Magnetic Relaxation via Competing Dynamics

Abstract

We study the magnetic relaxation of a system of finite identical non-interacting particles after a small applied magnetic field is reversed. We consider a “particle” consisting of N Ising spins, σ _i = ±1 at the sites of a square lattice with free boundaries. All the particles are in a completely ordered state at the beginning. When the magnetic field is reversed, the system is in a metastable state and the demagnetization starts due to the superposition of two processes: with probability p, there is a random spin-flip; with probability 1 − p we perform the flip according to the Metropolis’ rule at a temperature T. The preliminary results show that the demagnetization at low temperatures could occurs via avalanches for any p ≠ 0. The distribution of the sizes of this avalanches seems to follow a power law. The demagnetization shows, in a certain range, a lineal behavior with ln(t), defining a slope v for each T. The value of v becomes constant below a certain T _Q . This T-independent behavior has been reported before in experimental measurements from the demagnetization of real magnetic particles, associated to the quantum tunneling of magnetic vectors between two minima, in quantum coherence phenomena.