Metastability of the uniform magnetization in three-dimensional random-field Ising model systems. I. Fe0.7Mg0.3Cl2

Abstract

The uniform magnetization M of the random-field Ising model system ${\mathrm{Fe}}_{0.7}$ ${\mathrm{Mg}}_{0.3}$ ${\mathrm{Cl}}_2$ was measured using both optical Faraday rotation and superconducting quantum interference device (SQUID) techniques. Thermoremanent magnetization appears after cooling in a field $H_0$. It follows the relation μ∝$H_0^2$[T ln(t/τ${\left)\right]}^{\mathrm{-}1}$ at low temperatures. In close agreement with Nattermann and Vilfan, this describes the spin readjustments within the walls of immobile antiferromagnetic domains. Volume contributions to μ give rise to $H_0$ dependence of τ. At T≳0.6$T_c$(H), the decay of μ is additionally affected by domain growth. This also causes a decrease of the excess magnetization, ΔM, of the field-cooled domain state after switching the field off and on. Bruinsma-Aeppli-type instantaneous relaxation of ΔM is found in the broad-domain-wall limit very near to ${\mathit{T}}_{\mathit{c}}$(H). ΔM≳0 appears also in a zero-field cooled sample upon reversing the T scan below $T_c$ within the range of the dynamical rounding. It is attributed to frozen-in fluctuating disorder, which probably does not break the long-range order.