ROTATIONAL AND TRANSVERSE DYNAMICS OF SOME RANDOM ANISOTROPY MAGNETS (INVITED)

After rapid survey of the behavior of random anisotropy magnets, the time and frequency evolution of the magnetization vector of a rotating disk of amorphous DyNi is focused on, as measured by a two-SQUID detectors setup. The results show that the dynamics of this system is completely determined by two macroscopic variables: the magnetizations M1 and M2, respectively associated with short and long relaxation times. The first one is fixed with respect to the applied field and therefore responsible for dissipation (magnetic friction) whereas the second one is fixed with respect to the sample (rotating anisotropy). Furthermore, transverse ac susceptibility experiments are used to confirm some of the results obtained. In particular low-temperature dynamical scaling (T→0) permits the existence of a new type of frequency crossover, when the volume samples by the ac field becomes of the order of Imry and Ma domains, to be shown.