Jt Jürgen Kohlhepp

Large magnetoresistance using hybrid spin filter devices

A magnetic “spin filter” tunnel barrier, sandwiched between a nonmagnetic metal and a magnetic metal, is used to create a magnetoresistive tunnel device, somewhat analogous to an optical polarizer-analyzer configuration. The resistance of these trilayer structures depends on the relative magnetization orientation of the spin filter and the ferromagnetic electrode. The spin filtering in this configuration yields a previously unobserved magnetoresistance effect, exceeding 100%.

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.

Field-free magnetization reversal by spin-Hall effect and exchange bias

As the first magnetic random access memories are finding their way onto the market, an important issue remains to be solved: the current density required to write magnetic bits becomes prohibitively high as bit dimensions are reduced. Recently, spin–orbit torques and the spin-Hall effect in particular have attracted significant interest, as they enable magnetization reversal without high current densities running through the tunnel barrier. For perpendicularly magnetized layers, however, the technological implementation of the spin-Hall effect is hampered by the necessity of an in-plane magnetic field for deterministic switching. Here we interface a thin ferromagnetic layer with an anti-ferromagnetic material. An in-plane exchange bias is created and shown to enable field-free S HE-driven magnetization reversal of a perpendicularly magnetized Pt/Co/IrMn structure. Aside from the potential technological implications, our experiment provides additional insight into the local spin structure at the ferromagnetic/anti-ferromagnetic interface.

Band Structure and Density of States Effects in Co-Based Magnetic Tunnel Junctions

Utilizing Co/Al(2)O(3)/Co magnetic tunnel junctions with Co electrodes of different crystalline phases, a clear relationship between electrode crystal structure and junction transport properties is presented. For junctions with one fcc(111) textured and one polycrystalline (polyphase and polydirectional) Co electrode, a strong asymmetry is observed in the magnetotransport properties, while when both electrodes are polycrystalline the magnetotransport is essentially symmetric. These observations are successfully explained within a model based on ballistic tunneling between the calculated band structures (density of states) of fcc-Co and hcp-Co.

Precise control of domain wall injection and pinning using helium and gallium focused ion beams

In experiments on current-driven domain wall (DW) motion in nanostrips with perpendicular magnetic anisotropy (PMA), the initial DW preparation is usually not well controlled. We demonstrate precise control of DW injection using Ga and novel He focused ion beam (FIB) irradiation to locally reduce the anisotropy in part of a Pt/Co/Pt strip. DWs experience pinning at the boundary of the irradiated area. This DW pinning is more pronounced at the He irradiation boundary compared to Ga. This is attributed to a better He beam resolution, causing an anisotropy gradient over a much smaller length scale and hence, a steeper energy barrier for the DW. The results indicate that He FIB is a useful tool for anisotropy engineering of magnetic devices in the nanometer range.

Influence of photon angular momentum on ultrafast demagnetization in nickel

The role of pump helicity in laser-induced demagnetization of nickel thin films is investigated by means of pump-probe time-resolved magneto-optical Kerr effect in the polar geometry. Although the data display a strong dependency on pump helicity during pump-probe temporal overlap, this is shown to be of nonmagnetic origin and not to affect the demagnetization. By accurately fitting the demagnetization curves, we show that demagnetization time tM and electron-phonon equilibration time tE are not affected by pump helicity. Thereby, our results exclude direct transfer of angular momentum to be relevant for the demagnetization process and prove that the photon contribution to demagnetization is less than 0.01%. [on SciFinder (R)]

Spin-injection device based on EuS magnetic tunnel barriers

We propose a spin-valve device consisting of a nonmagnetic semiconductor quantum well, sandwiched between ferromagnetic semiconductor layers that act as barriers. The total conductance through such a trilayer depends on the relative magnetization of the two ferromagnetic-barrier layers which act as “spin filters.” With respect to practical realization, EuS/PbS heterostructures may be a suitable candidate. The magnetoresistance should exceed 100% for a wide range of the thicknesses of both the quantum well and the ferromagnetic barriers. From a fundamental physics point of view, the device may not only give insight into the spin lifetimes of the nonmagnetic layer, but the strong spin accumulation taking place in the quantum well may lead to novel optical and nuclear magnetic resonance properties.

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.

Tunnel conductance as a probe of spin polarization decay in Cu dusted Co/Al2O3/Co tunnel junctions

Tunneling magnetoresistance (TMR), dynamic resistance and bias dependence measurements were performed on Co/Al2O3/Co magnetic tunnel junctions with a thin Cu layer inserted at either the Co/Al2O3 (“bottom”) or Al2O3/Co (“top”) interfaces. Careful comparative analysis allows detailed growth characteristics to be elucidated, as well as providing information on the underlying mechanisms behind spin polarized transport in these structures. Conductance for top dusted junctions is indicative of parallel Co/Al2O3/Co and Co/Al2O3/Cu junctions, consistent with three-dimensional growth of Co and Cu on Al2O3, while conductance for bottom dusted junctions show novel behavior dissimilar to either type of junction. The bias dependence of the TMR, surprisingly, is unaffected by either type of dusting.