Abstract
Single crystalline magnetite Fe 3 O 4 was investigated at low temperatures in the charge ordered state by electric measurements and time-resolved diffraction with voltage applied in-situ. Dielectric spectroscopy indicates relaxor ferroelectric characteristics, with polarization switching observably only at sufficiently low temperatures and in a suitably chosen time-window. PUND measurements with a ms time scale indicate a switchable polarization of about 0 . 6 μ C / cm 2 . Significant switching occurs only above a threshold field of about 3 kV / mm , and it occurs with a time delay of about 20 μ s . The time-resolved diffraction experiment yields, for sufficiently high voltage pulses, a systematic variation by about 0 . 1 % of the intensity of the ( 2 , 2 ¯ , 10 ¯ ) Bragg reflection, which is attributed to structural switching of domains of the non-centrosymmetric C c structure to its inversion twins, providing proof of intrinsic ferroelectricity in charge ordered magnetite.
Highlights
Magnetite Fe3 O4 is a remarkable transition metal oxide
A high-quality single crystal of magnetite was investigated at low temperatures by electric measurements and time-resolved diffraction with voltage applied in-situ
PUND measurements suggest that a suitable time scale is a cycle of 1 ms, in which case a switchable polarization of about 0.6 μC/cm2 can be distinguished from non-switching contributions to the current response
Summary
Magnetite Fe3 O4 is a remarkable transition metal oxide It is the oldest known magnetic material, it gave rise to the terms “magnetic” and “magnetism”. It provides the classical example of a metal-insulator transition, which was found in 1939 around 120 K by Verwey [1], who attributed it to a charge ordering (CO) of the iron valences on the B sites of the inverted spinel structure of the compound. The low temperature CO structure of magnetite is extremely complex and has occupied generations of scientists, sparking plenty of controversy, including about the existence of CO [2]. The CO is almost centro-symmetric—exchanging the valences of only four iron pairs (hued red in the figure) would
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