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Multiferroicity and electron-glass state in charge ordered magnetite



Magnetite is famous for being the first magnetic material known to mankind and for showing the Vervey transition, a metal-insulator transition of so far unknown origin, which is believed to be closely related to the occurrence of charge order. Additionally indications for even another startling property of magnetite were found, namely the occurrence of ferroelectricity, which would make this material a multiferroic. (see, e.g., G. T. Rado and J. M. Ferrari, Phys. Rev. B 12, 5166 (1975) and Y. Miyamoto et al., Solid State Commun. 89, 51 (1994)) Multiferroics have attracted enormous interest in recent years, both from a theoretical and application viewpoint. Interestingly, for the generation of ferroelectricity in magnetite an exotic charge-order driven mechanism was shown to be applicable. (see D. I. Khomskii, J. Mag. Mag. Mat. 306, 1 (2006)).

relaxation process
Figure 1: Temperature dependence of the dielectric constant of magnetite for various frequencies obtained with silver-paint (symbols) and sputtered gold contacts (lines). Comparing both data sets reveals electrode-dominated behavior at high temperatures and an intrinsic nature of the results at low temperatures (T<40K). The frequency-dependent maxima at low temperatures show a behaviour, characteristic of relaxor ferroelectricity. (F. Schrettle et al., Phys. Rev. B 83, 195109 (2011))

 

However, a thorough characterization of the dielectric properties of magnetite is still missing and, thus, we have performed a detailed investigation of high-quality single-crystals using dielectric spectroscopy in a broad frequency and temperature range. Indeed, we found convincing evidence for ferroelectric ordering in magnetite. However, quite unexpectedly our results reveal that the ferroelectricity of magnetite is not of simple canonic nature but shows the typical characteristics of so-called relaxor-ferroelectrics where the dipolar ordering is of short-range nature only (Fig. 1). Most importantly, our investigations lead to the surprising conclusion that the dipolar dynamics in magnetite, which in contrast to other relaxor ferroelectrics is of electronic instead of ionic nature, slows down in a glasslike manner and finally becomes dominated by tunneling at low temperatures (Fig. 2). Thus, here we have electron dynamics reaching timescales as slow as 100s and exhibiting a glass transition at about 15 K. This new exotic ground state can be regarded as a new state of matter, a true "electron glass".

relaxation times

Figure 2: Dielectric constant ε' and dielectric loss ε" of magnetite as function of frequency. The lines are fits with the Havriliak-Negami-formula. The temperature dependence of the relaxation time obtained from the fits (inset) reveals glass-like slowing down of the relaxation dynamics. Obviously, timescales as slow as 100s are reached at the lowest temperatures investigated , which is an amazingly large value for electron dynamics. (F. Schrettle et al., Phys. Rev. B 83, 195109 (2011))

 

To learn more, see:

Relaxor ferroelectricity and the freezing of short-range polar order in magnetite
F. Schrettle, S. Krohns, P. Lunkenheimer, V. A. M. Brabers, and A. Loidl
Phys. Rev. B. 83, 195109 (2011)