Domain structures in single-crystal magnetite below the Verwey transition as observed with a low-temperature magnetic force microscope

Abstract

The magnetic domain structures on the {110} plane of magnetite (Fe3O4) below the Verwey transition (Tv=120K) were studied using a Low-Temperature Magnetic Force Microscope (LTMFM). At 298K, domain structures consisted of arrays of 180°, 109° and 71° walls, typical for magnetite with cubic anisotropy. At 77K (below Tv), the cubic style patterns disappeared and transformed into uniaxial patterns consistent with the uniaxial magnetocrystalline symmetry of the low-temperature monoclinic phase of magnetite. We also observed two distinct styles of domain patterns below Tv: (1) wide domains separated by straight 180° walls along the in-plane [100] easy axis; and (2) intricate wavy walls with reverse spike domains characteristic of out-of-plane easy axes. This intimate mixture of domain styles within adjacent areas of the crystal reflects variations in the direction of the magnetic easy axes in different regions produced by c-axis twinning of the crystal below Tv The thermal dependence of planar and wavy-wall patterns show little change from 77K until 110K, where patterns disappear. Upon cooling back to 77K, domain structures are different from the initial 77K states, indicating that renucleation of different domain states occurs by cycling near Tv.