Magnetic viscosity and barkhausen noise in NdFeB-type permanent magnets

Abstract

The Barkhausen noise and magnetic viscosity in sintered and melt-quenched needles of anisotropic NdFeB-type magnets are examined. In the sintered magnet, the time integral of the Barkhausen signal during magnetic viscosity is shown to correlate with the change in the bulk magnetisation as measured using a vibrating sample magnetometer. This is in contrast with similar measurements on soft magnetic materials by Tebble et al. [1], where the magnetisation change, as estimated from the time integral of the Barkhausen noise, was significantly less than that measured by magnetometric techniques. The activation volume in each of the two materials is estimated from measurements of the coefficient of magnetic viscosity, S v, and in the case of the sintered magnet is shown to be up to 13 orders of magnitude smaller that the largest Barkhausen volumes associated with the demagnetisation process. The magnitude of the Barkhausen volumes are indicative of magnetisation processes involving instabilities in the magnetisation of clusters of grains. It was not possible to identify heterogeneities in the microstructural or magnetic topology in these materials which would account for the magnitudes of the observed Barkhausen jumps.