Anomalous electric transport across Verwey transition in nanocrystalline Fe3O4 thin films

Abstract

Charge ordering (Fe +3 /Fe +2) is a key concept in the Verwey transition of Fe 3 O 4 because it frequently competes with functional properties (half-metallicity/ferromagnetism and structural transformation) and quantum confinement effect, especially at nanoscale dimensions. In this paper, we report the fabrication of nanocrystalline Fe 3 O 4 thin films via two different reduction routes, namely, vacuum annealing and wet H 2 annealing. While vacuum annealed films exhibit Verwey transition and resistivity values comparable to bulk Fe 3 O 4 , the same is not observed in electric transport properties of wet H 2 annealed films. However, this transition was visible in the magnetic characteristics exhibited by both the films though realized via different routes. This observation indicates the possibility of charge and spin ordering as two independent phenomena, and it is a coincidence that happens at the same Verwey transition region. It is seen that a crossover from thermally activated hopping (300-120 K) to Mott variable range hopping (VRH) (across Verwey transition) and then to Shklovskii-Efros VRH hopping (70-30 K) via the conduction mechanism takes place in vacuum annealed films in contrast to the typical semiconducting behavior (300-50 K) expected of wet H 2 annealed films. Different electric transport properties in both varieties of Fe 3 O 4 films could be ascribed to the electronic disorder/defects affecting charge ordering Fe +3 /Fe +2 and trimerons (Fe +3-Fe +2-Fe +3).

A Short Review on Verwey Transition in Nanostructured Fe3O4 Materials

“…This transition can also happen even if there is no structural transition but if there is a change in anisotropy. We now argue that charge order is triggered by the structural transition and it has nothing to do with magnetic ordering, as discussed by us elsewhere [28]. So the absence of VT in electronic transport may have a different origin like grain boundary volume, and defects/vacancies induced intergranular strain in nanocrystalline Fe 3 O 4 films that suppress the resistivity change caused during VT [28].…”

“…[23][24][25]. Previously, the author has worked extensively on oxide films, and we consider that the PLD [26][27][28] is the most suitable technique for stoichiometric growth of complex oxides. The deposition of Fe 3 O 4 has been performed in a reducing atmosphere (under the vacuum of 1 × 10 −5 mbar and substrate temperature, T S : RT-850°C) from the α-Fe 2 O 3 target on an amorphous quartz substrate [26].…”

Three-Dimensional Nanoconfinement Supports Verwey Transition in Fe3O4 Nanowire at 10 nm Length Scale

“…Basically, the fluctuation of electrons freezes below the Verwey transition and results in trimerons through the localization of electrons, which are distributed over the three-Fe-site unit [Fe 3+ –Fe 2+ –Fe 3+ ] with an approximate coherence length of 0.63 nm. Importantly, the low-temperature resistivity data of the Fe 3 O 4 nanowire with the volume of 50 × 50 × 10 nm 3 provide a better estimation of the coherence length of a trimeron than other reported values obtained from thin films , because of the accessibility of a small number of trimerons in an ultrasmall Fe 3 O 4 nanowire with a small number of defects. The observation of the coherence length of a trimeron in addition to the good physical properties of an ultrasmall Fe 3 O 4 nanostructure are very attractive from both fundamental and application perspectives.…”

Track the bands: Verwey phase transition in single magnetite nanocrystals

“…It has been described as a first-order metal-insulator transition accompanied by a structural phase transition during which the cubic symmetry of the Fe 3 O 4 crystal is broken by a small lattice distortion while cooling through the transition temperature of 123 K [2][3][4]. Verwey transition in Fe 3 O 4 nanocrystals has been observed by means of magnetization versus temperature measurements [5][6][7], photoelectron spectroscopy [8,9], electron paramagnetic resonance [10], electric transport characterization [11], and also by heat capacity measurements [12]. Recently, evo lution of magnetic domains [13] and anisotropic magnetoresistance [14] have been reported in epitaxial magnetite thin-films across the transition.…”