Time dependence in fine-particle systems

Abstract

The time dependent behaviour of the magnetisation of a non interacting ensemble of Stoner-Wohlfarth fine particles has been investigated. The particles typically represent the elongated single domain ferromagnetic particles that have widespread application in the magnetic recording industry. The particle ensemble considered exhibits both orientational texture and a distribution of particle volumes.
The coercivity, (i.e. the magnitude of the field that will just reduce the magnetisation of a saturated system to zero), is an important parameter used for characterising a recording medium, and consequently studies of its time dependence
are of great practical benefit. The work of Flanders et al (1987) and Oseroff et al (1987) considered two types of behaviour; the Rate Dependence due to a dynamic field process and the Time Dependence due to a static field process. Their observations have stimulated the computational and analytical studies described in this thesis.
The general case of the randomly orientated particles has been studied computationaly due to the complexity of the governing equation. The computational results for the static and dynamic field processes give good qualitative agreement with experiment and show a near logarithmic dependence of the coercive force on the field sweep rate. The simplified case in which the particle anisotropy axes are aligned with the applied field has been treated analytically.
This has provided a basis for the understanding of the experimental relationship between the rate-dependent and the time-dependent coercive fields. The parameters employed to study the irreversible changes in the dynamic and static field processes are the irreversible susceptibility Xirr = dli dh and the magnetic viscosity Sv = — dI/ dt. The ratio Sv/ Xirr is known as the fluctuation field H1, after Néel, the behaviour of which is not well understood.
The results from this study suggests that the orientational texture and volume ( or activation volume ) distribution of a material make an important contribution to this behaviour. Both these parameters contribute to a dependence on a distribution of particle energy barriers consistent with the analysis of Gaunt (1976).