arXiv subjects

Ching-Ming Lee

Publications and source records attributed to Ching-Ming Lee.

3 recordsLinked to original sources

arXiv2026

Hysteretic Excitation in Non-collinear Antiferromagnetic Spin-Torque Oscillators: A Terminal Velocity Motion Perspective

We present a theoretical framework for non-collinear antiferromagnetic spin torque oscillators (NC-AFM STO) by unifying spin dynamics under the Poisson Bracket formalism. Shifting from traditional torque-based descriptions to an operational symmetry perspective, we develop two complementary viewpoints: a vector perspective identifying infinite degenerate Rigid Body Precession (RBP) states where exchange energy depends solely on the total magnetic momentum, and a particle perspective decomposing dynamics into Center-of-Mass (CM) translation and Relative Motion (RM) oscillation. Using time-dependent rotational and translational transformation techniques, we analytically resolve the rapid (~10 ps) transient evolution into a stable RBP state driven by SOT and damping. We demonstrate that the out-of-plane anisotropy (OPA) lifts the exchange degeneracy, triggering a long-term (~1 ns) oscillatory decay toward a steady state characterized by uniform spin z-components and a 120-degree inter-spin locking angle. This state is accurately governed by our Terminal Velocity Motion (TVM) model [arXiv:2305.14013], where exchange coupling transforms into kinetic energy with a light effective mass. The model precisely predicts SOT-driven transients, hysteretic excitation, and the dynamic phase diagram. Finally, we account for the sub-critical current regime mismatch by identifying a 'Rigid-Body Breaking' effect: a surge in effective friction caused by the self-resonance of RM variables induced by CM translation, mediated by the in-plane anisotropy (IPA).

cond-mat.mes-hall↗

arXiv2023

Terminal Velocity Motion Model Used to Analyze the Mutual Phase-locking of STNOs

Using Legendre transformation, a standard theoretical approach extensively used in classical mechanics as well as thermal dynamics, two-dimensional non-linear auto-oscillators including spin torque nano-oscillators (STNOs) can be equivalently expressed either in phase space or in configuration space where all of them can be modeled by terminal velocity motion (TVM) particles. The transformation completely preserves the dynamic information about the canonical momenta, leading to very precise analytical predictions about the phase-locking of a coupled pair of perpendicular to plane STNOs (PERP-STNOs) including dynamical phase diagrams, (un)phase-locked frequencies, phase-locked angles, and transient evolutions, which are all solved based on Newton mechanics. Notably, the TVM model successfully solves the difficulty of the generalized pendulum-like model [Chen \textit{et al}. \textbf{J. Appl. Phys. 130}, 043904 (2021)] failing to make precise predictions for the higher range of current in serial connection. Additionally, how to simply search for the critical currents for phase-locked (PL) and asynchronized (AS) states by numerically simulating the macrospin as well as TVM model, which gets inspired through analyzing the excitations of a forced pendulum, is also supplied here. Therefore, we believe that the TVM model can bring a more intuitive and precise way to explore all types of two-dimensional non-linear auto-oscillators.

cond-mat.mes-hall↗

arXiv2021

Hysteretic Mutual Synchronization of PERP-STNO Pairs Analyzed by a Generalized Pendulum-like Model

At present, the Kuramoto model is the standard and widely accepted theoretical approach for analyzing the synchronization of spin-torque nano-oscillators (STNOs) coupled by an interaction. Nevertheless, the oscillatory decaying regime as well as the initial condition (IC)-dependence (hysteretic) that exist in the synchronization of many types of STNOs cannot be explained by this model. In order to more precisely elucidate the physical mechanisms behind the two phenomena, in this paper we develop a generalized pendulum-like model based on the two common features of non-linear auto-oscillators: one is the stability of the amplitude/energy of dynamic states; the other is the non-linear dynamic state energy of oscillators. In this new model, we find that the Newtonian-like particle with sufficient kinetic energy can overcome the barrier of phase-locking potential to evolve into a stable asynchronization (AS) state, leading to the (IC)-dependent synchronization. Furthermore, due to the presence of the kinetic energy, this particle can also oscillate around the minima of the phase-locking potential, leading to the oscillatory decaying regime. Thereby, in this work, we adopt this new model to analyze the IC-dependent mutual synchronization of perpendicularto-plane (PERP)-STNO pairs, and then we suggest that the initial conditions can be controlled to avoid such a phenomenon by using magnetic dipolar coupling.

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