Micromagnetic models of pseudo‐single domain grains of magnetite near the Verwey transition

Domain structures of small pseudo‐single domain (PSD) magnetite near the Verwey transition ( T υ ) at ≈120 K were modeled using an unconstrained three‐dimensional micromagnetic algorithm. The single‐domain (SD) threshold ( d 0 ) for the monoclinic phase below T υ 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 d 0 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 T υ was simulated. Domain structures were found to align along the monoclinic “easy” magnetocrystalline anisotropy axis, i.e., the c axis, on simulated cooling through T υ . 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 T υ , in agreement with experimental studies.