Low-temperature properties of a single crystal of magnetite oriented alongprincipal magnetic axes

We have measured saturation induced and remanent magnetizations and induced magnetization as a function of field at low temperatures, between 300 K an d 10 K, on an oriented 1.5-mm single crystal of magnetite. The induced magn etization curves along the cubic [001], [1<(1) over bar>0], and [1<(1) over bar>0] axes at 10 K have very different approaches to saturation. The crys tal is easy to magnetize along [001] but difficult along [110] and [110], t he hard directions of magnetization for monoclinic magnetite. The temperatu re dependence of saturation magnetization between the Verwey transition tem perature,T-v=119 K, and 10 K is also different along the three axes, indica ting that below T-v the crystal has uniaxial symmetry. The room-temperature saturation remanence (SIRM) produced along [001] decreases continuously in the course of zero-field cooling, levelling out at the isotropic temperatu re, T-i = 130 K, where the first magnetocrystalline anisotropy constant bec omes zero. At Ti 86% of the initial SIRM was demagnetized. The domain wall pinning responsible for this soft remanence fraction must be magnetocrystal line controlled. The remaining 14% of the SIRM is temperature independent b etween T-i and T-v and must be magnetoelastically pinned. This surviving ha rd remanence is the core of the stable magnetic memory. The Verwey transiti on at 119 K, where the crystal structure changes from cubic to monoclinic, is marked by a discontinuous increase in remanence, indicating that the cub ic [001] direction suddenly becomes an easy direction of magnetization. The formation of monoclinic twins may also affect the intensity of remanence b elow T-v. Reheating from 10 K retraces the cooling curve, with a decrease a t T-v back to the original remanence level, which is maintained to 300 K. W hen SIRM is not along [001], the initial SIRM is larger but the reversible changes across the Verwey transition are much smaller. The SIRM produced at 20 K is an order of magnitude larger than the 300 K SIRM, but the only cha nge during warming is a discontinuous and irreversible drop to zero at T-v. (C) 1999 Published by Elsevier Science B.V. All rights reserved.