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In the long history of permanent magnet research for more than 100 years, three-dimensional magnetic microscopy has been eagerly awaited to elucidate the origin of the magnetic hysteresis of permanent magnets. In this study, we succeeded in observing the three-dimensional magnetic domain structure of an advanced high-coercivity Nd-Fe-B-based permanent magnet throughout the magnetic hysteresis curve using a recently developed hard X-ray magnetic tomography technique. Focused-ion-beam-based three-dimensional scanning electron microscopy was employed to study the relationship between the observed magnetic domains and the microstructure of the magnet for the same observing volume. Thermally demagnetized and coercivity states exhibit considerably different magnetic domain structures but show the same periodicity of 2.3 μm, indicating that the characteristic length of the magnetic domain is independent of the magnetization states. Further careful examination revealed some unexpected magnetic domain behaviors, such as running perpendicular to the magnetic easy axis and reversing back against the magnetic field. These findings demonstrate a wide variety of real magnetic domain behaviors along the magnetic hysteresis inside a permanent magnet.

A spin-torque oscillator (STO) is a key device in microwave-assisted magnetic recording (MAMR). Conventionally, the magnetization dynamics of STOs has been analyzed using the high-frequency electrical signal from the STO (STO signal). However, the STO signal is sometimes undetectable because of the highly symmetric magnetization trajectory or contains several frequency components that do not correspond to the frequency of the actual magnetization oscillation [1] . Furthermore, when the STO is integrated into the write head, the STO signal is expected to attenuate heavily in the write head, making it difficult to extract the STO signal. These facts prevent identifying the oscillation frequency from the STO signal, and a new analysis method of the STO that does not rely on the STO signal is required.

Spin torque oscillators (STOs) that generate a persistent magnetization oscillation by spin-transfer torque are promising spintronic devices for various applications. In this study, we propose and experimentally demonstrate a method to characterize the magnetization dynamics of STOs using injection locking. By placing the STO near an antenna that generates a microwave magnetic field of a variable frequency ( fMW), injection locking of the STO to the microwave field occurs when fMW is in the locking range around the intrinsic oscillation frequency. During injection locking, the dc resistance of the STO exhibits a peak-and-valley dependence on fMW, which originates from the modification of the magnetization trajectory induced by injection locking. Based on this principle, the oscillation frequency can be estimated by measuring the dc resistance change. Because this method does not require measuring the high-frequency magnetoresistance signal from the STO, which is used in the conventional method, it is advantageous in cases where the high-frequency magnetoresistance signal is undetectable or exhibits additional frequency components different from the oscillation frequency.

We experimentally study the magnetization dynamics of an all-in-plane spin-torque oscillator (AIP-STO) by using injection locking to an external microwave magnetic field. The AIP-STO exhibits multiple frequency signals corresponding not only to the oscillation frequencies of the magnetic layers but also to the difference between these frequencies. The oscillation frequency is identified based on the principle that injection locking occurs only to the signal corresponding to the magnetization oscillation. We also analyze the magnetization dynamics during injection locking by micromagnetic simulations. The results demonstrate that injection locking is a powerful tool for elucidating the oscillation of the AIP-STO.