Effect of the magnetic field applied during cooling on magnetic hysteresis in the low-temperature phase of magnetite: First-order reversal curve (FORC) analysis

Abstract

The strength of a magnetic field applied during cooling through the Verwey transition can control the low-temperature (LT) magnetic hysteresis properties of magnetite. This effect is investigated using the first-order reversal curve (FORC) technique. LT FORC distributions were measured at 20 K from four polycrystalline samples after cooling from 300 K in zero magnetic field or in the presence of a magnetic field (HFC) from a 0.01 to 1.5 T range. The samples represented nearly single-domain (SD), pseudo-singledomain (PSD), and multidomain (MD) magnetic hysteresis behavior. The LT FORC distributions measured from PSD magnetite after zero field cooling are characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field (Hu = 0) axis on a FORC diagram. Application of an intermediate magnetic field (10 mT to 50 mT) during cooling results in a shift of the peaks to near symmetry with respect to the Hu = HFC line. The manifestation of the field-controlled offset of FORC distribution strongly depends on the direction of the field HFC; the effect is best expressed when a FORC diagram is measured parallel or antiparallel to the cooling field direction. For HFC > 100 mT, the upper peak disappears, and the LT FORC distribution becomes single-peaked and symmetrical with respect to the Hu = 0 axis. In contrast, no correlation between HFC and features of FORC distributions is observed for SD and MD magnetite. These experimental results provide further support to a phenomenological model which suggests that magnetic hysteresis behavior of monoclinic magnetite is controlled by an intrinsic interplay between the magnetic and crystallographic twin domain structures. Copyright 2007 by the American Geophysical Union.