Electrically driven phase transition inmagnetite nanostructurers (original) (raw)
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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 .
Magnetoresistance in magnetite: Switching of the magnetic easy axis
Journal of Alloys and Compounds, 2009
The influence of the external magnetic field B (B ≤ 4 T) on resistivity in magnetite single crystal was studied at few temperatures both below and above the Verwey transition temperature T V , and in two 1 0 0 type cubic directions. We have succeeded to confirm our predictions that the magnetic axis switching affects electronic transport. It was also found that the transverse resistivity (B ⊥ current direction) exceeds longitudinal one, but only for T > T V , with no obvious systematics below the transition temperature. This may indicate that atomic disorder, inherent and poorly defined in a material with structural domains, is the primary factor that governs transport properties below the transition temperature. Finally, and concomitant with the last statement, only very small magnetic field dependence in magnetite below T V may be inferred from our data.
Relaxor ferroelectricity and the freezing of short-range polar order in magnetite
Physical Review B, 2011
A thorough investigation of single crystalline magnetite using broadband dielectric spectroscopy and other methods provides evidence for relaxorlike polar order in Fe 3 O 4 . We find long-range ferroelectric order to be impeded by the continuous freezing of polar degrees of freedom and the formation of a tunneling-dominated glasslike state at low temperatures.
Speed limit of the insulator–metal transition in magnetite
Nature Materials, 2013
As the oldest known magnetic material, magnetite (Fe 3 O 4 ) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown 1 , magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulatormetal, or Verwey, transition has long remained inaccessible 2-8 . Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the lowtemperature insulating electronically ordered phase 9 . Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5 ± 0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics 10 .
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.
Charge order at magnetite Fe₃O₄(0 0 1): surface and Verwey phase transitions
Journal of physics. Condensed matter : an Institute of Physics journal, 2015
At ambient conditions, the Fe3O4(0 0 1) surface shows a (√2 × √2)R45° reconstruction that has been proposed as the surface analog of the bulk phase below the Verwey transition temperature, T(V). The reconstruction disappears at a high temperature, T(S), through a second order transition. We calculate the temperature evolution of the surface electronic structure based on a reduced bulk unit cell of P2/m symmetry that contains the main features of the bulk charge distribution. We demonstrate that the insulating surface gap arises from the large demand of charge of the surface O, at difference with that of the bulk. Furthermore, it is coupled to a significant restructuration that inhibits the formation of trimerons at the surface. An alternative bipolaronic charge distribution emerges below T(S), introducing a competition between surface and bulk charge orders below T(V).
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]
Charge-orbital ordering in low-temperature structures of magnetite: GGA+U investigations
Physical Review B, 2006
The atomic and electronic structure of magnetite ͑Fe 3 O 4 ͒ in the four possible low-temperature structures, namely, P2/c − Pmca ͑I͒, Pmca ͑II͒, Pmc2 1 ͑III͒, and Cc ͑IV͒, have been investigated by generalized gradient approximation+ Hubbard U ͑GGA+ U͒ electronic structure and structural optimization calculations. Chargeorbital ordering is found to exist in all the four structures. The charge-orbital ordering and hence the Verwey metal-insulator transition is shown to be driven by the on-site Fe d-electron correlation. The theoretical charge-orbital ordering patterns in the I, II, and III structures do not satisfy the Anderson criterion but are consistent with recent neutron and x-ray diffraction experiments. The IV ͑Cc͒ structure is found to be the ground state structure. In the IV structure, the charge-orbital ordering on 3 / 4 of the tetrahedra does not satisfy the Anderson condition, while on 1 / 4 of the tetrahedra it does. The observed entropy change at the Verwey transition, which has been a long standing puzzle, is analyzed and found to be consistent with the chargeorbital orders obtained here.
Aspects of the Magnetite Crystalline Structure a Brief Overview of Nanomagnetism
Nucleus, 2019
This article aims to present briefly the relation of the origin of the magnetic behavior at nanoscale with the influence of the crystalline structure of the respective particulate material in question-[magnetite (Fe 3 O 4)]. In general, we intend to give an insight into the understanding of the properties and fundamentals of the macroscopic interactions of magnetic solids in a microscopic picture.