An investigation of anisotropic magnetic properties of rare earth iron boron alloys

Abstract

In 1983 Sumitomo in Japan and General Motors in the USA both announced the production of new 'super magnets' based on a rare earth iron boron mixture. The Sumitomo magnets are based on an intermetallic compound with a composition Nd(_2)Fe(_14)B. This thesis describes an Investigation towards understanding the reason why permanent magnets made from this material have such favourable magnetic properties. A description is given of a new type of torque magnetometer designed to operate within the bore of a superconducting solenoid to make measurements at a field high enough for this type of material. The construction and testing of the instrument are described. Results are presented of uniaxial magnetocrystalllne anisotropy values calculated from torque measurements and also of magnetisation measurements for R(_2)Fe(_14)B, R=Nd,Ho,Gd and Dy from 4.2K to room temperature. The basal plane anisotropy calculated from torque measurements for Gd(_2)Fe(_14)B from l00K to 300K is also reported. These anisotropy results are related qualitatively to the crystal structure and electronic structure of the rare earth ions. Observations of regions of uniform magnetization, domains, within the material are also described, and the movement of the boundaries between such domains, domain walls, under varying magnetic fields. These movements’ characterise the material and help In understanding the processes Involved in making a magnet resistant to demagnetisation. From the observation of equilibrium domain wall patterns domain wall energies and domain wall thicknesses have been calculated.