Speed limit of the insulator–metal transition in magnetite (original) (raw)
Related papers
Electrically driven phase transition in magnetite nanostructures
Nature Materials, 2008
In 1939 Verwey[3] found that bulk magnetite undergoes a transition at T V ≈ 120 K from a high temperature "bad metal" conducting phase to a low-temperature insulating phase. He suggested[4] that high temperature conduction is via the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering upon cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial.[5, 6, 7, 8, 9, 10, 11]
Charge and Orbital Correlations at and above the Verwey Phase Transition in Magnetite
Physical Review Letters, 2008
The subtle interplay among electronic degrees of freedom (charge and orbital orderings), spin and lattice distortion that conspire at the Verwey transition in magnetite (Fe 3 O 4) is still a matter of controversy. Here, we provide compelling evidence that these electronic orderings are manifested as a continuous phase transition at the temperature where a spin reorientation takes place at around 130 K, i.e., well above T V % 121 K. The Verwey transition seems to leave the orbital ordering unaffected whereas the charge ordering development appears to be quenched at this temperature and the temperature dependence below T V is controlled by the lattice distortions. Finally, we show that the orbital ordering does not reach true long range (disorder), and the correlation length along the c-direction is limited to 100 Å .
Physical Review B, 2007
The Verwey phase transition in magnetite has been analyzed using the group theory methods. It is found that two order parameters with the symmetries X3X_3X3 and Delta5\Delta_5Delta5 induce the structural transformation from the high-temperature cubic to the low-temperature monoclinic phase. The coupling between the order parameters is described by the Landau free energy functional. The electronic and crystal structure for the cubic and monoclinic phases were optimized using the {\it ab initio} density functional method. The electronic structure calculations were performed within the generalized gradient approximation including the on-site interactions between 3d electrons at iron ions -- the Coulomb element UUU and Hund's exchange JJJ. Only when these local interactions are taken into account, the phonon dispersion curves, obtained by the direct method for the cubic phase, reproduce the experimental data. It is shown that the interplay of local electron interations and the coupling to the lattice drives the phonon order parameters and is responsible for the opening of the gap at the Fermi energy. Thus, it is found that the metal-insulator transition in magnetite is promoted by local electron interactions, which significantly amplify the electron-phonon interaction and stabilize weak charge order coexisting with orbital order of the occupied t2gt_{2g}t2g states at Fe ions. This provides a scenario to understand the fundamental problem of the origin of the Verwey transition in magnetite.
Journal of the Less Common Metals
The principal features of the Verwey transition in magnetite have been simulated by adopting elements of order-disorder theory. In certain limiting cases we obtain the model of Strlssler and Kittel which had previously been used to rationalize the electronic and thermodynamic properties of magnetite. According to the present microscopic model the discontinuous Verwey transition in magnetite is driven by a change in a highly correlated electron system with temperature from a charge-ordered small-polaron state associated with local lattice deformations to a disordered state in which electrons resonate between Fez+ and Fe3+ ions located on the octahedral cationic sites.
Order-disorder phase transition on the (100) surface of magnetite
Physical Review B, 2013
Using low-energy electron diffraction, we show that the room-temperature ( √ 2 × √ 2)R45 • reconstruction of Fe3O4(100) reversibly disorders at ∼450 • C. Short-range order persists above the transition, suggesting that the transition is second order and Ising-like. We interpret the transition in terms of a model in which subsurface Fe 3+ is replaced by Fe 2+ as the temperature is raised. This model reproduces the structure of antiphase boundaries previously observed with STM as well as the continuous nature of the transition. To account for the observed transition temperature, the energy cost of each charge rearrangement is 82 meV. arXiv:1310.5979v1 [cond-mat.mtrl-sci]
Electrically driven phase transition inmagnetite nanostructurers
In 1939 Verwey[3] found that bulk magnetite undergoes a transition at T V ≈ 120 K from a high temperature "bad metal" conducting phase to a low-temperature insulating phase. He suggested[4] that high temperature conduction is via the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering upon cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial.[5, 6, 7, 8, 9, 10, 11]
Mechanism of the Verwey Transition in Magnetite
Physical Review Letters, 2006
By combining ab initio results for the electronic structure and phonon spectrum with the group theory, we establish the origin of the Verwey transition in Fe3O4. Two primary order parameters with X3 and Δ5 symmetries are identified. They induce the phase transformation from the high-temperature cubic to the low-temperature monoclinic structure. The on-site Coulomb interaction U between 3d electrons at Fe ions plays a crucial role in this transition—it amplifies the coupling of phonons to conduction electrons and thus opens a gap at the Fermi energy.
Origin of the Verwey Transition in Magnetite
Physical Review Letters, 2006
Comprehensive x-ray powder diffraction studies were carried out in magnetite in the 80–150 K and 0–12 GPa ranges with a membrane-driven diamond anvil cell and helium as a pressure medium. Careful data analyses have shown that a reversible, cubic to a distorted-cubic, structural transition takes place with increasing pressure, within the (P,T) regime below the Verwey temperature TV(P). The experimental documentation that TV(P)=Tdist(P) implies that the pressure-temperature-driven metal-insulator Verwey transition is caused by a gap opening in the electronic band structure due to the crystal-structural transformation to a lower-symmetry phase. The distorted-cubic insulating phase comprises a relatively small pressure-temperature range of the stability field of the cubic metallic phase that extends to 25 GPa.
Studies of the Verwey Transition in Magnetite
Acta Physica Polonica A, 2004
Studies of the specific heat and simultaneous AC magnetic susceptibility (χ) and electric resistance of stoichiometric magnetite single crystal are presented. The temperature hysteresis of the Verwey transition is of 0.03 K found from the specific heat data confirming its first-order character. The continuous temporal change of χ at T V can be switched off by an external magnetic field without affecting the transition. The electrical resistance decreases continuously with increasing temperature with a rapid change of slope at the point when the phase transition is completed. It was concluded that the magnetic degrees of freedom do not actively participate in the transition and that the entropy released at TV may come from ordering electrons.
Phase separation in the non-equilibrium Verwey transition in magnetite
We present equilibrium and out-of-equilibrium studies of the Verwey transition in magnetite. In the equilibrium optical conductivity, we find a step-like change at the phase transition for photon energies below about 2 eV. The possibility of triggering a non-equilibrium transient metallic state in insulating magnetite by photo excitation was recently demonstrated by an x-ray study. Here we report a full characterization of the optical properties in the visible frequency range across the nonequilibrium phase transition. Our analysis of the spectral features is based on a detailed description of the equilibrium properties. The out-of-equilibrium optical data bear the initial electronic response associated to localized photo-excitation, the occurrence of phase separation, and the transition to a transient metallic phase for excitation density larger than a critical value. This allows us to identify the electronic nature of the transient state, to unveil the phase transition dynamics, and to study the consequences of phase separation on the reflectivity, suggesting a spectroscopic feature that may be generally linked to out-of-equilibrium phase separation.