DOMAIN STATES IN FINE PARTICLE MAGNETITE AND TITANOMAGNETITE

The domain state (single domain versus multidomain) of fine magnetic particles has long been associated with their remanent and hysteresis properties. Both domain observations and micromagnetic theory indicate that a large number of local energy minimum (LEM) domain states are available to single domain and multidomain particles. Theoretical calculations based on a simple model of domain structure are made to determine the particle size, mineralogical, stress and temperature dependence of the LEM domain states available to fine particles of magnetite and titanomagnetite (TM56). The global (or absolute) energy minimum (AEM) domain state depends primarily on the balance between magnetostatic and exchange energies and shows only a weak temperature dependence. The range of local energy minima (LEM) domain states depends primarily on the anisotropy energies (magnetocrystalline and magnetoelastic) and is strongly temperature dependent. From the evolution of domain structure as a function of temperature and grain size implied from the domain state calculations, the relative stabilities of Thermoremanent Magnetization (TRM), grain growth Chemical Remanent Magnetization (CRM) and trans-domain Viscous Remanent Magnetization (VRM) are discussed. The magnetic behavior of an ensemble of fine particles will depend on the LEM states initially occupied by the particles.