High-temperature magnetic stability of small magnetite particles

Abstract

The stability of magnetic domain structures of small grains of magnetite were examined between room temperature and the Curie temperature using a high-resolution three-dimensional micromagnetic algorithm. At all times the minimum resolution used was determined by calculating the exchange length. Using an unconstrained model, the single domain (SD) to multidomain (MD) threshold grain size d 0 was found to be nearly independent of temperature up to ?450\,^{\circ}C. Above this temperature, d 0 was observed to rise sharply. Energy barriers between metastable domain states trapped in local energy minimums (LEM) were determined using a constrained algorithm. Three types of domain structure were considered: SD, vortex, and double vortex (effectively three domain), in a range of grain sizes with side length between 30 and 300 nm. In addition, the effect of varying shape was also considered by examining asymmetric grains with aspect ratios up to 1.4. From the numerical solutions energy barriers between LEM states were determined. It was found that MD grains 300 nm in size display higher stability than smaller SD grains (?50 nm). Double vortex states were found to be less stable than single vortex states at nearly all temperatures. Blocking temperatures as function of grain size for both symmetric and asymmetric grains were determined and agree well with experimental results. Transdomain thermoremanence analysis indicated that there are a limited number of grain sizes and shapes which will nucleate domain wall-type structures during cooling. Such nucleation events would cause the total measured remanence to decrease with cooling in conflict with N{é}el's analytical theory for remanence cooling behavior but in agreement with experimental observations.