mn, Au or Cu; The ferromagnetic layer is provided on the atomic-level intercalation layer and has perpendicular magnetic anisotropy; the material of the ferromagnetic layer is Co, CoNi, CoPt or FePt, etc.; 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.

[0005] 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.

[0006] 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, preparing 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.

[0007] 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.

[0008] The present invention has the following beneficial effects: The thickness of the antiferromagnetic layer in the multilayer film structure prepared by molecular beam epitaxy in the present invention is 1-3 nm, which is an ultra-thin antiferromagnetic layer. The present invention introduces anisotropic epitaxial strain in the antiferromagnetic layer to increase its anisotropic energy, thereby reducing the antiferromagnetic layer thickness threshold, and achieves a significant vertical exchange bias effect at room temperature at a relatively thin thickness. At the same time, the anisotropic epitaxial strain induces the generation of global uncompensated magnetic moments in the antiferromagnetic layer, reducing the energy barrier of the vertical exchange bias reversal, and further realizes the current-driven exchange bias reversal without an external field through the spin-orbit moment effect, overcoming the dependence of the existing technology on the external magnetic field, and significantly improving the integrability and application potential of spin electronic devices. This technology lays a foundation for the design and development of efficient and high-density magnetic storage and logic devices.

[0009] The atomic-level intercalation in the multilayer film structure provided by the present invention is of atomic-level thickness, and the spatial inversion symmetry breaking is introduced by the intercalation, so as to promote the magnetic moment of the ferromagnetic layer in the multilayer film structure to achieve magnetization reversal driven by the spin-orbit moment without an external auxiliary field. The multilayer film structure of the present invention can reverse its exchange bias field by applying a pulse current without an external auxiliary magnetic field. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Schematic diagram of the multilayer film structure without atomic-level intercalation; Figure 2 for Figure 1 High-angle annular dark field image of the multilayer film structure and energy spectra of different components; Figure 3 Schematic diagram of a multilayer film structure containing atomic-level intercalation; Figure 4 is the room temperature vertical exchange bias effect of the device; Figure 5 It is the reflected high-energy electron diffraction spectrum and line scan curve of each layer in the device; Figure 6 is the X-ray diffraction curve of the IrMn3 layer in the device; Figure 7 This is a local high-resolution transmission electron microscope image of the device; Figure 8 Schematic diagram of anisotropic epitaxial strain in epitaxial IrMn3 layer; Fig. 9 is the global uncompensated magnetic moment in the epitaxial IrMn3 layer; Fig.10 The operation flow chart of current-induced vertical exchange bias flipping; Fig.11 The perpendicular magnetic anisotropy of the Cr intercalation device; Fig.12 Field-free magnetization reversal behavior driven by spin-orbit moment of Cr intercalation device; Fig.13 The operation flow chart of full current induced vertical exchange bias flipping; Fig.14 To detect and count the stability of the vertical exchange bias of the device; Figure numerals: 1 - heavy metal base layer, 2 - atomic level intercalation layer, 3 - ferromagnetic layer, 4 - anti-ferromagnetic layer. DETAILED DESCRIPTION

[0011] The present invention is further described below in conjunction with the embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0012] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional means in the prior art in the art, and therefore, they are not described in detail; the raw materials used in the following embodiments are all commercially available products and can be purchased commercially.

[0013] 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 intercalation, and its structural schematic diagram is shown in FIG. Figure 1 As shown, from bottom to top, it includes a heavy metal base layer 1, a ferromagnetic layer 3 and an antiferromagnetic layer 4, and the materials and thicknesses of each layer are Pt (3 nm) / Co (1 nm) / IrMn3(2 nm) respectively. The preparation method of the multilayer thin film structure includes the following steps: using molecular beam epitaxy (MBE) to sequentially deposit a heavy metal base layer (Pt layer), a ferromagnetic layer (Co layer) and an antiferromagnetic layer (IrMn3 layer) on a single crystal substrate Al2O3(0001), that is, to prepare a multilayer thin film structure, and the prepared multilayer film must have a flat interface and a good epitaxial structure to maintain the anisotropic epitaxial stress in the antiferromagnetic layer.

[0014] Figure 2 Figure 1 is a high-angle annular dark field image of the multilayer film structure and the energy spectra of different components. Figure A is a high-angle annular dark field image (HAADF) of the multilayer film structure, Figure B is the energy spectrum of Pt in the heavy metal base layer, Figure C is the energy spectrum of Co in the ferromagnetic layer, Figure D is the energy spectrum of Mn in the antiferromagnetic layer, and Figure E is the energy spectrum of Ir in the antiferromagnetic layer. Figure 2 It can be seen that the interface between the multilayer films is smooth, the epitaxial quality is good, and the epitaxial stress in the IrMn layer can be effectively maintained.

[0015] As an embodiment of the present invention, the structural schematic diagram of the multilayer film is as follows Figure 3 As shown, Figure 1 Compared with the multi-layer film structure in the embodiment, the multi-layer film structure in this embodiment has an additional atomic level intercalation layer 2, the material of the atomic level intercalation layer 2 is Cr, and the thickness is 0.2 nm. The other structures are the same as Figure 1The same. The multilayer film structure prepared in this embodiment includes a heavy metal base layer 1, an atomic level intercalation layer 2, a ferromagnetic layer 3 and an antiferromagnetic layer 4 from bottom to top, and the material and thickness of each layer are Pt (3 nm) / Cr (0.2 nm) / Co (1 nm) / IrMn3 (2 nm). The preparation method of the multilayer film structure includes the following steps: using molecular beam epitaxy (MBE) 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 Al2O3 (0001), that is, to prepare a multilayer film structure, and the prepared multilayer film must have a flat interface and a good epitaxial structure to maintain the anisotropic epitaxial stress in the antiferromagnetic layer.

[0016] Performance testing and analysis Device preparation: Through ultraviolet exposure and argon ion etching technology, the above Figure 1 and Figure 3 The multilayer film structure in is etched into Hall bar devices, respectively denoted as device 1 and device 2. Device 1 and device 2 have current arms (characteristic size 15 μm) and voltage arms (characteristic size 4 μm) to allow pulse current to pass and detect the reversal of vertical exchange bias through the anomalous Hall effect.

[0017] Verify the vertical exchange bias effect of device 1 at room temperature: Verify the vertical exchange bias effect of the device at room temperature through the anomalous Hall curve and hysteresis loop, such as Figure 4 shown. Figure 4 The figure in the lower right corner 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, a current I is passed through the current arm, and the Hall voltage V is measured in the voltage arm, and the Hall resistance of the device can be obtained. Figure 4 It can be seen that under the premise that the thickness of the IrMn3 layer is only 2nm, the structure has good perpendicular magnetic anisotropy and exhibits considerable exchange bias effect at room temperature.

[0018] Verify the anisotropic epitaxial stress of the IrMn3 layer in device 1: Intensity line scan by reflection high-energy electron diffraction pattern, Figure 5 are the reflected high-energy electron diffraction patterns and line scan curves of each layer. According to the distances between the first-order diffraction beam and the reflected beam of the Pt layer and the IrMn3 layer (393 pixels and 397 pixels, respectively), it can be inferred that the IrMn3 layer has an average in-plane tensile strain of 2.7%, as shown in Figure 5 As shown; Figure 6 is the X-ray diffraction pattern of the multilayer film, in which the peak position of the IrMn3 layer with a thickness of 2 nm is 40.68°, as shown in Figure 6As shown in the local enlarged view of the IrMn3 peak in the upper left corner, based on its standard spectrum peak position (41.36°), it can be inferred that the layer has an average tensile strain of 1.35% in the normal direction of the film, as shown in Figure 6 As shown; through the atomic image 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 inner direction and 2.3% in the normal direction of the film, as shown Figure 7 The ultra-thin IrMn3 layer has anisotropic epitaxial strain in the in-plane direction and normal direction, as shown in Figure 8 shown.

[0019] Verify the global uncompensated magnetic moment of the IrMn3 layer in device 1: The magnetic-depth curve of the device is detected by polarized neutron reflection technology, such as Fig. 9 As shown, there is an uncompensated magnetic moment throughout the 2 nm IrMn3 layer in the device. Fig. 9 The MgO and Cr layers in the CrO layer are protective layers to protect the core structure below from being exposed to air and oxidized. x This is the result of the Cr layer being oxidized by air. Because these two protective layers are not core functional layers, they are only used to protect the core structure, so they are not mentioned above.

[0020] With the assistance of an external magnetic field, the feasibility of current-induced vertical exchange bias reversal in the device 1 structure was verified, as shown in Fig.10 As shown: a. Using the anomalous Hall curve to examine the initial exchange bias field of the film H PEB =-454 Oe, as Fig.10 As shown in Figure G, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interface IrMn3 layer is downward. Fig.10 As shown in Figure B; b. If Fig.10 As shown in Figure A, an external magnetic field of -5000 Oe is applied along the normal direction of device 1 for 5 seconds and then removed. At this time, the magnetic moment of the Co layer is downward, and the uncompensated magnetic moment of the interface IrMn3 layer is downward, as shown in Figure 1. Fig.10 As shown in Figure C; c. A pulse current with an amplitude of 2.7×10 7 A / cm 2 , the pulse width is 80 μs, at this time, the magnetic moment of the Co layer is downward, and the uncompensated magnetic moment of the interface IrMn3 layer is flipped upward, such as Fig.10 As shown in Figure D; at the same time, the vertical exchange bias field of the film is reversed H PEB =+558 Oe, as Fig.10 As shown in Figure J; d. Apply an external magnetic field of +5000 Oe along the normal direction of the film and remove it after 5s. At this time, the magnetic moment of the Co layer is upward, and the uncompensated magnetic moment of the interface IrMn3 layer is upward, such as Fig.10 As shown in Figure E; e. A pulse current with an amplitude of 2.7×10 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, such as Fig.10 As shown in Figure F, at the same time, the vertical exchange bias field of the film is reversed. H PEB =-437 Oe, as Fig.10 As shown in Figure K.

[0021] The above tests verified that the epitaxial stress in the multilayer film structure is effective, and with the assistance of an external magnetic field, the exchange bias can be reversed by current.

[0022] On the basis of device 1, an atomic-level Cr intercalation layer with a thickness of 0.2 nm was introduced to optimize the multilayer film structure, namely the device 2 mentioned above. The perpendicular magnetic anisotropy of device 2 was verified by the anomalous Hall curve, as shown in Fig.11 shown. Fig.11 Medium Cr =0.2 nm means that device 2 has an atomic-level Cr intercalation layer with a thickness of 0.2 nm. Fig.11 It can be seen that after adding the Cr intercalation layer, the device still has good perpendicular magnetic anisotropy and maintains a considerable perpendicular exchange bias effect at room temperature.

[0023] 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. Fig.12 shown.

[0024] 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: Fig.13 As shown: (1) If Fig.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 / cm2 , 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 Fig.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; (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 Fig.13 As shown in Figure C; (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 Fig.13 As shown in Figure D.

[0025] 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. Fig.14 Figure 2 is the stability test and statistics of the vertical exchange bias of device 2. Figure A shows the vertical exchange bias field of 10 devices randomly extracted from five groups of multilayer film samples with the same structure (sample 1, sample 2, sample 3, sample 4, sample 5) prepared repeatedly, and Figure B shows the exchange bias field of a total of 50 devices ( H PEB ) size. Fig.14 It can be seen that in five groups of repeatedly prepared multilayer film structure samples with the same structure, a vertical exchange bias field of about 400 Oe can be stably achieved at room temperature, which is suitable for the needs of large-scale integrated circuits.

[0026] The flipping process of the vertical exchange bias of the multilayer film structure provided by the present invention is fully current controlled and does not require an auxiliary magnetic field, thereby improving the feasibility of application and being suitable for new magnetic storage and logic devices. In addition, the preparation process is mature and can be expanded to a variety of magnetic multilayer film systems according to demand.

[0027] 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: 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 can specifically be 1nm, 1.5nm, 3nm, etc.

[0028] For the material of the ferromagnetic layer, CoNi, CoPt, FePt, etc. can also be selected; for the thickness of the ferromagnetic layer, different requirements are required for different materials, provided that the material has good perpendicular magnetic anisotropy at the corresponding thickness; For the atomic level intercalation material, Mn, Au, and Cu can also be selected; for the atomic level intercalation thickness, 0.5 to 3 angstroms are preferred; As for the material of the heavy metal primer layer, Pd, Ir, W, etc. can also be selected; as for the thickness of the heavy metal primer layer, it is preferably 2 to 6 nanometers.

[0029] For the above processes, those skilled in the art can make appropriate selections according to actual needs, and all of them can achieve the purpose of the present invention.

[0030] Obviously, the described embodiments are only some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

Claims

1. A multilayer thin film structure for realizing vertical exchange bias without external field assisted full current reversal, characterized in that: include: Heavy metal base; Atomic-level intercalation, placed on a heavy metal substrate, is used to produce spatial inversion symmetry breaking and achieve field-free magnetization reversal driven by spin-orbit moment; The ferromagnetic layer, which is placed on the atomic-level intercalation layer, has perpendicular magnetic anisotropy; The antiferromagnetic layer is arranged on the ferromagnetic layer, is used to form an antiferromagnetic / ferromagnetic interface and introduce a vertical exchange bias effect, and has anisotropic epitaxial stress.

2. The multilayer thin film structure for realizing vertical exchange bias without external field assisted full current switching according to claim 1, characterized in that: The material of the heavy metal primer layer is Pt, Pd, Ir or W.

3. The multilayer thin film structure for realizing vertical exchange bias without external field assisted full current switching according to claim 1, characterized in that: The atomic level intercalation material is Cr, Mn, Au or Cu.

4. The multilayer thin film structure for realizing vertical exchange bias without external field assisted full current switching according to claim 3, characterized in that: The thickness of the atomic level intercalation layer is 0.1-0.3 nm.

5. The multilayer thin film structure for realizing vertical exchange bias without external field assisted full current switching according to claim 1, characterized in that: The material of the ferromagnetic layer is Co, CoNi, CoPt or FePt.

6. The multilayer thin film structure for realizing vertical exchange bias without external field assisted full current switching according to claim 1, characterized in that: The material of the antiferromagnetic layer is IrMn3, IrMn, PtMn3 or Mn3Sn; the thickness of the antiferromagnetic layer is 1-3 nm.

7. The multilayer thin film structure for realizing vertical exchange bias without external field assisted full current switching according to any one of claims 1 to 6, 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.

8. A method for preparing a multilayer thin film structure, characterized in that: The multilayer thin film structure is a multilayer thin film structure as described in any one of claims 1 to 6, and the preparation method of the multilayer thin film structure comprises 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 to obtain a multilayer thin film structure.

9. The method for preparing a multilayer thin film structure according to claim 8, characterized in that: The material of the single crystal substrate is Al2O3, MgO, AlN or SrTiO3.

10. Application of a multilayer thin film structure in the preparation of a magnetic memory or logic device, characterized in that: The multilayer film structure is the multilayer film structure according to any one of claims 1 to 6.

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