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.

Changes in remanence, coercivity and domain state at low temperature in magnetite

“…5). This is approximately true even for the profound changes in magnetic properties and domain structure that occur in crossing the Verwey transition [3,29]. From Table 1, H c for our 1.3 mm crystal increases from 0.13 to 1.1 mT between 295 and W15 K, while M rs /M s increases by a similar factor, from 0.003 to 0.013.…”

“…First, the domain structure of monoclinic magnetite below T V is di¡erent from that of cubic magnetite above T V [3,29]. Below T V , spins lie along the [001] c-axis, domains are lamellar without closure structures, and walls are strongly pinned by crystal defects because of the high magnetostriction and crystalline anisotropy.…”

“…If all parts of the crystal have the same monoclinic c-axis, the entire crystal must distort. To reduce such megascopic strain, di¡erent parts of the crystal tend to form twins with di¡erent c-axes [40,41] and contrasting domain patterns [3]. Strain is accommodated at the boundaries between twinned regions, which are consequently under stress [42].…”

“…Moloni et al [3] were successful in imaging domains between 77 and 110 K on the {110} plane of discs cut from a large synthetic single crystal of magnetite, using a novel low-temperature magnetic force microscope. They found two types of domains, both typical of substances with strong uniaxial anisotropy.…”

A low temperature ultrahigh vaccum scanning force microscope

“…In literature we find a large variety of sensors which have been used in LTSFM instrumentation. Below we give a brief historical overview on LTSFM instrumentation, describing designs using tunneling, 9,10 fiber-optic interferometry, [11][12][13][14][15][16] piezoresistive cantilevers, 17 and laser beam deflection. 18 The first successfully working LTSFM built by Gießibl et al 9 was equipped with a tunneling tip as deflection sensor.…”