Magnetization reversal via internal spin waves in magnetic nanoparticles

Abstract

By numerically solving the equations of motion for atomic spins we show that internal spin-wave processes in large enough magnetic particles, initially in unstable states, lead to complete magnetization reversal and thermalization. The particle's magnetization $m$ strongly decreases in the middle of reversal and then recovers. The closer is the initial orientation of $\mathbf{m}$ to the energy minimum, the slower is the relaxation toward it and the smaller is the decrease in $m$ in the course of relaxation. We identify two main scenarios, exponential and linear spin-wave instabilities. For the latter, the longitudinal and transverse relaxation rates have been obtained analytically. Orientation dependence of these rates leads to a nonexponential relaxation of the particle's magnetization at long times.