Magnetically induced structural reorientation in magnetite studied by nuclear magnetic resonance

Abstract

Phenomenon related to low symmetry phase of magnetite Fe3O4 below the Verwey transition is the switching of magnetic easy axis by external magnetic field connected with a structural transition. Results of nuclear magnetic resonance (NMR) studies of axis switching are presented, preceded by careful characterization by magnetic measurements. We detect changes in the F57e NMR spectra that evidence the structural transition interrelated with the axis switching. We also observe the switching process in time and analyze the dynamics of the phenomenon. Phenomenological approach according to Kolmogorov–Johnson–Mehl–Avrami is employed to find a link between the experimental data from magnetization measurements and NMR, showing that the effect observed by these two methods has the same origin. Additionally, NMR identifies separately the unswitched and switched phase during the switching process.

Short-Range Correlations in Magnetite above the Verwey Temperature

“…Similar low-temperature (low-T ) processes were revealed by microscopic probes [40,41] for samples prepared in a different way. Thus, electronic processes that cause diffuse scattering result from the intrinsic properties of magnetite, and not from particular preparation conditions.…”

Magnetite (Fe3O4): a new variant of relaxor multiferroic?

“…These might be either electron tunneling and electron hopping processes [64], glassy polar degrees of freedom [46] or thermally activated structural processes [39]. A comparison of the activation energies of the dielectric Debye process (31 meV (this work), 40 meV [49]), of easy-axis switching (25-33 meV [10,39]) and of domain-wall motion in single crystals (40 meV [65]) indicates a connection between these phenomena. Since the easy-axis direction is stabilized by the film-substrate strain, it would be easier to observe a ferroelectric polarization in thin films than in bulk.…”

“…• Studies of the easy-axis switching in magnetite [10,[38][39][40] revealed thermally activated behavior with an activation energy in the range 25-33 meV.…”

A Short Review on Verwey Transition in Nanostructured Fe3O4 Materials

“…However, it is surprising that the electric polarization does not already develop at the VT. This is because in between 40 K and T V , the electric polarization is suppressed by relaxation processes due to electron tunnelling and electron hopping processes [53], glassy polar degrees of freedom [54], or thermally activated structural processes [55]. Since the easy-axis direction is generally stabilized by the film-substrate strain, it would be easier to observe a ferroelectric polarization in thin films than in bulk.…”