Spin dynamics of fine magnetic particles

Abstract

A computer model of fine magnetic particles has been developed based on the Landau-Lifshitz equation. It is a dynamic model which allows the detailed behaviour of the magnetic moments during magnetisation changes to be investigated.
The model was initially used to study gyroremanent magnetisation (GRM) which is observed in magnetic rocks. Stephenson proposed that GRM was caused by the gyroscopic precession of the moments. In collaboration with Stephenson a model was developed to show the complex motion of the moments in an alternating field and it was found that positive and negative cycles of the field produced an asymmetry in the motion.
This asymmetry leads to one of the easy magnetisation directions being favoured so that when the field is cycled to zero a remanence is produced, the observed GRM.
The model was then extended to include thermal agitation through the addition of a random field term in the simulation. It was found that in the absence of thermal agitation two particles always moved coherently but when thermal agitation was included other modes appeared. This work showed the importance of ncluding thermal agitation in any micromagnetic
model.
The model was then used to simulate ferromagnetic resonance on a single particle and the results precisely matched the results predicted by analytical theory. The effect of particle anisotropy and alignment on the response was then investigated and the computed results agreed with experimental measurements. The model was further extended to include dipolar
interactions firstly in a chain of spheres and then on an array of particles. Results agreed very well with experimental measurements on tapes with different packing densities and have helped to indicate the local fields acting in an assembly of fine magnetic particles.