Stability of out-of-plane precession with the application of a tilted magnetic field in spin torque oscillator having a planar free layer and a perpendicular polarizer
Ryo HiramatsuHitoshi KubotaSumito TsunegiShingo TamaruKay YakushijiFukushima AkioRie MatsumotoHiroshi ImamuraShinji Yuasa
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Polarizer
Spin-transfer torque
Spin valve
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The authors derive an analytical form of the critical current density for the gyromagnetic precession in spin-transfer torque (STT) devices. The STT device which is injected by a perpendicular-to-plane polarized current has very different properties. We find that the critical current needed to generate a large cone-angle precession motion depends on the Gilbert damping constant, in-plane anisotropy field, stray field, and demagnetizing filed. Compared to the most used STT device, which is injected by a current with in-plane spin polarization, the critical current is always smaller.
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Precession
Spin-transfer torque
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The paper discusses the magnetization dynamics of spin-torque oscillator with negative K u under alternative head field for microwave assisted magnetic recording. Magnetization switching of spin-injection layer (SIL) is essential to have the same precession mode of the field generation layer before and after the head field change directions. The SIL needs to be properly chosen in order to have short switching time. The layer stack and material selection of spin torque oscillator (STO) is also discussed in terms of stability against the stray field. The results discussed in this paper helps to understand the design fundamentals of STO for microwave assisted magnetic recording.
Magnetization dynamics
Precession
Micromagnetics
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We present an overview in the understanding of spin-transfer torque (STT) induced magnetization dynamics in spin-torque nano-oscillator (STNO) devices. The STNO contains an in-plane (IP) magnetized free layer and an out-of-plane (OP) magnetized spin polarizing layer. After a brief introduction, we first use mesoscopic micromagnetic simulations, which are based on the Landau–Lifshitz–Gilbert equation including the STT effect, to specify how a spin-torque term may tune the magnetization precession orbits of the free layer, showing that the oscillator frequency is proportional to the current density and the z-component of the free layer magnetization. Next, we propose a pendulum-like model within the macrospin approximation to describe the dynamic properties in such type of STNOs. After that, we further show the procession dynamics of the STNOs excited by IP and OP dual spin-polarizers. Both the numerical simulations and analytical theory indicate that the precession frequency is linearly proportional to the spin-torque of the OP polarizer only and is irrelevant to the spin-torque of the IP polarizer. Finally, a promising approach of coordinate transformation from the laboratory frame to the rotation frame is introduced, by which the nonstationary OP magnetization precession process is therefore transformed into the stationary process in the rotation frame. Through this method, a promising digital frequency shift-key modulation technique is presented, in which the magnetization precession can be well controlled at a given orbit as well as its precession frequency can be tuned with the co-action of spin polarized current and magnetic field (or electric field) pulses.
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Magnetization dynamics
Larmor precession
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Spin-transfer torque
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Steady magnetization precession can be excited by a DC spin-polarized current in a spin-valve with combined in-plane (IP) and out-of-plane (OP) dual polarizers. The precession frequency is proportional to the strength of spin-torque generated by the OP polarizer but less sensitive to the IP polarizer, which can be successfully interpreted by an analytic model. A single current pulse with its duration as short as 150 ps is able to drive the magnetization switching. The switching probability is dominated by the value of the free layer angle φc at which the free layer magnetization rotation transforms to a damped small angle precession after stopping the current pulse.
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Precession
Larmor precession
Spin-transfer torque
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Through the anomalous Hall effect (AHE), a ferromagnet can generate spin current in the direction transverse to an applied charge current, and hence exert a torque on another ferromagnet. The authors observe, however, that when in-plane current is passed through a spin-valve structure, an out-of-plane effective magnetic field is created, which exerts torque on the free layer. This effective field, originating from the broken mirror symmetry of the spin valve, completely dominates the dampinglike torque that one expects from the AHE. Such a strong out-of-plane field would be useful for switching perpendicular magnetic bits in spintronic high-density nonvolatile memory applications.
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Spin valve
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This paper aims to explore potential mechanisms for sustaining steady-state precession in MgO-based magnetic tunnel junctions (MTJ) with an in-plane polarizer and an out-of-plane free layer. The Landau-Lifshitz-Gilbert-Slonczewski equation is analytically and numerically solved for a nano-pillar MTJ with circular cross-section under constant perpendicular applied current and field. It is demonstrated that the spin torque angular asymmetry is sufficient to sustain the spin transfer torque-driven dynamics of spin-torque nano-oscillators.
Precession
Spin-transfer torque
Tunnel magnetoresistance
Polarizer
Larmor precession
Tunnel junction
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In this article, we have theoretically devised a Spin Torque Nano Oscillator (STNO) with perpendicular polarizer using macro spin model. The devised spin valve structure is heterogeneous (i.e.) it is made of two different ferromagnetic materials [Co and its alloy CoFeB]. The dynamics of magnetization provoked by spin transfer torque is studied numerically by solving the famous Landau-Lifshitz-Gilbert-Slonczewski [LLGS] equation. The results are obtained for the perpendicular polarizer and for that particular out of plane orientation we vary the free layer angle from 10° to 90°. The obtained results are highly appealing, because frequency range is available in all the tilt angles of free layer and it is exceptionally tunable in all free layer tilt angles with zero applied field. Moreover, the utmost operating frequency of about 83.3 GHz and its corresponding power of 4.488 µW/mA2/GHz is acquired for the free layer tilt angle θ = 90° with the solid applied current density of 10 × 1010 A/m2. Also, our device emits high quality factor of about 396, which is remarkably desirable for making devices. These pioneering results provides a significant development for future spintronic based devices.
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Tunnel magnetoresistance
Spin-transfer torque
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