Terahertz conductivity at the Verwey transition in magnetite

Abstract

The complex conductivity at the (Verwey) metal-insulator transition in Fe3O4 has been investigated at THz and infrared frequencies. In the insulating state, both the dynamic conductivity and the dielectric constant reveal a power-law frequency dependence, the characteristic feature of hopping conduction of localized charge carriers. The hopping process is limited to low frequencies only, and a cutoff frequency ν1 ≃ 8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielectric constant abruptly decreases and becomes negative. Together with the conductivity spectra this indicates a formation of a narrow Drude-peak with a characteristic scattering rate of about 5 meV containing only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral weight from the hopping process to the free-carrier conductivity. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy.

Glass-like and Verwey transitions in magnetite in details

“…1, best fitting gives dDðTÞ=dT % 400 meV=K and D % 40 meV. This result is consistent with measurements of the infrared complex conductivity which show that above T V the thermally activated quasi-free carriers (gap % 40 meV) occupy a narrow band and charge transport is Drude like conductivity in a high mobility semiconductor [2]. Spin glass-like AC response is observed in temperature range 25250 K for ''audio frequency'' driving AC field, see Fig.…”

“…In summary, ''zero-field'' measurements of AC susceptibility show linear gap opening on warming through Verwey transition consistent with recent experimental and theoretical results [2,6]. Below transition the thermally activated charge hopping is linked with nonzero susceptibility.…”

“…On cooling, the crystal structure slightly distorts from cubic to monoclinic at Verwey transition. Verwey transition is attended by increase of the semiconductor gap on cooling rather than MIT [2]. Consequently, charge transport and spin response change considerably.…”

ESR of the quasi-two-dimensional antiferromagnet CuCrO2with a triangular lattice

“…The high frequency branch of the spectra was studied using the quasi-optical technique 17,18 . In the case of magnetic excitations the transmission through the sample can be obtained as a function of either temperature, frequency, or external magnetic field 19 .…”

Magnetic susceptibility studies of the spin-glass and Verwey transitions in magnetite nanoparticles

“…The origin of this drop in electrical resistance is still under discussion, but it is assumed that there occurs a charge order-disorder transition in crystal structure. [6][7][8] The other interesting transformation in magnetite is the so called spin-glass transition, where a transition from high temperature ferromagnetic state to low temperature paramagnetic state happens at the grain surfaces. This is, below temperature of spin-glass transition, T sg , grain-boundary spins freeze, and a drop in magnetization would take place.…”