Temperature dependence of hysteresis in 0.04–0.22 μm magnetites and implications for domain structure

Abstract

Initial susceptibility χ 0, second Rayleigh coefficient B 0, coercive force H c, remanent coercive force H R, and near-saturation remanent and induced magnetizations J rs and J s have been measured at a series of temperatures between ambient and 575° C for dispersed submicron magnetites with mean crystal sizes of 0.037, 0.076, 0.10 and 0.22 μm.χ 0 and B 0 reach a Hopkinson peak just below the Curie point, and χ 0( T) data are well explained by reversible domain-wall displacements. Saturation remanence has three distinctive characteristics: (1) its dependence on particle size d changes smoothly from d −0.75 at room temperature to d −0.6 at 500° C; (2) J rs( T) varies as H c( T) up to 300° C and as H K( T) at higher temperatures ( H K is microscopic coercive force corrected for thermal fluctuations); (3) J rs/ J s decreases steadily as the temperature rises. Neither failure of domain-wall nucleation following saturation nor domain-wall moments can reconcile these observations, but they are explained in a natural way by domain wall displacements, particularly if the average number of domain walls per particle increases at high temperature. Other experimental evidence indicates that the equilibrium single-domain to two-domain transition size remains essentially constant up to 500° C. However, apparent demagnetizing factors N = H K/ J rs suggest a decrease in the 2-domain to 3-domain and 3-domain to 4-domain transition sizes at high temperatures.