We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by the circularly polarized cosine chirp microwave pulse (CCMP). The numerical findings, based on the Landau-Lifshitz-Gilbert equation, reveal that the CCMP is by itself capable of driving fast and energy-efficient magnetization reversal. The microwave field amplitude and initial frequency required by a CCMP are much smaller than that of the linear down-chirp microwave pulse. This is achieved as the frequency change of the CCMP closely matches the frequency change of the magnetization precession which leads to an efficient stimulated microwave energy absorption (emission) by (from) the magnetic particle before (after) it crosses over the energy barrier. We further find that the enhancement of easy-plane shape anisotropy significantly reduces the required microwave amplitude and the initial frequency of CCMP. We also find that there is an optimal Gilbert damping for fast magnetization reversal. These findings may provide a pathway to realize the fast and low-cost memory device.

In this paper, we investigate the magnetic-domain wall (DW) dynamics in uniaxial/biaxial-nanowires under a thermal gradient (TG). The findings reveal that the DW propagates toward the hotter region in both nanowires. In uniaxial nanowire, the DW propagates accompanying a rotation of the DW-plane. In biaxial nanowire, the DW propagates in the hotter region, and the so-called Walker breakdown phenomenon is observed. The main physics of such DW dynamics is the magnonic angular momentum transfer to the DW. The hard (shape) anisotropy exists in biaxial-nanowire, which contributes an additional torque; hence DW speed is larger than that in uniaxial-nanowire. But the rotational speed is lower initially as hard anisotropy suppresses the DW-rotation. After certain TG, DW-plane overcomes the hard anisotropy and so the rotational speed increases slightly. With lower damping, the DW velocity is smaller and DW velocity increases with damping which is a contrary to usual desire. The reason is predicted as the formation of the standing spin-waves (by superposing the spin waves and its reflection from the boundary) which do not carry any net energy to DW. However, for larger damping, DW velocity decreases with damping since the magnon-propagation length decreases. Therefore, the above findings might be useful to realize the spintronics (i.e. racetrack-memory) devices.

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We study the thermal gradient (TG) induced domain wall (DW) dynamics in a uniaxial nanowire in the framework of the Stochastic-Landau-Lifshitz-Gilbert equation. TG drives the DW in a certain direction, and DW (linear and rotational) velocities increase with TG linearly, which can be explained by the magnonic angular momentum transfer to the DW. Interestingly, from Gilbert damping dependence of DW dynamics for fixed TG, we find that the DW velocity is significantly smaller even for lower damping, and DW velocity increases with damping (for a certain range of damping) and reaches a maximal value for critical damping which is contrary to our usual desire. This can be attributed to the formation of standing spin wave (SSW) modes (from the superposition of the spin waves and their reflection) together with travelling spin wave (TSW) modes. SSW does not carry any net energy/momentum to the DW, while TSW does. Damping $\alpha$ compels the spin current polarization to align with the local spin, which reduces the magnon propagation length and thus $\alpha$ hinders to generate SSWs, and contrarily the number of TSWs increases, which leads to the increment of DW speed with damping. For a similar reason, we observe that DW velocity increases with nanowire length and becomes saturated to maximal value for a certain length. Therefore, these findings may enhance the fundamental understanding as well as provide a way of utilizing the Joule heat in the spintronics (e.g., racetrack memory) devices.

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