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Spin-torque oscillators (STOs) are unique spintronic devices in which spin current induces the precession of magnetization in the oscillation layer. In this study, we demonstrate coupled out-of-plane oscillation between two oscillation layers within an STO. The coupled oscillation state is observed in the magnetoresistance spectra as well as its degradation due to the mixing of the free oscillation state depending on the bias current. These behaviors are analytically described and numerically reproduced based on the Landau–Lifshitz–Gilbert equation. Since the STO is fabricated in the write head of a hard disk drive, the coupled oscillation state and the subsequent microwave magnetic field are applicable for microwave-assisted magnetic recording for high-density recording.

We study microwave-assisted magnetization switching (MAS) of a perpendicularly magnetized nanomagnet with a diameter of 50 nm in a circularly polarized microwave magnetic field. The MAS effect appears when the rotation direction of the microwave field matches that of the ferromagnetic resonance excitation, and a large switching field decrease from 7.1 kOe to 1.5 kOe is demonstrated. In comparison with a linearly polarized microwave magnetic field, the circularly polarized microwave field induces the same MAS effect at half the microwave field amplitude, thereby showing its efficiency. We also examine MAS in the subnanosecond region and show that the magnetization switching can be induced by a microwave field with the duration of 0.2 ns.

We demonstrate microwave-assisted magnetization switching of a perpendicular magnetic nanodot in a microwave stray field from a spin-torque oscillator (STO). The switching field decreases when the STO is operated by applying a current. The switching field reduction is almost the same as that in a microwave magnetic field generated by a signal generator despite the fluctuations of the STO oscillation. The switching field distribution, however, is broader when the STO is used. We also examine the magnetization switching process in the nanosecond region by applying a nanosecond-order pulse current to the STO and measuring the STO signal waveform. The onset of the STO oscillation and subsequent assisted switching occur within a few nanoseconds.

Synchronized magnetization motion among the several magnetic layers composing a spin-torque oscillator (STO) is considered an effective way to improve spectral purity. To utilize this scheme in a MgO-based STO, we have fabricated a dual free-layer STO composed of a CoFeB free layer (FL), a MgO barrier layer, and a CoFe/Ru/CoFeB synthetic ferrimagnet free layer (SyF). Unlike conventional MgO-based STOs, this structure does not have an antiferromagnetic layer that pins the SyF, leading to a large-amplitude oscillation of magnetization in the SyF. The dual free-layer STO exhibits coherent microwave emissions with power spectrum density beyond 800 nW/GHz and narrow spectral linewidth below 5 MHz (Q-factor ≈ 2000). Macrospin simulations confirm that the stable oscillations originate from the synchronized magnetization motion of the FL and the SyF through dynamical dipolar coupling.

Angular dependence of power spectra is measured for tunnel-type spin-transfer oscillators by changing the direction of the external field in the plane. The oscillation frequency shows the red shift with increasing the current in the case that the field directions are away from the hard axis of the free layer. The frequency shift changes from red to blue at a specific direction near the hard axis, where the linewidth is much reduced. These results are explained by taking into account the nonlinearity arising from the in-plane anisotropy as well as that arising from the demagnetizing effect.