van der Paa Paul Heijden

On the construction of an Fe3O4 based all-oxide spin-valve

Abstract The progress made in constructing an all-oxide spin valve based on the intrinsic, fully spin-polarized electron transport in Fe 3 O 4 is discussed. Two possible oxidic spacer layers, MgO and Mn 3 O 4 , have been investigated. Interlayer coupling studies indicate that MgO is the more suitable spacer layer of the two. Thus far a limited magnetoresistive effect ⩽0.4% is found in the all oxide, Fe 3 O 4 -based, spin-valves which we have made. Possible causes for this low magnetoresistive effect are discussed.

Exchange biasing in MBE grown Fe3O4/CoO bilayers: The antiferromagnetic layer thickness dependence

Exchange biasing has been studied for a series of [100] and [111] oriented, epitaxial Fe3O4/CoO bilayers grown by oxidic MBE. The low‐temperature exchange biasing versus CoO layer thickness is compared to theoretical models for exchange biasing. We argue that the Malozemoff random field model does not apply to this system. The exchange biasing calculated according to the Meiklejohn–Bean model, assuming nearest‐neighbor exchange coupling across a flat and magnetically uncompensated interface, differs for [100] oriented bilayers by a factor of ≂8 from the experimental value.

Anti-phase domains and magnetism in epitaxial magnetite layers

Recent studies show that the magnetic properties of epitaxial thin films of magnetite (Fe3O4) deviate strongly from bulk behavior: it is difficult to saturate thin films, ultrathin films may become super paramagnetic, their saturation magnetization drops to zero, and the local magnetic moments are oriented out of plane in zero field. The possible relationship between this anomalous behavior and the occurrence of anti-phase boundaries (APBs) is discussed. Transmission electron microscopy images confirm that APBs are present in our Fe3O4 films grown by molecular beam epitaxy on MgO(100). Only APBs with out-of-plane shift vectors are visible. The much higher APB density found in sputtered films suggests that preparation conditions are important. To explain the deviating saturation and the superparamagnetic behavior of thin Fe3O4 films at the same time, the magnetic coupling over the APB must be dramatically reduced due to spin disorder along the boundaries.

A ferromagnetic resonance study on ultra-thin Fe3O4 layers grown on (0 0 1)MgO

Ferromagnetic resonance experiments have been performed on Fe3O4 layers with thicknesses in the range of 1.5–115 nm, which have been grown by molecular beam epitaxy on (0 0 1)MgO substrates. For large layer thicknesses, the magnitude as well as the temperature dependence of the magnetic anisotropy resemble the bulk Fe3O4 behavior. At room temperature, the magnetic anisotropy decreases with decreasing Fe3O4-layer thickness and vanishes below 5 nm. Low-temperature FMR experiments show a clear Verwey transition, characteristic for Fe3O4, above layer thicknesses of 5 nm and an anomalous anisotropy behavior at 50 K below 10 nm.

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.

Magnetic interface anisotropy of MBE-grown ultra-thin (001) Fe3O4 layers

Uniform as well as wedge-shaped Fe3O4 layers of 0 to 50 nm were grown by MBE on single crystalline (001) MgO substrates. A magnetic interface anisotropy contribution favouring perpendicular magnetisation for thick Fe3O4 layer thicknesses was observed. However, a perpendicular magnetisation direction was not realised at low Fe3O4 thicknesses. Magnetic characterisation also indicated that the Fe3O4 layers exhibit a reduced magnetic moment at the interfaces.

Fabry–Pérot line shape analysis on an expanding cascaded arc plasma in argon

Fabry–Perot line profile measurements have been used to obtain heavy particle temperatures and electron densities for an expanding cascaded arc plasma in argon. This was done for the argon 415.9 and 696.5 nm neutral lines as a function of the distance from the onset of the expansion. Temperatures in the range of 2000–12 000 K were obtained. The electron density in the beginning of the expansion appeared to be 5.6×1021 m−3. The 696.5 nm line profiles appeared to be asymmetric because of self‐absorption by cool metastables around the plasma. The density and temperature of these metastables could be determined by fitting the measurements to a theoretical model, and appeared to be around 1017 m−3 and around 3000 K, respectively.

Study of the Verwey transition of Fe3O4 films and Fe3O4/MgO multilayers grown by MBE

Thin magnetite (Fe304) films and Fe304/MgO multilayers have been epitaxially grown by Molecular Beam Epitaxy (MBE) on MgO(100) and MgA12O4(100) substrates. The epi taxial growth on MgO(100) substrates, with a slightly larger bulk lattice parameter than that of magnetite, resulted in an in-plane expansion of the magnetite lattice, accompanied by a perpendicular compression. For films grown on MgAlsO4(100), with a smaller lattice parameter, the substrate misfit is relaxed by the incorporation of misfit dislocations at the interface. It is shown that the substrates have a large effect on the magnetic and electronic properties of the films. The characteristic Verwey transition is shifted towards lower temperatures, broadened, and reduced in amplitude, more so for thinner films. This can not be quantitatively ex plained by substrate induced stress alone, but is more likely due to a rigid structural coupling between the magnetite film and the cubic lattice of the substrate. Hereby the orthorhombic deformation accompanying the Verwey transition may be supressed. When growing at reduced oxygen pressure, the length scale for the rigid coupling will be reduced by the introduction of vacancies. This leads to more bulk-like resistivity and Verwey transition characteristics, but also to deviations from stoichiometry, as suggested by magnetization and Ferromagnetic Resonance (FMR) experiments.

MAGNETIZATION OF FE3O4/MGO MULTILAYERS STUDIED WITH MOSSBAUER SPECTROSCOPY

The magnetic behavior and structure of Fe3O4/MgO multilayers have been investigated with Mossbauer spectroscopy in field, magnetization measurements, and scanning tunneling microscopy. It is shown that detailed structural knowledge of the size of the structural domains and the width of the antiphase boundaries are indispensable in understanding the magnetization behavior of the films and the contradictory results reported in the literature.

Influence of annealing on the structural and magnetic properties of Co/Ag multilayers with thin Co layers

Abstract The structural and magnetic properties of sputtered Co Ag multilayers were studied as a function of anneal treatment. Smoothing of the interfaces, strain relaxation, and phase separation of Co and Ag were observed. The magnetoresistance showed a maximum upon annealing.