Dynamic Crossover in Dipolar Ferromagnets

Abstract

Ferromagnets were among the first systems where dynamical critical phenomena with non classical features were observed experimentally. Despite of this long history a series of seemingly contradictory experimental results remained. At first sight one could expect that the exchange interaction plays the major role in the spin dynamics. Taking into account solely the exchange interaction results in the well known dynamical scaling law for the linewidth Γ(q,ξ) = ∧q ^z γ(x) with the scaling function γ(x) depending on the single scaling variable x = 1/qξ. The critical exponent deduced from the wave vector dependence of neutron scattering data right at T _c is 5/2 down to very small wave vectors [1-3]. But nevertheless the data for the linewidth above the critical temperature could not be described by a single scaling function [2]. The situation looks even more contradictory by looking at hyperfine-interaction (HFI) experiments. There a crossover in the dynamical critical exponent z from z = 5/2 to z = 2 was observed indicating a crossover to a dynamics with a non conserved order parameter [4]. This is confirmed by electron-spin-resonance (ESR) and magnetic relaxation experiments, where one finds a non vanishing Onsager coefficient at zero wave vector [5].