1/f Nyquist magnetic noise, magnetic viscosity and hysteresis

We discuss the connection between magnetic hysteresis, viscosity, and linear-response susceptibility, under the assumption that the system can be described by a collection of elementary two-level systems and that all the mentioned phenomena are controlled by the same two-level-system distribution. The approach is in close relation to the Preisach hysteresis model. In particular, we show that the spectral density of thermal-equilibrium Nyquist noise is proportional to the Preisach switching field distribution. This result suggests that, under appropriate conditions, it should be possible to predict the equilibrium noise of a magnetic system from its large-field hysteresis properties