Micromagnetic models of pseudo-single domain grains of magnetite near the Verwey Transition

Abstract

Domain structures of small pseudo-single domain (PSD) magnetite near the Verwey transition (Tv) at ≈120 K were modeled using an unconstrained three-dimensional micromagnetic algorithm. The single-domain (SD) threshold (do) for the monoclinic phase below Tv was calculated to be ≈0.14 μm at 110 K. However, it is postulated that as a result of the very high energy barriers in the monoclinic phase, grains near do in size and in vortex states are unlikely to denucleate domain walls to become SD. Low-temperature cycling of saturation isothermal remanence (SIRM), thermoremanence (TRM), and partial TRM (pTRM) through Tv was simulated. Domain structures were found to align along the monoclinic "easy" magnetocrystalline anisotropy axis, i.e., the c axis, on simulated cooling through Tv. This process was found to "destroy" SIRM structures giving rise to demagnetization; however, for TRM and pTRM structures only "closure" domains were removed increasing magnetostatic leakage giving rise to a reversible anomaly in rough agreement with experimental studies. SIRM displayed a smaller anomaly at Tv, in agreement with experimental studies. Copyright 1999 by the American Geophysical Union.