Yiming Shi

Slow magnetization relaxation and reversal in magnetic thin films

The slow magnetization dynamics in magnetic thin films has been investigated in this paper. It is shown that the experimental results of the time-dependent magnetization can be well described by the extended exponential function, exp(−(t/τ)β) with β>0, in a number of thin film systems. By investigating the characteristics of the magnetization process and examining the limitations of the present models, an explanation is provided based on the structural and dynamical properties of the magnetic domains. Meanwhile, some questions have been clarified in the study towards understanding the magnetization relaxation phenomenon in thin films.

Microwave generation by a direct current spin-polarized current in nanoscale square magnets

Theoretical calculation for a simple nanoscale magnetoelectronic device to function as a microwave generator based on the spin-transfer torque effect is presented. The device is unique because the output amplitude and frequency can be continuously tuned by the electrical current in the microwave frequency range. Analysis and discussion of the device structure, function, and realization are provided.

Spin-current effect on ferromagnetic resonance in patterned magnetic thin film structures

We have theoretically investigated the ferromagnetic resonance in the magnetic thin film structures under the influence of spin-transfer torque using a modified Landau–Lifshitz–Gilbert equation in the linearization regime. The study shows that spin currents do not shift the resonance field but rather change both the resonance amplitude and the linewidth. Ferromagnetic resonance under this circumstance can be characterized by an effective damping constant. Depending upon its direction, the spin current can pump energy into or dissipate energy from the magnetic system. In addition, the quality factor of the resonance can be tuned by changing the current intensity. Ferromagnetic resonance excited by ac electrical currents is also theoretically demonstrated and discussed in this article.

Precessional dynamics of single-domain magnetic nanoparticles driven by small ac magnetic fields

The magnetic and mechanical dynamics of nanometre-sized magnetic particles with a uniaxial anisotropy excited by ac magnetic fields is studied using the classical theory approach in the Lagrangian formalism. Through the magnetic anisotropy between the magnetic moment and the lattice, the nanoparticle is coupled with the magnetic moment and then driven into a mechanical precession mode by the ac magnetic fields. Mechanical resonance can be achieved along with the magnetic resonance at frequencies and with resonance amplitudes that depend strongly upon both the magnetic and the mechanical properties of the nanoparticles. The dynamics manifests itself in magnetic susceptibility and energy dissipation, which are discussed in great detail in this article.

Circular domain wall motion driven by spin-polarized currents in confined square nanomagnets

The spin-transfer torque effect on the magnetization in a pillar structure has been investigated using micromagnetic simulation. The study shows that a spin-polarized dc current induces a domain wall formation from the initial inhomogeneous magnetization configuration of the square magnet and drives it to rotate in the constrained structure. Based on the result, the structure has potential use in magnetoelectronics as a rotor with no moving parts in the nanometer scale.

Magneto-elastic coupling and Peierls-like phase transition of one-dimensional magnetic nanoparticle chains

The fine structure of magnetic nanoparticle periodic chains is studied by considering the competition between magnetic dipolar interaction and elastic interaction. A Peierls phase with a doubled unit cell length can be formed under proper conditions and a Peierls transition can be induced by temperature and by external magnetic field. Due to the coupling between the magnetic state and the structural state, the nanoparticle chains would possess distinct properties and exhibit interesting behaviour.

Hysteretic magnetization behaviour influenced by spin-currents in magnetic thin films with perpendicular anisotropy

While most recent studies of the spin-transfer torque effect mainly deal with magnetic structures with in-plane anisotropies and magnetizations, we theoretically investigate the magnetization behaviour driven by spin-currents in a trilayer structure with perpendicular anisotropy and magnetization using a modified Landau?Lifshitz?Gilbert equation. This study is particularly conducted for hysteresis loop measurements under spin-currents and current?voltage measurements in the presence of external magnetic fields, by which the unique magnetization rotations and reversals associated with the spin-transfer torque in the magnetic configuration can be captured.