Verwey Phase Research Articles

Order parameters in the Verwey phase transition

The Verwey phase transition in magnetite is analyzed on the basis of the Landau theory. The free energy functional is expanded in a series of components belonging to the primary and secondary order parameters. A low-temperature phase with the monoclinic P2/c symmetry is a result of condensation of two order parameters X3 and Δ5. The temperature dependence of the shear elastic constant C44 is derived and the mechanism of its softening is discussed.

Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study

The Verwey phase transition in magnetite has been analyzed using the group theory methods. It is found that two order parameters with the symmetries ${X}_{3}$ and ${\ensuremath{\Delta}}_{5}$ 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 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 $U$ and Hund's exchange $J$. 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 interactions 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 ${t}_{2g}$ states at Fe ions. This provides a scenario to understand the fundamental problem of the origin of the Verwey transition in magnetite.

Study of the Verwey transition in magnetite by low field and magnetically modulated non-resonant microwave absorption

We have investigated the Verwey phase transition (VPT) by two novel non-resonant microwave absorption techniques: low-field absorption (LFA) and magnetically modulated microwave absorption spectroscopy (MAMMAS). Measurements were carried out on sintered polycrystalline samples of Fe 3O 4, in the 77–300 K temperature range. LFA refers to the microwave absorption around the zero DC field range (−1000< H DC<+1000 G). LFA measurements showed an hysteretic behavior; a minimum in the hysteresis loop width was observed at 126 K, which can be associated with a minimum in magnetic anisotropy, as a consequence of charge localization below the VPT. MAMMAS experiments are based on the variations in microwave absorption (at constant H DC), as a function of temperature, and seem particularly well adapted to detect a wide range of phase transitions. In the magnetite case, a continuous increase in the microwave power absorption level was observed as temperature decreased, reaching a strong maximum at 130 K and a minimum at 100 K. An inflection point at 126 K was found, in very good agreement with LFA measurements. These results are discussed in detail.

Preparation of Magnetite Nanoparticles by Thermal Decomposition of Hematite Powder in the Presence of Organic Solvent

ABSTRACTMagnetite nanoparticles have been synthesized by thermal decomposition of hematite (Fe2O3) powder in the presence of high boiling point solvent. The mixture of hematite and 1- octadecene solvent was heated and stirred in nitrogen gas at the temperature of 320 °C for the desired time (∼2 to 28 hrs). The influence of the reaction time on transformation process was analyzed with X-ray diffraction (XRD), Mössbauer spectroscopy (MS), and magnetic measurements. XRD patterns show that the phase of intermediate was composed of spinel phase and corundum phase (α-Fe2O3). The 57Fe Mössbauer spectra show that the spinel phase originated from the magnetite particles. The structure transformation proportion of hematite to magnetite strongly depends on reaction times. After reflux for 28 hrs the hematite-magnetite transformation was complete. The mean crystallite size of pure phase of magnetite particles is about 40 nm. The saturation magnetization increases with the reaction time, which corresponds to an increase of concentration of magnetite in the samples. A pronounced feature of the Hc and σr/σs observed in samples is the steplike change which appears at 125 K and is characteristic of the Verwey transition. The hyperfine parameters of Mössbauer spectrum measured at low temperature also indicate that the Verwey phase transition occurs. In other words, the Verwey transition is an indication that the magnetite particles exactly grew up in the synthesized compounds. This thermal decomposition process provided a method to prepare pure magnetite as well as magnetite/hematite nanocomposites useful for various magnetic applications.

Two paramagnetic iron states at the Verwey phase transition in magnetite

At the Verwey phase transition (VPT) region a wide line of the ferromagnetic resonance (FMR) and a narrow EPR spectrum were observed. The EPR line at 125 K occurred and then disappeared below 85 K. This unexpected phenomenon is observed only if iron charge state transformation is present. Simulation of both FMR as well as EPR spectra has indicated the presence of two different iron clusters in the Fe 3O 4 structure. Best fit of both spectra yielded g eff = 2.4 and 4.1 for FMR, and g EPR = 2.8 and 3.4 for EPR. The presence of two-component (EPR, FMR) spectra at about T V is due to a frustration of charge distribution in the B-sublattice as a result of the orthorhombic Pmca pseudosymmetry constraints on the atomic positions in monoclinic symmetry cell P2/c. The paramagnetic (EPR) center observed is an admixtured Fe 2+ state in antiferromagnetically ordered A-sublattice ferromagnetically coupled with the B-lattice. Two paramagnetic defects observed (EPR) of localized spin states are evoked by two valency states of Fe 2.4 and Fe 2.6 below T v . They can originate from two types of Fe 4O 4 cubes: “electron-rich” and” electron-poor” at a ratio 3:1 [J.P. Wright, J.P. Attfield, P.G. Radaelli, Phys. Rev. Lett. 87 (2001) 266401; Phys. Rev. B 66 (2002) 214422] or from two types of bond dimerization (H. Seo, M. Ogata, H. Fukuyama, Phys. Rev. B65 (2002) 85107).

Effect of multiple scattering in the interaction of low-energy ions with the magnetite surfaces covered by xenon

The energy spectra of 5.5 keV Ne + and Ar + ions scattered from a cleavage surface of a single-crystal magnetite covered by condensed xenon layers as a function of target temperature and of xenon density were investigated. Under the small-angle geometry, the energy spectra contain one broad peak (the substrate peak) in case of the “clean” surface (i.e. without xenon cover) while two broad peaks were observed for the xenon covered surfaces. The substrate peak was located at lower energy whereas the xenon peak was at higher energy, but both the peaks were distinctly shifted relative to the single scattering energy. A large increase of the intensity of the xenon peak with increasing xenon layer thickness was observed. The substrate peak, however, was found to be almost independent on the xenon pressure for both bombarding ions. Relatively large energy losses, which were found, can be well explained in the framework of the zigzag collision model including the last re-ionizing collision. A very large change in the scattered ion yields at temperature of 120 K, i.e. the Verwey phase transition temperature of magnetite, was observed for the clean target. Some reduction of such a phase transition effect by the xenon layers was found, but the anomaly at the phase transition temperature still occurred. The presence of the separated xenon peak and its energy position suggests that some arrangement of the first xenon layer on the target surface exists.

Low energy ion scattering from the single crystal magnetite and from magnetite covered by condensed xenon at the Verwey transition

Energy spectra of He+, Ne+, Ar+ and Kr+ ions, scattered from a cleavage surface of a single crystal of magnetite, have been measured in small-angle geometry and in the temperature range of 85–300K. The large energy losses of scattered ions and a very deep minimum around 120K in the temperature dependence curve of ion yield ( R+(T) ) have been observed for all bombarding ions. The large energy losses were well explained in the frame of the zigzag collisions of incoming ions with Fe ions forming the surface semi-channels. The minimum in the R+(T) curve corresponding to the Verwey phase transition of magnetite can be explained by a change of the neutralization probability of incoming and outgoing ions, by a change of the target material transparency and by a change in the number of the trajectories containing ionizing collisions. The trajectory effect is visible especially for Ne+ and Ar+ ion interactions. Large suppression of the phase transition effect on ion scattering was observed on the magnetite surface covered by layers of condensed xenon.

PAC measurements of the Verwey transition in magnetite

Perturbed angular correlation (PAC) experiments with implanted 111In tracers have recently been used to investigate magnetic phase transitions in metal oxides. Here we report on PAC measurements for 111Cd in polycrystalline Fe3O4 in the neighborhood of the Verwey phase transition (TV≈ 120 K). Perturbed angular correlation spectra were taken for implanted 111In probes at temperatures between 9 and 850 K. The two observed Larmor frequencies are attributed to the two possible cation sites in the cubic inverse spinel lattice. For T > 120 K, the temperature dependence of both Larmor frequencies follows a Curie–Weiss power law ωL(T)/ ωL(0)=(1-T/TN)β, with the parameters TN=848(2) K and β= 0.392(2). At the Verwey temperature we find a rapid change of both Larmor frequencies, with ωL1 increasing from 178(2) MHz at 120 K to 191(4) MHz at 100 K, and ωL2 decreasing from 173(4) MHz to 151(5) MHz. The Verwey transition also affects the widths of the frequency distributions, which more or less double below TV. This possibly indicates the presence of several components with Larmor frequencies similar to those found in the previous Mossbauer data, or for electronic after-effects correlated with the semiconductivity of magnetite below TV.

Investigations of the interplay between crystalline and magnetic ordering in Fe3O4/NiO superlattices

Using SQUID magnetometry and both x-ray- and neutron-diffraction techniques, we have studied the structural and magnetic ordering of a series of Fe3O4/NiO superlattices grown by MBE. X-ray diffraction reveals that the superlattices are coherent, single phase crystals with narrow interfaces. Symmetry differences between the Fe3O4 spinel and NiO rocksalt structures lead to interfacial stacking faults, manifested in some diffraction intensities. Analysis of the neutron-diffraction spectra show that the NiO antiferromagnetic ordering is coherent through several superlattice bilayers, while the Fe3O4 magnetic ordering is confined to individual interlayers by stacking faults in all superlattices but those with thinnest (≤10 Å) NiO interlayers. Neutron diffraction and SQUID magnetometry have been used to study the Fe3O4 Verwey phase transition in thin-layered superlattices. The charge ordering in superlattices such as [Fe3O4 (75 Å)‖NiO (9 Å)]500, below the Verwey transition, directly observable in (4, 0, 1/2) neutron intensities, indicates a shift to higher temperature of the charge ordering transition from the bulk Fe3O4 TVerwey at 123 K. We also describe ongoing efforts to extract the moment distribution in these superlattices from field dependent high-angle neutron diffraction.

Magnetic and transport properties of magnetite in the vicinity of the Verwey transition

Systematic investigations concerning the variation of the Verwey phase transition in magnetite Fe 3(1−δ)O 4 on the metal-to-oxygen ratio by electrical resistivity, Seebeck coefficient, and magnetization measurements are reported, together with electrical properties of Fe 3− x Zn x O 4 zinc ferrites. In conformity with earlier heat capacity measurements it has been documented that the transition temperature is significantly reduced with increasing δ. In the range 0 < δ < δ c = 0.0039 the transition is of first order, whereas for δ c < δ ≤ 0.012 it is of second (or higher) order. The electrical properties and Verwey transition of Fe 3− x Zn x O 4 and Fe 3(1−δ)O 4 were found to match very well with the compositional correspondence x ⇔ 3 δ. Implications of the above findings are discussed.

Influence of nonstoichiometry on the Verwey transition in Fe3(1−δ)O4

A systematic investigation of the dependence of the Verwey phase transition in magnetite on the metal-to-oxygen ratio, by thermomagnetic analysis of the initial permeability, reveals that the transition temperature is a maximum for pure stoichiometric Fe3O4 and is strongly depressed by departures from ideal stoichiometry. Homogeneous single crystalline samples were prepared by subsolidus controlled oxygen fugacity annealing at 1400 °C. Heat capacity measurements by relaxation calorimetry techniques indicate a drastic reduction in the entropy, as well as temperature, of the transition with departures from ideal stoichiometry. For high levels of cation deficiency, a second, instead of a first-order, transition is observed.

Mössbauer and μSR studies on magnetite

Abstract Experimental results obtained in Mössbauer studies on pure and doped magnetite are discussed. First results of a μSR study on natural and synthetic single crystals of magnetite are reported. The results are interpreted and related to theoretical and other experimental evidence, concerning the characteristics of the Verwey phase transition and the conduction band mechanism of magnetite.