Spin Precession and Avalanches

In many magnetic materials, spin dynamics at short times are dominated by precessional motion as damping is relatively small. We describe how avalanches evolve under these conditions. The growth front is spread out over a large region and consists of rapidly fluctuating spins often above the ferromagnetic transition temperature. In the limit of no damping the system will transition to an ergodic state if the initial instability is large enough, but otherwise can die out. This dynamic nucleation phenomenon is analyzed theoretically and the implications for real materials are discussed.

Figure 1

Snapshots of systems with different damping $\gamma$ starting with the same configuration, during an avalanche, taken at different times when the avalanches are roughly the same size. (a) For $\gamma =0.8$ and (b) For $\gamma =0.01$. The intensity represents the local spin motion.

Figure 2

The boundary between avalanche propagation and extinction for the one dimensional model discussed in the text $\nu =0$. The + symbols are the numerical values and the solid line is an analytical fit to the data. The × symbols are for systems with quenched disorder with $-1<{\eta }_i<1$.