Spin moment over 10–300K and delocalization of magnetic electrons above the Verwey transition in magnetite

Abstract

In order to probe the magnetic ground state, we have carried out temperature dependent magnetic Compton scattering experiments on an oriented single crystal of magnetite (Fe3O4), together with the corresponding first-principles band theory computations to gain insight into the measurements. An accurate value of the magnetic moment µS associated with unpaired spins is obtained directly over the temperature range of 10-300K. µS is found to be non-integral and to display an anomalous behavior with the direction of the external magnetic field near the Verwey transition. These results reveal how the magnetic properties enter the Verwey energy scale via spin-orbit coupling and the geometrical frustration of the spinel structure, even though the Curie temperature of magnetite is in excess of 800 K. The anisotropy of the magnetic Compton profiles increases through the Verwey temperature Tv and indicates that magnetic electrons in the ground state of magnetite become delocalized on Fe B-sites above Tv.

X-ray magnetic circular dichroism study of epitaxial magnetite ultrathin film on MgO(100)

“…The presence of integer m spin and vanishing orbital moment (m orb ) of magnetite are expected as an indication for a B-site minority electron conduction mechanism, and its accompanied full spin polarization at the E F . However, concerning with that, controversial results have been reported utilizing techniques such superconducting quantum interference device (SQUID) magnetometer, 11,38 XMCD, 12,27,30,39,38 and magnetic Compton scattering (MCS) 33,34 and calculations with local density approximation (LDA), 30 LDA+U, 30 local spin density approximation (LSDA)+U, 2 and moment analysis etc.. 12 The fundamental magnetic properties of Fe 3 O 4 show strong dependence on the sample preparation methodology. By simultaneous oxidation, Babu et al 32 34 and all the way down to −0.001μ B /f.u.…”

Large hidden orbital moments in magnetite

“…From a simplified view one would always expect the presence of a net orbital magnetic moment related to the Fe 2þ electrons at the B site, while all Fe 3þ is in a nominally filled majority shell and a 3d 5 configuration, with Hund's rules compensated orbital moments. By the use of X-ray magnetic circular dichroism (XMCD) sum rules [13] and also by Compton scattering [14], such an orbital moment has indeed been observed. On the other hand, more recent results, again utilizing XMCD [5,15,16] and Compton scattering [17], only found a clear vanishing orbital moment.…”

Dependence of magnetization on crystal fields and exchange interactions in magnetite

“…Thus, all the A site moments are oriented antiparallel to all the B site moments. Also, it may be related to the effect of geometrical frustration on magnetic properties above T v [21]. Magnetite reaches the maximum of conductivity, which was found experimentally up to 300 K [22] and theoretically at 383 K [23].…”