H. J. M. Swagten

Antiferromagnetic interlayer coupling in ferromagnetic semiconductor EuS/PbS(001) superlattices

Antiferromagnetic coupling between ferromagnetic layers has been observed for the first time in an all-semiconductor superlattice structure EuS/PbS(001), by neutron scattering and magnetization measurements. Spin-dependent superlattice band structure effects are invoked to explain the possible origin and the strength of the observed coupling.

Specular reflection in spin valves bounded by NiO layers

We have experimentally investigated the possibility of increased electron reflectivity induced by an insulating NiO layer that is used to exchange bias a metallic spin-valve. For this purpose Ni/sub 80/Fe/sub 20//Cu/Ni/sub 80/Fe/sub 20/ and Ni/sub 66/Fe/sub 16/Co/sub 18//Cu/Co/sub 90/Fe/sub 10/ spin-valves were grown and subsequently covered by insulating NiO or by metallic FeMn. In all cases the giant magnetoresistance of the NiO spin-valves is systematically larger upon a variation of the exchange-biased ferromagnetic layer thickness, also after correcting the data for the conductivity of FeMn which we have determined from a separately grown series of samples with variable FeMn thickness. The increased giant magnetoresistance ratios can be qualitatively understood on the basis of a semiclassical calculation in which (partial) specular reflectivity at the NiO interface has been included.

Optical and in situ characterization of plasma oxidized Al for magnetic tunnel junctions

An optical polarization modulation technique was adapted to provide a simple, fast, and flexible method for studying the kinetics and growth characteristics of thin oxide layers, using Al2O3 as an example. The optical technique allows precise determination of the amount of remaining metallic Al as a function of the initial Al thickness, while scanning a laser spot across the wedge. Optical data suggest that the oxide growth rate for the ultrathin layers may be dependent on the specific microstructure. In situ x-ray photoelectron spectroscopy performed on homogenous samples confirmed the interpretation of the optical results.

Fe:O:C grown by focused-electron-beam-induced deposition: magnetic and electric properties

We systematically study the effect of oxygen content on the magneto-transport and microstructure of Fe:O:C nanowires deposited by focused-electron-beam-induced (FEBID) deposition. The Fe/O ratio can be varied with an Fe content varying between ∼ 50 and 80 at.% with overall low C content (≈16 ± 3 at.%) by adding H(2)O during the deposition while keeping the beam parameters constant as measured by energy dispersive x-ray (EDX) spectroscopy. The room-temperature magnetic properties for deposits with an Fe content of 66-71 at.% are investigated using the magneto-optical Kerr effect (MOKE) and electric magneto-transport measurements. The nanostructure of the deposits is investigated through cross-sectional high-resolution transmission electron microscopy (HRTEM) imaging, allowing us to link the observed magneto-resistance and resistivity to the transport mechanism in the deposits. These results demonstrate that functional magnetic nanostructures can be created, paving the way for new magnetic or even spintronics devices.

Magnetic study of the diluted magnetic semiconductor Sn1−xMnxTe

New measurements of the magnetic carrier‐induced properties of Sn1−xMnxTe for x≤0.10 are presented. The results of these and previously reported measurements for other compositions and carrier concentrations will be compared to model calculations of the low‐temperature magnetic state. A magnetic phase diagram will be presented.

Thermodynamic properties of iron-based II-VI semimagnetic semiconductors

The specific heat, magnetization and low field susceptibility is calculated for iron-based Semimagnetic Semiconductors. Calculations are performed in Extended Nearest Neighbour Pair Approximation taking into account both Heisenberg-type d-d exchange interaction and spin-orbit interaction. Comparison with available experimental data for ZnFeSe and CdFeSe is presented.

Giant anomalous Hall effect in Fe-based microwires grown by focused-electron-beam-induced deposition

We report the temperature dependence of the resistivity, the anisotropic magnetoresistance and the Hall effect of iron microwires grown by focused-electron-beam-induced deposition. By modifying the growth conditions in a controllable way, we study wires with iron compositions varying from 45% to 70%, which present different electrical conduction mechanisms, with resistivity values differing over three orders of magnitude. The magnetoresistance depends highly on the composition, and it can be understood by a subtle interplay between the anisotropic magnetoresistance and intergrain magnetoresistance due to their complex microstructure, consisting of an iron–carbon–oxygen amorphous matrix. A giant value for the anomalous Hall effect is found, which we explain by a large contribution of the skew scattering mechanism. The present results emphasize the correlation between the exotic microstructure of the microwires, and their magnetotransport properties.

Enhanced field-driven domain-wall motion in Pt/Co68B32/Pt strips

It is now commonly accepted that materials exhibiting high perpendicular magnetic anisotropy are excellent candidates for devices based on current-induced domain-wall (DW) motion. A major hindrance of these materials however, is that they exhibit strong DW pinning. Here we report a significant increase in the field-driven DW velocity in Pt(4 nm)/Co68B32(0.6 nm)/Pt(2 nm) layers patterned into 900 nm wide strips. We compare the DW velocity between Co and Co68B32 films and discuss the observed effects using the morphology of the films investigated by high-resolution transmission electron microscopy.

Intrinsic thermal robustness of tunneling spin polarization in Al/Al2O3/Co junctions

Through a direct observation, based on the spin-polarized tunneling technique, we explore the thermal stability of tunneling spin polarization in Al/Al2O3/Co junctions. Thermal robustness of this parameter, which is of key importance for magnetic tunnel junction performance, is established for in situ postdeposition anneal temperatures up to 500 °C. This stability is consistent with detailed in situ x-ray photoelectron spectroscopy measurements on the Al2O3/Co system which show no structural changes during the anneal. Our results imply that, for comparable magnetic tunnel junction devices, thermal stability is not limited by intrinsic processes in the Al2O3 barrier and its interfaces. With ex situ postdeposition annealing in an Ar-atmosphere, which leads to severe degradation of the spin polarization above 250 °C, we demonstrate that the spin polarization is extremely vulnerable to diffusion of impurities.

Carrier-induced breakdown of ferromagnetism

Magnetization, susceptibility and specific-heat data for PbSn(Mn)Te and Sn(Mn)Te will be reported for various carrier concentrations. For the first time a carrier-concentration-induced breakdown of a ferromagnetic state has been observed. Based on a simple geometrical argument, a phase diagram for this new transition in RKKY-driven systems will be derived and will be compared with the ferromagnetic-to-spin-glass transition predicted by the well-known Sherrington-Kirkpatrick model.