T. Hibma

Magnetic versus crystal-field linear dichroism in NiO thin films

We have detected strong dichroism in the Ni L-2,L-3 x-ray absorption spectra of a monolayer NiO film. The dichroic signal appears to be very similar to the magnetic linear dichroism observed for thicker antiferromagnetic NiO films. Detailed analysis reveals, however, that the dichroism is caused by crystal-field effects in the monolayer film, which is a nontrivial effect because the high spin Ni 3d(8) ground state is not split by low-symmetry crystal fields. We present a practical method for identifying the independent magnetic and crystal-field contributions to the linear dichroic signal in spectra of NiO films with arbitrary thicknesses and lattice strains. Our findings are also relevant for 3d(5) and 3d(3) systems such as LaFeO3, Fe2O3, VO, LaCrO3, Cr2O3, and Mn4+ thin films.

Controlling orbital moment and spin orientation in CoO layers by strain

We have observed that CoO films grown on different substrates show dramatic differences in their magnetic properties. Using polarization dependent x-ray absorption spectroscopy at the Co L2,3 edges, we revealed that the magnitude and orientation of the magnetic moments strongly depend on the strain in the films induced by the substrate. We presented a quantitative model to explain how strain together with the spin-orbit interaction determine the 3d orbital occupation, the magnetic anisotropy, as well as the spin and orbital contributions to the magnetic moments. Control over the sign and direction of the strain may, therefore, open new opportunities for applications in the field of exchange bias in multilayered magnetic films.

Growth and properties of strained VOx thin films with controlled stoichiometry

We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001) substrates. The oxygen content as a function of oxygen flux was determined using O-18(2)-Rutherford backscattering spectrometry and the vanadium valence using x-ray-absorption spectroscopy. The upper and lower stoichiometry limits found are similar to those known for bulk material (0.8

Magneto-resistance and superparamagnetism in magnetite films on MgO and MgAl2O4

Abstract The present study is a comparison of the resistive and magnetic properties of ultrathin epitaxial Fe 3 O 4 films grown on MgO(0xa00xa01) and MgAl 2 O 4 (0xa00xa01). Such films on MgO are known to contain anti-phase boundaries, which cause magneto-resistance in these films, an increased resistivity with respect to the bulk and superparamagnetic behaviour for films below 5xa0nm thickness. Ultrathin films on MgAl 2 O 4 show a very similar behaviour as the resistivity is also increased with respect to the bulk, but is still lower than the resistivity of films grown on MgO. This indicates a larger domain size in the films grown on MgAl 2 O 4 . This is supported by the observed magneto-resistance that is larger than for films grown on MgO. Mossbauer spectroscopy analysis of a 1.7xa0nm thick film on MgO indicates that all magnetic moments in the film are fluctuating, whereas for the film on MgAl 2 O 4 a sextet with a parabolic background is observed. This shows that some domains are small enough for the moment to fluctuate while in larger ones, the moments are fixed; this also indicates that the domains in the film on MgAl 2 O 4 are larger than in the film grown on MgO.

Composition of MBE-grown iron oxide films

A wide range of iron oxides have been grown epitaxially on MgO(100) substrates using a dual beam technique in which the deposited iron is oxidised by a beam of NO 2 particles. At high fluxes magnetite (Fe 3-δ O 4 ) phases with compositions between near-stoichiometric magnetite (Fe 3 O 4 , δ = 0) and maghemite (γ-Fe 2 O 3 , δ = 1/3) are obtained and at low fluxes wustite (Fe 1-x O 1-y N y ) phases. The nature and composition of these phases was determined by combining the results of Mossbauer spectroscopy, RHEED, XPS and ion scattering techniques.

Giant positive magnetoresistance in metallic VOx thin films

We report on giant positive magnetoresistance (MR) effect observed in VOx thin films, epitaxially grown on SrTiO3 substrate. The MR effect depends strongly on temperature and oxygen content and is anisotropic. At low temperatures its magnitude reaches 70% in a magnetic field of 5 T. Strong electron-electron interactions in the presence of strong disorder may qualitatively explain the results. An alternative explanation, related to a possible magnetic instability, is also discussed.

Spin-valve behaviour of anti-ferromagnetic boundaries in ultrathin magnetite films

Magneto-resistance (MR) measurements on epitaxial Fe3O4 films grown on polished MgO have been performed. The measurements presented here are interpreted by a model that describes the MR behaviour as spin-polarised transport across anti-ferromagnetic (AF) interfaces. The Fe3O4 films consist of structural domains, separated by anti-phase boundaries where an AF coupling is present. These AF interfaces enhance the resistance of the films. Upon application of a magnetic field the AF-spins rotate towards each other and the resistance decreases. The AF interfaces are thus behaving as spin-valves. In agreement with the model, the observed magneto-resistance is negative and shows linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane respectively. Above the anisotropy field, the slopes of the two MR curves are expected to be equal, which is observed at 60 K. Above the Verwey transition, the shape of the normalised MR curves is independent of temperature. Below the Verwey transition the MR curve becomes more linear with decreasing temperature. A large difference between parallel and perpendicular MR is observed at the Verwey transition.

Comparison of a stoichiometric analysis of Fe3-delta O4 layers by magneto-optical Kerr spectroscopy with Mossbauer results

The stoichiometry of a series of 300 Angstrom thick Fe3-deltaO4 layers grown by means of molecular beam epitaxy on MgO(100) has been investigated both by magneto-optical Ken spectroscopy and by Mossbauer spectroscopy, The layers consisted of a 200 Angstrom thick Fe3-deltaO4 layer grown with the Fe-57 isotope, sandwiched between two 50 Angstrom thick layers grown with natural Fe, It was found that delta varied between 0 and 0.33. From the fits of the polar Ken spectra of the Fe3-deltaO4 layers measured between 1.4 and 4.0 eV we find, within experimental accuracy, a stoichiometry that is identical to that determined by Mossbauer spectroscopy. This confirms that the magneto-optical Ken spectra can also be used to determine the stoichiometry of thin Fe3-deltaO4 films.

Epitaxial growth of solution deposited Bi2Sr2CaCu2Ox films

Abstract.The epitaxial growth of Bi2Sr2CaCu2Ox (Bi2212) high temperature superconducting thin films was studied. The films were solution-deposited from a polymer-containing precursor onto SrTiO3 (001) substrates. Bi2212 formed an epitaxial phase with the c-axis parallel to the substrate normal and an in-plane orientation with the a-axis parallel to the SrTiO3 [110] and [

Magnetism of /sup 57/Fe spins close to the /sup 57/Fe/sub 3/O/sub 4//CoO interface

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