Relaxation measurements on ferromagnetic rare earth hydroxides

The hydroxides (RE(OH)3) of the rare earths Tb, Dy, and Ho are known to be ferromagnetic at liquid He temperatures. They show an extreme magnetic anisotropy. Measurements of their ac susceptibility in their real and imaginary parts from 1.5 to 700 Hz are reported. The low-field measurements reveal Cole-Cole semicircles with well defined relaxation times, leading to activation energies of a few 10−3 eV. The high-field ac susceptibility shows an abrupt jump at a well defined amplitude of the driving field. As demonstrated by switching experiments it takes tens of minutes for the magnetization to follow a change of the dc magnetic field. The time variation of the magnetization can be approximated by a logarithmic law. It can be concluded from a phenomenological description that two different relaxation processes are observed the first of which is thermally activated and the second one is similar to a thermal after-effect.