Dynamics of the Random-Field Ising Model

Ising magnets with long, needle-like domains can be treated as single extended spins which interact via dipole--dipole forces. Typically such dipole interactions cancel out due to spatial symmetry, but the combination of randomly packed grains and the application of a magnetic field transverse to the easy axis of magnetization can break the symmetry. This results in a site-random-magnetic field that points along the easy axis and varies from grain to grain, described by the Random--Field Ising Model (RFIM). We report a series of magnetization measurements in longitudinal and transverse magnetic fields that demonstrate RFIM behavior in the room--temperature, rare--earth ferromagnet Nd$_2$Fe$_{14}$B, and analyze our data in terms of predicted scaling relations.