Noise of ferromagnetic materials

The main types of noise in ferromagnetic materials are thermal or Nyquist noise, excess noise, and Barkhausen noise. They differ in magnitude as well as in the frequency dependence of their associated power spectra. The comparatively weak Nyquist noise is generated by the thermal energy of the conduction electrons and spin waves acting on the domain configuration in the ferromagnetic materials. Excess noise occurs during temperature changes of the sample. It is generated by magnetization processes caused by a changing mechanical strain inside the sample. Strongest by far can be the Barkhausen noise. This is due to a pronounced tendency to form clusters of Barkhausen jumps. The temporal and spatial behavior of such clusters can be well described as a diffusion process; a simple model of this process is given.