Magnetization thermal noise in soft ferromagnets at liquid He temperatures

Abstract We are investigating the spontaneous equilibrium fluctuations of magnetization in soft ferromagnets at liquid He temperatures using a rf SQUID magnetometer. 1/ f noise behaviour is found both in amorphous and polycrystalline materials. We show that the observed magnetization noise is entirely of thermodynamical origin and we discuss the link between 1/ f noise and magnetic viscosity.