Spin torque oscillator

Abstract

To provide a spin-torque oscillation element capable of oscillation in a sub-GHz band under no magnetic field and achieving a high oscillation output than before.SOLUTION: There is provided a spin-torque oscillation element 10 which is capable of oscillation in a sub-GHz band and includes: a first electrode 11 and a second electrode 12; and a stacked body including a magnetization fixed layer 13, a tunnel barrier layer 15 comprising MgO, a magnetization free layer 14 and a spin injection layer 18 which are stacked between the first electrode and the second electrode. The spin injection layer is a vertical magnetization film with magnetization vertical to a film surface. The magnetization free layer comprises a FeB layer. The spin-torque oscillation element is capable of obtaining an oscillation output by applying a predetermined DC voltage when there is no magnetic field to be applied to the film surface in a vertical direction.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a spin torque oscillator having a thin film laminated structure. [Background technology]

[0002] Mimicking neuronal and synaptic activity in animal brains Artificial neural networks, which originated from this, are being actively researched in order to realize artificial intelligence. In recent years, progress has been made in developing computing units that can operate artificial neural networks at low power consumption and high speed by replacing some of the functions of neurons and synapses with artificial devices. Among these, computing units that use nonlinear oscillators as artificial neurons have attracted particular attention as they can become high-speed computing units through parallel operation.

[0003] As with animal brains, this computing unit is capable of more complex calculations as the number of neurons increases, so using a small oscillator is desirable for miniaturizing the computing unit. In addition, since the electrical output of the oscillator is observed to confirm the calculation results, if the oscillator output is small, amplification is required, so in order to miniaturize the computing unit and save power, it is desirable for the oscillator output to be large. Furthermore, since the calculation speed of the computing unit is proportional to the oscillation period of the oscillator, an oscillator with a high oscillation frequency may seem suitable at first glance, but the manufacturing costs of equipment that transmits, controls, and observes high frequencies increase as the frequency increases. In addition, the power consumption of such equipment, including the oscillator, also increases. For these reasons, oscillators with an oscillation frequency in the sub-gigahertz band are suitable for computing unit applications.

[0004] A spin-torque oscillator (STO) is an oscillator that can be miniaturized to sub-micrometer size. In particular, a magnetic vortex STO with a self-oscillating magnetization free layer that has a magnetic vortex structure has an oscillation frequency of sub-gigahertz, making it suitable as an artificial neuron for computing devices.

[0005] The oscillation frequency and oscillation output of STO can be controlled by the magnetic field and voltage applied to the element. In conventional STO, a high magnetic field of 100 mT or more must be applied to the element to excite oscillation, and a large electromagnet power supply or permanent magnet is required to stably apply this magnetic field to the element. Therefore, in order to put STO into practical use, such as by incorporating it into computing devices, there was a demand for an STO that would oscillate stably in the absence of a magnetic field.

[0006] STO that oscillates in the absence of a magnetic field has been proposed (see, for example, Patent Document 1 or Non-Patent Document 1). Patent Document 1 discloses that a high-frequency signal in the millimeter wave band can be obtained at about 100 mT or in the absence of a magnetic field. Patent Document 1 also describes that STO is a technology premised on achieving oscillation in the millimeter wave band and at extremely high frequencies, and that it is difficult to reduce the frequency to sub-gigahertz.

[0007] Non-Patent Document 1 discloses an STO that can excite microwaves with an oscillation frequency of 180 MHz and an output of 0.6 μW in the absence of a magnetic field in an element that has a magnetic vortex film in the magnetization free layer (oscillating layer), an in-plane magnetized film in the fixed layer, and a magnetic tunnel junction using a perpendicular spin injection layer microfabricated into a pillar shape with a diameter of 330 nm. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 5321851 Specification [Non-patent literature]

[0009] [Non-Patent Document 1] A. Dussaux et. al. “Large spin amplitude torque vortex oscillations at zero external field using a perpendicular spin polarizer”, Applied Physics Letters 105, 022404 (2014) Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a spin torque oscillator that is capable of oscillating in the sub-gigahertz range in the absence of a magnetic field and has improved oscillation output compared to conventional spin torque oscillators. [Means for solving the problem]

[0011] According to one aspect of the present invention, there is provided a spin torque oscillator capable of oscillating in the sub-gigahertz band, comprising a first electrode and a second electrode, and a laminate including a magnetization fixed layer between the first electrode and the second electrode, a tunnel barrier layer made of MgO, a magnetization free layer, and a spin injection layer, wherein the spin injection layer is a perpendicular magnetization film whose magnetization is perpendicular to the film surface, and the magnetization free layer is made of an FeB layer, and the spin torque oscillator is capable of oscillating when a predetermined DC voltage is applied in the absence of a magnetic field applied perpendicular to the film surface.

[0012] According to the above aspect, a spin torque oscillator can be provided in which a spin injection layer of a perpendicular magnetization film that supplies a spin polarized current with a component perpendicular to the film surface is laminated on a magnetization free layer of a magnetic vortex structure, the spin polarized current causes self-oscillation of the magnetic vortex, and the magnetoresistance effect is improved by making the magnetization free layer an FeB layer, thereby improving the oscillation output in the absence of an applied external magnetic field. Note that in this specification and claims, the term "sub-gigahertz band" refers to a band of 10 MHz to 1000 MHz. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic cross-sectional view of a spin torque oscillator according to an embodiment of the present invention. [Diagram 2] FIG. 13 is a diagram showing the relationship between the oscillation frequency and the oscillation spectrum with respect to the strength of an external magnetic field in the spin torque oscillator of the example. [Diagram 3]FIG. 13 is a graph showing the relationship between the external magnetic field strength and the oscillation output of the spin torque oscillator of the example. [Figure 4] FIG. 11 is another diagram showing the relationship between the external magnetic field strength and the oscillation output of the spin torque oscillator of the example. [Diagram 5] FIG. 13 is a diagram showing the relationship between the input DC voltage and the oscillation output when there is no external magnetic field in the spin torque oscillator of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that elements common to multiple drawings are given the same reference numerals, and detailed descriptions of those elements will be omitted.

[0015] Fig. 1 is a schematic diagram of a spin torque oscillator according to an embodiment of the present invention. Referring to Fig. 1, the spin torque oscillator 10 is a laminated body including a magnetization fixed layer 13, a magnetization free layer 14, a tunnel barrier layer 15 between them, and a nonmagnetic layer 16 and a spin injection layer 18 on the magnetization free layer 14, between a lower electrode 11 and an upper electrode 12. The spin torque oscillator 10 is formed in a cylindrical shape with a diameter of 200 nm to 1500 nm. This diameter is preferably 250 nm to 600 nm. In addition to being cylindrical, the cross section perpendicular to the stacking direction of the spin torque oscillator 10 may be elliptical, square, or rectangular.

[0016] The lower electrode 11 and the upper electrode 12 are made of a metal such as Cu (copper), Au (gold), or aluminum (Al).

[0017] The magnetization fixed layer 13 has a ferromagnetic layer with a fixed magnetization direction, or a ferromagnetic layer / intermediate layer / ferromagnetic layer with a synthetic ferrimagnetic (SAF) structure. In the SAF structure, two ferromagnetic layers are antiferromagnetically coupled, and the magnetizations of each layer are antiparallel. The intermediate layer is a thin conductive layer. The magnetization directions of the magnetization fixed layer 13 are fixed in approximately the same direction. The ferromagnetic material of the magnetization fixed layer 13 can be CoFe, CoFeB, CoPt laminated film, etc.

[0018] A buffer layer (not shown) may be formed between the lower electrode 11 and the magnetization fixed layer 13. The buffer layer is an antiferromagnetic layer, such as IrMn.

[0019] The tunnel barrier layer 15 is formed on the magnetization fixed layer 13 and is made of an insulating material, MgO. The thickness of the tunnel barrier layer 15 is preferably set to 0.9 nm to 1.2 nm in order to obtain a high magnetoresistance ratio and a high output.

[0020] The magnetization free layer 14 is made of a ferromagnetic FeB layer. The composition of the FeB layer is preferably selected from the range of 10 at (atomic) % to 30 at %, in that the saturation magnetic flux density is higher than that of NiFe. By using an FeB layer for the magnetization free layer 14, the magnetoresistance ratio of the spin torque oscillator 10 can be made 80% or higher. Since the oscillation output of the spin torque oscillator 10 is proportional to the square of the magnetoresistance ratio, the oscillation output can be improved by improving the magnetoresistance ratio. The thickness of the magnetization free layer 14 is preferably set to 3 nm to 10 nm, in that a magnetic vortex structure is stably realized and a high oscillation output is obtained.

[0021] The magnetization free layer 14 is formed on the MgO layer of the tunnel barrier layer 15 having the above thickness, and by forming it within the above thickness range, the magnetization free layer 14 has a magnetization structure (magnetic vortex) consisting of magnetization oriented in a spiral shape around the center of the magnetization free layer 14. Such a spin torque oscillator is called a magnetic vortex type spin torque oscillator. When a direct current is applied to the spin torque oscillator 10, the center of the magnetic vortex in the magnetization free layer 14 rotates (precesses). When this rotation occurs, the electrical resistance changes due to the magnetoresistance effect, and an AC voltage with a frequency in the sub-gigahertz band is generated.

[0022] The non-magnetic layer 16 is formed on the magnetization free layer 14. The non-magnetic layer 16 functions as an underlayer for (111) crystal orientation of the spin injection layer 18 formed thereon. The non-magnetic layer 16 weakens the magnetic coupling between the magnetization free layer 14 and the spin injection layer 18, and achieves both the magnetization free layer 14 with a magnetic vortex structure and the perpendicularly magnetized spin injection layer 18. The non-magnetic layer 16 is made of a Cu layer. The Cu layer is expected to grow with a (111) crystal orientation on the FeB layer of the magnetization free layer 14. The Cu layer is preferable in that it is less affected by the crystal orientation when the FeB layer that is the underlayer has an amorphous structure, and the crystal orientation of the Cu layer itself is easily formed. Note that a thin film with a face-centered cubic lattice (fcc) structure with a thickness of 0.2 nm to 1.0 nm, for example a Co layer or NiFe layer, may be inserted between the magnetization free layer 14 and the non-magnetic layer 16 to promote the (111) crystal orientation of the non-magnetic layer 16.

[0023] The spin injection layer 18 is formed on the non-magnetic layer 16. By injecting a sufficient amount of spin polarized current with a component perpendicular to the film surface from the spin injection layer 18 into the magnetization free layer 14, the differential motion of the magnetic vortex in the magnetization free layer 14 is excited. The spin injection layer 18 is a perpendicular magnetization film with magnetization perpendicular to the film surface, and is made of, for example, a Co / Ni multilayer film. The Co / Ni multilayer film is formed by alternately stacking Co layers and Ni layers to form a (111) crystal orientation in the stacking direction. The thicknesses of each Co layer and Ni layer are 0.2 nm and 0.5 nm, respectively. The number of repetitions of stacking the Co layers and Ni layers is preferably 4 to 20 times in terms of obtaining perpendicular magnetic anisotropy. The spin injection layer 18 may be a Co / Pt multilayer film or a TbFeCo layer in addition to the Co / Ni multilayer film.

[0024] The spin injection layer 18 functions as a spin injection layer for self-oscillating the magnetic vortex of the magnetization free layer 14. In order to realize a state in which the magnetic vortex of the magnetization free layer 14 self-oscillates, it is necessary to apply a spin-polarized current with a component perpendicular to the film surface to the magnetization free layer 14 with a magnetic vortex structure, and to cancel the damping of the magnetization free layer 14 by the spin torque. The spin-polarized current with a perpendicular component can be obtained by passing a current through the spin injection layer 18 of the perpendicular magnetization film.

[0025] A cap layer (not shown) made of a conductive material may be formed on the spin injection layer 18. The cap layer is, for example, a Ta layer / Ru layer.

[0026] The spin torque oscillator 10 is formed by depositing the above-mentioned layers by, for example, sputtering, and processing them into a cylindrical shape by photolithography and electron beam lithography.

[0027] According to this embodiment, a spin injection layer 18 made of a perpendicular magnetization film that supplies a spin-polarized current with a component perpendicular to the film surface is laminated on the magnetization free layer 14 having a magnetic vortex structure, and the magnetic vortex is self-oscillated by the spin-polarized current. In addition, by making the magnetization free layer 14 an FeB layer, the magnetoresistance effect is improved, and a spin torque oscillator 10 with improved oscillation output in the absence of an applied external magnetic field can be provided.

[0028] [Example] As an example of the spin torque oscillator 10, a lower electrode 11: a 5 nm thick Ta layer / 30 nm thick Cu layer / 10 nm thick Ta layer, a buffer layer: a 5 nm thick IrMn layer, a magnetization fixed layer 13: a SAF structure having a 2.5 nm thick CoFe layer / 0.9 nm thick Ru layer / 2.5 nm thick CoFeB layer, a tunnel barrier layer 15: a 1.1 nm thick MgO layer, a magnetization free layer, and Layer 14: 6 nm thick FeB layer / 0.54 nm thick Co layer, non-magnetic layer 16: 5 nm thick Cu layer, spin injection layer 18: 0.2 nm thick Co layer / 0.5 nm thick Ni layer alternately stacked 7 times, cap layer: 7 nm thick Ru layer, upper electrode 12: 5 nm thick Cr layer / 200 nm thick Au layer, and these were processed into a cylindrical shape with a diameter of 400 nm by photolithography and electron beam lithography.

[0029] While applying a DC external magnetic field perpendicular to the film surface to the spin torque oscillator of the embodiment, a DC voltage was applied between the lower electrode and the upper electrode to pass a DC current, and the AC voltage signal generated by the spin torque oscillator was measured. The external magnetic field was changed in the range of -100 mT (millitesla) to 100 mT.

[0030] Fig. 2 shows the relationship between the oscillation frequency and the oscillation spectrum of the spin torque oscillator of the example versus the strength of the external magnetic field. Fig. 2 shows that the spin torque oscillator oscillates when there is no external magnetic field (zero external magnetic field strength). It also shows that oscillation occurs when the external magnetic field strength is in the range of -100 mT to 60 mT. It also shows that the frequency at which the power spectrum density is highest (corresponding to the part with the highest density in the figure) varies depending on the external magnetic field strength, but is between 260 MHz and 280 MHz.

[0031] 3 shows the relationship between the external magnetic field strength and the oscillation output of the spin torque oscillator of the example. The DC voltage applied between the lower electrode and the upper electrode of the spin torque oscillator was set to 270 mV. The oscillation output was calculated by integrating the oscillation spectrum obtained by a spectrum analyzer.

[0032] As shown in Figure 3, the spin torque oscillator achieved an oscillation output of 1.19 μW when there was no external magnetic field. Furthermore, when the external magnetic field strength was in the range of -100 mT to 70 mT, the oscillation output was 0.85 μW to 1.37 μW. This oscillation output far exceeds the 0.6 μW in Non-Patent Document 1, the prior art document mentioned above, and a spin torque oscillator with improved oscillation output in the absence of a magnetic field was achieved.

[0033] 4 is another diagram showing the relationship between the external magnetic field strength and the oscillation output of the spin torque oscillator of the example. The DC voltage applied between the lower electrode and the upper electrode of the spin torque oscillator was set to 240 mV. The oscillation output was determined in the same manner as the oscillation output shown in FIG.

[0034] Referring to Figure 4, it can be seen that the spin torque oscillator oscillated when the applied voltage was 240 mV and the external magnetic field strength was in the range of -100 mT to 90 mT. When there was no external magnetic field, the spin torque oscillator achieved an oscillation output of 0.93 μW. When the external magnetic field strength was in the range of -100 mT to 70 mT, the oscillation output was 0.68 μW to 1.07 μW. This oscillation output far exceeded the 0.6 μW in Non-Patent Document 1, the prior art document mentioned above, and a spin torque oscillator with improved oscillation output in the absence of a magnetic field was achieved.

[0035] 5 shows the relationship between the applied voltage and the oscillation output of the spin torque oscillator of the example when there is no external magnetic field. In FIG. 5, a DC voltage was applied between the lower and upper electrodes of the spin torque oscillator of the example when no external magnetic field was applied to the spin torque oscillator of the example, and the oscillation spectrum was measured. The oscillation output was calculated in the same manner as the oscillation output shown in FIG.

[0036] 5, the spin torque oscillator did not oscillate under an external magnetic field when the applied voltage was 210 mV or less or 320 mV or more, but oscillated in the range of 220 mV to 310 mV. Within the range of applied voltages where oscillation occurred, a minimum oscillation output of 0.68 mW (applied voltage 230 mV) and a maximum oscillation output of 1.67 μW (applied voltage 310 mV) were obtained.

[0037] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the present invention described in the claims. [Explanation of symbols]

[0038] 10 Spin torque oscillator 11 Lower electrode 12 Upper electrode 13 Magnetization fixed layer 14 Magnetization free layer 15 Tunnel Barrier Layer 16 Nonmagnetic layer 18 Spin injection layer

Claims

1. A spin torque oscillator capable of oscillating in the sub-gigahertz band, a first electrode and a second electrode, and a laminate in which a magnetization fixed layer, a tunnel barrier layer made of MgO, a magnetization free layer, and a spin injection layer are laminated between the first electrode and the second electrode; the spin injection layer is a perpendicular magnetization film having magnetization perpendicular to a film surface, and is a multi-layered film of a Co layer and a Ni layer; a Cu layer is formed as an underlayer of the spin injection layer, the magnetization free layer is made of an FeB layer having a magnetization structure with a spiral magnetization direction, a Co layer is formed between the FeB layer and the Cu layer, The spin torque oscillator is capable of oscillating when a specific DC voltage is applied in the absence of a magnetic field applied perpendicular to the film surface of the spin torque oscillator.

2. The spin torque oscillator according to claim 1 , wherein the free magnetization layer is an FeB layer and has a thickness of 3 nm to 10 nm.

3. The spin torque oscillator according to claim 1, wherein the magnetization fixed layer contains CoFeB.

4. A spin torque oscillator according to any one of claims 1 to 3, wherein an oscillation output of 0.68 μW or more can be obtained by applying the specified DC voltage in the absence of a magnetic field applied perpendicular to the film surface of the spin torque oscillator.

5. A spin torque oscillator as described in any one of claims 1 to 4, wherein the spin torque oscillator can obtain an oscillation output of up to 1.67 μW by applying the specified DC voltage in the absence of a magnetic field applied perpendicular to the film surface.

6. The spin torque oscillator according to any one of claims 1 to 5, having an oscillation output of 0.68 μW or more when a magnetic field applied perpendicular to the film surface is in the range of -100 mT to 90 mT.

7. A spin torque oscillator capable of oscillating in the sub-gigahertz band, comprising: a first electrode and a second electrode, and a laminate in which a magnetization fixed layer, a tunnel barrier layer made of MgO, a magnetization free layer, and a spin injection layer are laminated between the first electrode and the second electrode; the spin injection layer is a perpendicular magnetization film having magnetization perpendicular to the film surface, the magnetization free layer is made of an FeB layer having a magnetization structure with a spiral magnetization direction, The spin torque oscillator is capable of oscillating when a predetermined DC voltage is applied in the absence of a magnetic field applied perpendicular to its film surface, and has an oscillation output of 0.68 μW or more when the magnetic field applied perpendicular to its film surface is in the range of -100 mT or more and 90 mT or less.

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