Magnetotransport anisotropy effects in epitaxial magnetite(Fe3O4)thin films

Epitaxial films of magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ have been grown on ${\mathrm{SrTiO}}_3$ (100) and sapphire (α-${\mathrm{Al}}_2{\mathrm{O}}_3,$ 0001) substrates by pulsed laser deposition, and they exhibit crystal orientations of [100] and [111], respectively. Films of both orientations show a clear Verwey transition near 120 K. The temperature dependence of magnetoresistance of the films is examined at fields up to 8.5 T applied parallel to the film plane. The [100] oriented films show a pronounced extremum in magnetoresistance (MR) at the Verwey transition temperature ${(T}_v),$ while no such feature is observed in the case of the [111] oriented films. However, the [111] oriented films exhibit a sudden change in the MR dependence on $T$ at $T_v$ and films of both orientations show a rapid increase of MR as the temperature is decreased below about 110 K. An attempt is made to analyze these data in terms of the known anisotropy in magnetostriction and optical magnon-phonon dispersions, and the small polaron band and hopping conductivity mechanisms.