Reptation and hysteresis in disordered magnets

(Received 1 st September 1992, accepted in final form 21 September 1992)

Abstract
In most disordered magnets, the hysteresis cycles obtained by the repetitive application of a magnetic field are not exactly reproducible. At low temperature, a logarithmic growth of the remanent magnetization with the number of cycles ( n) is observed. This phenomenon is known as magnetic reptation. A review of the experimental studies of reptation in disordered ferromagnets and spin-glasses is presented. We emphasize the simple phenomenological scaling laws governing the field and the temperature dependence of this phenomenon. We consider two microscopic mechanisms which may be responsible for this process. The first one relies on the random dependence of the energy barriers on applied magnetic field resulting from the balance between the fluctuations in the magnetic pressure, the random pinning potential and the surface tension of a domain wall. In systems with long range forces (dipolar interactions in ferromagnets, RKKY exchange in metallic spin-glasses), a second reptation process induced by the Onsager reaction field appears to be particularly relevant to experiments.