Pjh Pascal Bloemen

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.

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.

ON THE FREQUENCY DEPENDENCE OF THE MAGNETIC PERMEABILITY OF FEHFO THIN FILMS

The frequency dependence of the magnetic permeability as well as of the electrical impedance have been investigated for soft-magnetic granular FeHfO thin films. The impedance measurements indicate that capacitive effects resulting from the inhomogeneous structure of the layers are of no importance for the roll off of the permeability of the present films. The frequency behavior at various FeHfO thicknesses shows that eddy current effects start to play a role above thicknesses of 10 μm. Below this thickness ferromagnetic resonance dominates the roll off.

Temperature dependence of the resistivity and tunneling magnetoresistance of sputtered FeHf(Si)O cermet films

We have studied the tunneling resistivity and magnetoresistance of reactive sputter deposited FeHfO and FeHfSiO thin granular films. Maximum magnetoresistance ratios at room temperature of 2% and 3.2% were observed for films with compositions of Fe47Hf10O43 and Fe40Hf6Si6O48, respectively. The magnetoresistance shows a decrease with temperature, which cannot be explained by spin-dependent tunneling only. We propose that spin-flip scattering in the amorphous FeHf(Si)O matrix causes this decrease as function of temperature. A two current model for the tunnel magnetoresistance, taking into account spin-flip scattering, is presented which can describe the observed temperature dependence of the magnetoresistance.

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.

The temperature dependence of the magnetization of magnetic multilayers

The magnetization of Co/Pt and Co/Au multilayers has been measured as a function of temperature. For the Co/Pt multilayers with fixed cobalt sublayer thickness of 4, 6 and 8 A, the magnetization decreases faster with temperature as the platinum layers are made thicker. In contrast the magnetization of the Co/Au multilayers is nearly independent of the Au thickness in a range from 8 to 40 A with a Co thickness of 6 A and remain almost constant up to 500 K. Interlayer coupling and interdiffusion are discussed as possible causes for the observed behaviour.

Antiparallel and perpendicular magnetization alignment for Co / Ru multilayers

Several Co / Ru multilayers were grown in which the magnetization directions of consecutive Co layers were aligned antiparallel and along the film normal. The magnetization curves for these multilayers agree with the curves obtained by minimizing the total energy including anisotropy and interlayer coupling. These multilayers were part of a series with Co thicknesses of 11 and 22 A and Ru thicknesses varying between 5 and 20 A. Torque and magnetization measurements showed that the multilayers with 11 A Co had a perpendicular easy axis while for the 22 A Co series an in-plane preferential orientation was observed. A strong antiferromagnetic interlayer coupling was obtained for Ru thicknesses of about 8 A.

Magnetic layer thickness dependence of the interlayer exchange coupling in (001) Co/Cu/Co

A dependence of the strength of the antiferromagnetic coupling across Cu on the Co layer thickness has been observed. The Co thickness dependence displays two clear peaks consistent with the recently predicted oscillation period of 6.2 A Co. Apart from the two peaks also several small peaks are visible on a scale of about 1 monolayer Co. Free‐electron calculations indicate that these rapid variations in strength may result from slight differences between the slopes and starting points of the two Co wedges that were involved in the experiment.

Magnetic layer thickness dependence of the interlayer exchange coupling

Recent progress in the research on interlayer exchange coupling, focusing on the effect of the magnetic layer, is reviewed. Several theoretical works have shown that the exchange coupling does not just involve an interaction localized at the interfaces, but the electronic properties of the sandwich as a whole. The important prediction that the strength of the exchange coupling should oscillate as a function of the magnetic layer thickness has been confirmed recently for the Co/Cu(100) and the Fe/Cr(100) system. The oscillation periods are strongly believed to be related to wave vectors that span extremal parts of the spin-resolved Fermi surfaces of the ferromagnets.