Interactions and thermal effects in systems of fine particles: a Preisach analysis of CrO2 audio tape and magnetoferritin

We present an analysis and comparison of magnetization and remanence data from two particulate systems: commercial CrO₂ audiotape and a frozen suspension of magnetoferritin particles. The measurements were performed over a range of temperatures, 25°C⩽T⩽120°C for CrO₂ and 5 K⩽T⩽10 K for magnetoferritin, and over a range of fields sufficient to saturate the remanence at each temperature. The CrO₂ tape was prepared by both ac demagnetization and by thermal demagnetization from above the Curie temperature, while the magnetoferritin was thermally demagnetized from above its highest blocking temperature. The data were analyzed within the framework of a scalar Preisach model that included thermally activated relaxation and a reversible nonlinearity. In the case of CrO ₂, our fits showed that changes in hysteresis with temperature were due primarily to thermal variations in the spontaneous moment and the anisotropy of the particles, and in their magnetostatic interaction fields, and hence to changes in the free-energy landscape. In magnetoferritin, the temperature dependence of the magnetic response was due almost exclusively to changes in the relative populations of blocked and unblocked particles which relax by thermal activation over a fixed distribution of energy barriers. Our analysis of interaction effects was based on the Wohlfarth-Henkel parametric plot of the principal magnetizing and demagnetizing remanences. These plots showed curvature indicative of demagnetizing-like interactions in both thermally demagnetized magnetoferritin and ac-demagnetized CrO₂ , but essentially no curvature in thermally demagnetized CrO₂ . Our analysis revealed that there were no long-range, mean field interactions in either system, but that thermal demagnetization of CrO ₂ produced a random initial state that was insensitive to fluctuations in the interaction field due to disorder, provided that mean field effects were absent, while thermal demagnetization of magnetoferritin produced a highly asymmetric initial state resembling that induced by ac demagnetization, which was sensitive to these fluctuations