Vam Vic Brabers

One-Dimensional Reconstruction Observed on Fe3o4(110) by Scanning-Tunneling-Microscopy

The (110) surface of magnetite Fe3O4 single crystals was studied by scanning tunneling microscopy (STM). A clean and regular surface was obtained after sputtering and annealing at 1200 K. This preparation procedure resulted in a one-dimensional reconstruction consisting of rows running in the (110) direction. The row spacing was found to vary on different terraces, where the most frequent row separation was determined to be 25 A. Current-versus-voltage curves display a transition from semiconducting to metallic character when the tip–sample distance is reduced. These results, together with complementary low energy electron diffraction (LEED) measurements, are compared with a bulk termination of Fe3O4 and the other known iron-oxide phases.

High-energy photoemission on Fe3O4: Small polaron physics and the Verwey transition

We have studied the electronic structure and charge ordering (Verwey) transition of magnetite (Fe3O4) by soft X-ray photoemission. Due to the enhanced probing depth and the use of different surface preparations we are able to distinguish surface and volume effects in the spectra. The pseudogap behavior of the intrinsic spectra and its temperature dependence give evidence for the existence of strongly bound small polarons consistent with both dc and optical conductivity. Together with other recent structural and theoretical results our findings support a picture in which the Verwey transition contains elements of a cooperative Jahn-Teller effect, stabilized by local Coulomb interaction.

The electrical conduction of titanomagnetites

Electrical conductivity and thermoelectric power measurements on single crystalline titanomagnetites are reported for the temperature range of 80 up to 1100 K. The results can be well explained in terms of hopping conduction in disordered systems; the conduction is governed by electron-electron and electron-titanium interactions, leading to a randomization of the site-energies. Both interaction parameters are in the order of 0.1 eV. From the experimental results a tentative energy level scheme has been deduced for the complete mixed series Fe3−xTixO4.

A study of the magneto-optical Kerr spectra of bulk and ultrathin Fe3O4

An analysis of the complete dielectric tensor of bulk magnetite, Fe3O4, and of Fe3O4 substituted with a variable degree of Mg2+ or Al3+ allowed the assignment of the main magneto‐optical (MO) transitions in Fe3O4 between 0.5 and 4.0 eV. This assignment is consistent with our reinterpretation of the MgFe2O4 and Li0.5Fe2.5O4 MO‐Kerr spectra. An analysis of the stoichiometry of a thin Fe3O4 layer is made, based on the Kerr spectrum, which is compared to an analysis based on the Verwey transition. The Kerr spectrum is also used to compare wavelengths for MO‐Kerr effect studies of wedge‐shaped samples.

Terahertz conductivity at the Verwey transition in magnetite

The complex cond. at the (Verwey) metal-insulator transition in Fe3O4 was investigated at terahertz and IR frequencies. In the insulating state, both the dynamic cond. and the dielec. const. reveal a power-law frequency dependence, the characteristic feature of hopping conduction of localized charge carriers. The hopping process is limited to low frequencies only, and a cutoff frequency n1.simeq.8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielec. const. abruptly decreases and becomes neg. Together with the cond. spectra this indicates the formation of a narrow Drude peak with a characteristic scattering rate of about 5 meV contg. only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral wt. from the hopping process to the free-carrier cond. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy. [on SciFinder (R)]

Thermally-activated vortex dynamics in Bi2CaSr2Cu2O8+δ studied by complex susceptibility measurements

Abstract Complex AC magnetic susceptibility has been measured on Bi2CaSr2Cu2O8+δ single crystals with hnc, Hdc|c-axis. It will be shown that field, frequency and temperature dependence of both χ′ and χ″ in a constant but sufficiently large DC magnetic field can quantitatively be described by thermally-activated flux flow (TAFF) provided that a narrow distribution of activation energies is implemented. From the frequency dependence of the ACχ transition, a power law for the DC field dependence of the activation energy has been found, with an exponent −1 4 .

Structural and physical properties of the tetragonal superconductors LaBaACu3O7-δ (A = Ca and Sr)

Abstract The compounds LaBaACu 3 O 7-δ (A = Ca and Sr) have been prepared and characterized by X-ray diffraction and electron microscopy. Their structure is similar to that of YBa 2 Cu 3 O 7 , except that they are tetragonal. Electron diffraction of LaCaBaCu 3 O 7-δ reveals the presence of a weak superstructure having doubled, a , b and c axes. The oxygen content and superconducting transition temperature were found to vary significantly, depending on the post-annealing conditions. Oxygen- and air-annealed samples have T c values of 76 K and 60 K for A = Ca and 50 K and 33 K for A = Sr respectively. Air-quenched samples exhibit semiconductor-like behaviour in both cases. The d.c. resistivity and a.c. susceptibility vs. temperature data for LaBaSrCu 3 O 7-δ are presented. The absence of an orthorhombic unit cell for these compounds is discussed and a model for the oxygen-vacancy ordering is proposed to explain the observed superstructure in LaBaCaCu 3 O 7-δ .

A Mössbauer study of Al and Ga substituted magnetite

A Mossbauer study of Fe3−xAlxO4 and Fe3−xGaxO4 has been undertaken. Measurements at low temperatures in magnetic fields enabled us to better separate the Fe(A) and Fe(B) subspectra and to determine the cation distribution on tetrahedral and octahedral sites. Whereas Al up to x=1.4 shows a strong preference for B sites, giving rise to a normalization of the spinel structure with Fe2+ located mainly in tetrahedral sites, Ga distributes over both A and B places without tendency towards normalization at least below x≲1.2.

Microstructure and properties of BiCaSrCuO superconductors

Electric and magnetic properties of sintered and compact-zone-melted samples of BiCaSrCuO were measured. The results were interpreted with a theoretical model, in which the samples consist of small superconducting granules weakly coupled by Josephson junctions.

Thermal expansion of Ti-substituted barium hexaferrite

Thermal expansion measurements in the range of 20–500 °C were carried out on both poly- and single crystalline samples of the hexagonal magnetoplumbite ferrite with composition BaTiFe11O19. The continuous scanning of the thermal expansion reveals the existence of a λ-type anomaly near the Curie temperature. In contrast to the usual observed anisotropic behavior for magnetoplumbite hexaferrites, the averaged thermal expansion coefficient for the a direction is larger than for the c direction, i.e., 12.8×10−6 and 9.5×10−6 °C−1, respectively.