Ronald Martin Wolf

FE L2,3 LINEAR AND CIRCULAR MAGNETIC DICHROISM OF FE3O4

Abstract Measurements of circular and linear magnetic dichroism at the Fe 2p edges of Fe 3 O 4 are presented. The dichroism results from the ferrimagnetic ordering of Fe d 5 and d 6 ions in octahedral and tetrahedral sites. Atomic calculations taking into account the different crystal fields have been fitted to all L -edge spectra of this compound. A chemical shift of 1.5 eV between the dominant L 3 peak of the octahedral Fe 2+ and Fe 3+ ions gives the best fit of linear and circular dichroism. We investigate whether these parameters can be transferred to predict the Fe 2p dichroism in other ferrites.

A study of the magnitude of exchange biasing in [111] Fe3O4/CoO bilayers

Abstract Exchange biasing has been studied for a series of [111]-oriented Fe 3 O 4 /CoO bilayers with constant Fe 3 O 4 and varying CoO thicknesses. The magnitude of exchange biasing in this oxidic system is compared with the value calculated under the assumption of nearest-neighbour exchange at a flat and magnetically uncompensated interface.

An STM study of Fe3O4 (100) grown by Molecular Beam Epitaxy

Abstract STM imaging of MBE-grown pseudomorphic (100) Fe 3 O 4 surfaces reveals terrace widths that are typically a few hundred angstroms broad, and can be as broad as 1000 A. These terraces are separated by steps that are 1 4 of the spinel lattice constant high, corresponding to the distance (2.1 A) between planes of oxygen (or equivalent iron) atoms. The images show that the p(1 × 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a [110] direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 A spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 A step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred angstroms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order.

Investigation of the stoichiometry of MBE-grown Fe3O4 layers by magneto-optical Kerr spectroscopy

The polar Kerr spectra between 0.7 and 4.0 eV of single-crystalline layers of Fe3O4 grownepitaxially on MgAl2O4, SrTiO3 and MgO (100) substrates by meansof oxidic molecular beam epitaxy are compared to the spectrum of a bulk synthetic single crystal of magnetite to asses the stoichiometry of the layers. Model calculations of the magneto-optical spectra of thin layers are used to explain the differences between the spectra of bulk Fe3O 4and thin layers of Fe3O4. The observed differences are attributed to interference effects and oxidation of the surface of the thin layers. The results are compared to magnetisation and Verwey transition data.

ROLE OF THE ANTIFERROMAGNET IN EXCHANGE-BIASED FE3O4/COO SUPERLATTICES (INVITED)

Using neutron diffraction techniques, we have directly characterized the antiferromagnetic spin structure of CoO in exchange-biased Fe3O4/CoO (001) superlattices. The CoO antiferromagnetic order is long-range and extends coherently across intervening ferrimagnetic Fe3O4 layers. The ordering is influenced significantly by not only epitaxial growth constraints but also by coupling to the Fe3O4 layers. The antiferromagnetic order in these Fe3O4/CoO (001) samples exhibits significant dependence on field cooling preparations. The CoO spins show a strong tendency to align perpendicular to the net ferrimagnetic moment with some changes in the average domain size associated with field cooling. The CoO spin structure in the exchange-biased state illustrates important aspects of recent biasing theories but with unusually large domain sizes.

Polarized neutron reflectometry studies of magnetic oxidic Fe3O4/NiO and Fe3O4/CoO multilayers

The magnetic properties of [1 0 0] oriented Fe3O4/NiO and Fe3O4/CoO multilayers, MBE-grown on MgO(0 0 1) substrates, have been studied by polarized neutron reflectometry. In both samples, the Fe3O4 layer exhibits a depth-dependent magnetic profile characterized by a reduction in the magnetization near the interfaces. The possible origins of this behaviour, such as domain wall formation in the ferrimagnetic layer and deviations in stoichiometry, are discussed.

A scanning tunneling microscopy investigation of MBE-grown Fe3O4(001)

We present STM images of Fe3O4 epitaxially grown on MgO(001) showing details of the surface structure including the surface reconstructions, the presence of monolayer steps on the surface, and the terrace widths. The p(1×1) surface is formed by a grouping of 2 to 4 atoms in the unit cell, to form a cluster that is elongated along a [110] direction. Stacking fault defects are revealed by 90° rotations of the surface reconstruction orientation observed on a single terrace, and by the presence of bands devoid of the surface reconstruction separating the regions of different surface reconstruction orientation. Although the oxygen atom positions in MgO and Fe3O4 are lattice matched to within 0.3%, the difference in symmetry of the Fe and Mg atoms leads directly to these faults.

Magnetic Properties of Epitaxial Mbe-Grown thin Fe 3 O 4 Films on MgO (100)

Coherent epitaxial Fe3O4 layers in the range of 0 to 400 angstrom have been grown by molecular beam epitaxy on single crystal MgO(100) substrates. The magnetic properties were studied by local magneto-optical Kerr effect experiments on a wedge shaped Fe3O4 layer, by ferromagnetic resonance and SQUID. The results show that the magnetic behavior of the Fe3O4 thin films resembles bulk Fe3O4 in the investigated thickness range.

The blocking and Néel temperature in exchange-biased Fe3O4/CoO multilayers

Abstract The blocking temperature TB and the Neel temperature of the antiferromagnet, TN, are compared in exchange-biased Fe3O4/CoO. Neutron diffraction shows that the observed reduction in TB for CoO layer thicknesses A is not due to the reduction of TN of these layers.

Semiconductive Behavior of Sb Doped SnO 2 Thin Films

Sb doped Sn02 has been deposited on polished ceramic A1203 substrates by Pulsed Laser Deposition. Conductivity, charge carrier density and mobility of these thin films have been meas ured as a function of temperature. A model for the electrical properties of the films is proposed. Since Sb doped Sn02 is a transparent, high mobility material, it is shown that it can be used as channel material for an all-oxide thin film transparent field-effect transistor with a linear dielectric.