The anisotropy of bonded ferrite magnets obtained by centrifugal casting in magnetic field

Abstract

• The orientation of bonded magnets in an external magnetic field depends on the powder particle size. • For powders composed of multidomain particles, the orientation occurs in the circumferential direction. • The orientation of the powder occurs in a way that leads to a minimization of the energy. Bonded magnets were produced by centrifugal casting of a powder mixture comprising a magnetically hard material (strontium ferrite) and a chemically hard polymer (epoxy resin) in an external, constant magnetic field perpendicular to the axis of rotation. The tests were carried out for commercial strontium ferrite powder, with micrometre-sized particles. At 5% and 10% powder content by volume, the magnets exhibited a gradient structure, i.e. the proportion of magnetic powder was highest at the surface and decreased towards the sample’s axis. At higher powder content, the viscosity of the slip was high enough to prevent particles to move over significant distances, and no marked change in the proportion of ferrite in the sample volume was observed. Measurements of the magnetic properties in the characteristic directions of the cylindrical sample (radial, axial and circumferential) showed that the powder particles tend to align their easy axis with the axis of the cylinder, i.e. perpendicular to the direction of the magnetic field. These results indicate that the magnetization anisotropy obtained is different from that described in earlier work, where strontium ferrite powder with a particle size above 100 μm was oriented circumferentially (the easy axis was perpendicular to the radius and axis of the cylinder). These differences are due to the different proportion of single and multidomain particles in the two powders and to the different mechanisms of their remagnetization. The results are explained on the basis of the presented model assuming that the single domain particles rotate in such a way that their remagnetization during mould centrifugation is achieved by rotating the magnetization vector in a plane perpendicular to the easy axis, which minimises the energy loss in this process.