Magnetisation reversal in permanent magnet materials

Abstract

This thesis presents an investigation of the reversal of magnetisation in sintered anisotropic Nd-Fe-B, isotropic melt-spun Nd-Fe-B powder, and in Sm-(Co,Cu,Fe,Zr) 2:17 anisotropic powder. Magnetisation measurements have been performed using vibrating sample magnetometers, a permeameter, and an adapted pulsed field magnetometer. Magnetisation reversal is explained in terms of domain wall formation and motion, and irreversible switching of moments across energy barriers.
For sintered Nd-Fe-B, consisting of multi-domain grains in the virgin state, it is suggested that the pinning of domain walls at grain boundaries plays an important role in hindering reversal of magnetised grains. The type of pinning mechanism appears to depend on the initial magnetic state of the material. In the melt-spun Nd-Fe-B and in Sm-(Co,Cu,Fe,Zr) 2:17 powders, it is suggested that since domains remain correlated across several grains, the reversal mechanism is best described as bulk pinning of domain walls by grain boundaries. The nature of this mechanism is investigated. In Sm-(Co,Cu,Fe,Zr) 2:17 an unexpected variation of remanent coercivity and fluctuation field with magnetic orientation and texture is observed. Data on magnetic interactions and their effects on the reversal mechanisms is analysed.
Weight changes of melt-spun Nd-Fe-B powders during ageing in different environments are related to the observed changes in the reversal mechanism. A preliminary study physically modelling the alignment of anisotropic polymer bonded Sm-(Co,Cu,Fe,Zr) 2:17 powders is outlined and related to a simple analytical model.