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This paper presents a steady auto-oscillation in a spin-torque oscillator using MgO-based magnetic tunnel junction (MTJ) with a perpendicular polarizer and a perpendicular free layer. As the injected d.c. current varied from 1.5 to 3.0 mA under a weak magnetic field of 290 Oe, the oscillation frequency decreased from 1.85 to 1.3 GHz, and the integrated power increased from 0.1 to 74 pW. A narrow linewidth down to 7 MHz corresponding to a high Q factor of 220 was achieved at 2.7 mA, which was ascribed to the spatial coherent procession of the free layer magnetization. Moreover, the oscillation frequency was quite sensitive to the applied field, about 3.07 MHz/Oe, indicating the potential applications as a weak magnetic field detector. These results suggested that the MgO-based MTJ with perpendicular magnetic easy axis could be helpful for developing spin-torque oscillators with narrow-linewidth and high sensitive. [ABSTRACT FROM AUTHOR]

We propose a novel setup for a spin-torque oscillator reader in magnetic hard disk drive technology. Two adjacent bit tracks are to be read simultaneously, leading to high data transfer rate and increased resilience to noise as the lateral size of the oscillator device is allowed to remain larger than the bit width. We perform micromagnetic simulations of an example system and find that the magnetization response has a clear unimodal character, which enables for detection of two bit values at the same time. We analyze the frequency of the device under the influence of two different external fields and conduct a simulation of a successful dynamic readout. We estimate the signal linewidth and signal-to-noise ratios of the setup and show that it may be potentially beneficial for magnetic readout applications. [ABSTRACT FROM AUTHOR]

Current-induced spin-transfer torque can induce magnetization precession in gigahertz ranges, which enables various microwave devices. In most cases, this current-induced precession requires an additional external magnetic field, which is detrimental for device applications. In this work, we show that the current-induced precession is realized even without an external field when the magnetic layer has conical magnetization, caused by the second order easy-axis magnetic anisotropy. We theoretically derive the oscillation frequency and output power and confirm their validity by macrospin simulations. We find that the ratio of the second to the first anisotropy constants is key to determine the maximum frequency and power. Our results will be helpful for the applications of microwave devices utilizing spin-transfer torques and provide a design rule of such devices. [ABSTRACT FROM AUTHOR]

Microwave-assisted magnetic recording (MAMR) employs a spin-torque oscillator (STO) to assist a standard magnetic writer field in switching grains of a magnetic recording medium. The STO field has different polarization angles at different positions, resulting in different mixes of in-media-plane and out-of-media-plane (vertical) field components. We find that the vertical component of the STO field degrades the writing process, increasing jitter, decreasing low-frequency signal-to-noise ratio, decreasing magnetic write width, and decreasing areal density capability. This is not due to postwrite erasure, but rather is part of the write process itself. Simple vector considerations illuminate the cause for this degradation. Consequently, experimental and modeling approaches that estimate MAMR switching-field reduction using only in-media-plane oscillating fields will erroneously overestimate the impact of the ac fields in the magnetic recording process. [ABSTRACT FROM AUTHOR]

This paper formulates a general analytic approach to the theory of microwave generation in magnetic nano-structures driven by spin-polarized current and reviews analytic results obtained in this theory. The proposed approach is based on the universal model of an auto-oscillator with negative damping and nonlinear frequency shift. It is demonstrated that this universal model, when applied to the case of a spin-torque oscillator (STO) based on a current-driven magnetic nano-pillar or nano-contact, gives adequate description of most of the experimentally observed properties of STO. In particular, the model describes the power and frequency of the generated microwave signal as functions of the bias current and magnetic field, predicts the magnitude and properties of the generation linewidth, and explains the STO behavior under the influence of periodic and stochastic external signals: frequency modulation, phase-locking to external signals, mutual phase-locking in an array of STO, broadening of the generation linewidth near the generation threshold, etc. The proposed nonlinear auto-oscillator theory is rather general and can be used not only for the development of practical nano-sized STO, but, also, for the description of nonlinear auto-oscillating systems of any physical nature. [ABSTRACT FROM PUBLISHER]

We present a micromagnetic modeling study of microwave assisted magnetic recording (MAMR) on a typical exchange-coupled four-layer media. With the help of an alternating current (ac) magnetic field, the switching field can be reduced, especially at the media resonant frequency. Typical hysteresis loop measurements employ a depth-invariant ac field; however, the field generated from a spin-torque oscillator (STO) varies with spacing. Considering this depth variance, the switching field reduction becomes less than observed with a depth-invariant ac field. Furthermore, we find an even smaller switching field reduction when using a realistic magnetic writer and STO fields to record a dc track on randomly magnetized grains. Consequently, we conclude that MAMR hysteresis loops employing a depth-invariant ac field significantly overestimate the ac field’s contribution to grain switching in an actual recording situation. [ABSTRACT FROM AUTHOR]

We studied the effect of injecting a low-frequency (0.5–1 GHz) ac signal on the natural oscillations of a nano-oxide layer-based nanocontact magnetoresistive spin-torque oscillator with multiple Fe50Co50 nanocontacts. We observed a frequency modulation effect with odd-numbered side bands having low amplitudes. Also, no frequency shift at main spectral peak's frequency, typical with nonlinear frequency modulation, was observed. We could not completely explain this behavior, but we propose that this could be related to the internal dynamics of constricted domain walls. [ABSTRACT FROM AUTHOR]