Effect of surface polishing and oxidization induced strain on electronic order at the Verwey transition in Fe3O4

International audienceFollowing the controversy between two previous publications (Lorenzo et al 2008 Phys. Rev. Lett. 101 226401 and Garcia et al 2009 Phys. Rev. Lett. 102 176405), we report on the influence of mechanical polishing, and subsequent sample storage, on the electronic order at the Verwey transition of highly pure magnetite, Fe3O4, by resonant x-ray scattering. Contrary to expectations, mechanically polishing the surface induces an inhomogeneous micron deep dead layer, probably of powdered Fe3O4. In addition, we have found that polishing the sample immediately before the experiment influences and favors the appearance of long range order electronic correlations, whereas samples polished well in advance have their electronic order quenched. Conversely, lattice distortions associated with the Verwey transition appear less affected by the surface state. We conclude that mechanical polishing induces stresses at the surface that may propagate into the core of the single crystal sample. These strains relax with time, which affects the different order parameters, as measured by x-ray resonant diffraction

Inhomogeneity-induced second-order phase transitions in Potts model on hierarchical lattices

The thermodynamics of the $q$-state Potts model with arbitrary $q$ on a class of hierarchical lattices is considered. Contrary to the case of the crystal lattices, it has always the second-order phase transitions. The analytical expressions fo the critical indexes are obtained, their dependencies on the structural lattice pararmeters are studied and the scailing relations among them are establised. The structural criterion of the inhomogeneity-induced transformation of the transition order is suggested. The application of the results to a description of critical phenomena in the dilute crystals and substances confined in porous media is discussed.Comment: 9 pages, 2 figure

Heat Capacity and Neutron Diffraction of Low Doped Fe3-xZnxO4

We report on the heat capacity and neutron diffraction studies of Fe3-xZnxO4 (w<0.04) series. Adiabatic heat capacity measurements, performed on single crystals, confirm our earlier observations that in the range x<0.012 the Verwey transition is of first order, whereas for 0.012<x<0.035 the transition character changes to second order. The change of transition order is accompanied by the clear difference in heat capacity baselines below Verwey transition temperature Tv. The time-of-flight neutron diffraction data were collected at the High Resolution Powder Diffractometer in ISIS in the T range 4-240 K for two Fe3-xZnxO4 samples with x falling within first and second order regime. The comparison of spectra for both samples shows that second order sample undergoes much smaller structure change than that observed for first order sample what was suggested by our heat capacity measurements results

Charge ordering and elastic constants in Fe 3-x Zn x x O4

PACS. 71.30.+h Metal-insulator transitions and other electronic transitions, 62.65.+k Acoustical properties of solids, 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.),

Experimental and theoretical studies of vibrational density of states in Fe3O4 single-crystalline thin films

This paper presents experimental and theoretical studies of lattice vibrations in a single-crystalline Fe3O4(001) thin film. The investigations were carried out in order to see how the lattice dynamics changes at the Verwey transition. Vibrational densities of states (DOS) were obtained from nuclear inelastic scattering (NIS) of synchrotron radiation in the temperature range 25 to 296 K, while theoretical DOS were calculated ab initio within density functional theory. Experimental phonon density of states shows good agreement with calculated DOS, reproducing both the general features of main line groups as well as the groups' structure. This is also in qualitative accord with heat capacity data, provided that experimental DOS is augmented with that calculated for oxygen atoms. We have observed a gradual change in the NIS raw data as well as the relevant DOS while lowering the temperature. In particular, the main peak in the energy region 15-25 meV shows increasing splitting on cooling. The Lamb-Mossbauer factor calculated in the course of DOS evaluation shows a pronounced drop in the vicinity of the Verwey transition that may be partly connected to the observed abrupt lowering of the count rate at approximately 7 meV for T <T-V. Since this is an indication of lattice stiffening below T-V, we conclude that we have found experimental evidence for lattice participation in the mechanism leading to the Verwey transition