Domain growth in the three-dimensional random-field Ising magnet Fe0.5Zn0.5F2.

Complementary neutron-scattering and superconducting quantum interference device (SQUID) magnetometry measurements have been carried out to study the time dependence of the field-cooled metastable domains in Fe{sub 0.5}Zn{sub 0.5}F{sub 2}. These neutron-scattering data provide direct evidence of a logarithmic increase in the correlation length upon quenching the system to {ital T}{le}{ital T}{sub {ital c}} at {ital H}=5.5 T. At low temperatures the magnetic domains are frozen on experimental time scales. SQUID measurements performed at a series of fields and temperatures confirm the evolution of the excess magnetization {ital M}{sub ex} arising from domain surfaces and allow quantitative analysis of its time dependence. Specifically, we found that {ital M}{sub ex}{similar_to}{ital H}{sup {nu}}{sub {ital M}}[ln({ital t}/{tau})]{sup {minus}{psi}}, where {nu}{sub {ital M}}=2.7{plus_minus}0.1 and {psi}=1.1{plus_minus}0.3 for {ital H}{ge}3.45 T; {psi} is much less than 1 for lower fields. In particular, {psi}=0.42{plus_minus}0.14 at {ital H}=1.5 T, in agreement with a prediction by Nattermann and Vilfan. At {ital H}=5.5 T, {ital M}{sub ex}({ital T}) and the inverse correlation length {kappa}({ital T}) exhibit identical time dependences. Overall, these measurements reveal the varied time dependence of a field-cooled, strongly anisotropic dilute antiferromagnet in different applied fields, and shed light on the mechanisms of domain evolution.