[en] We studied epitaxially grown single-crystalline chemically disordered ultrathin Ni_xPd_1-x alloy films on Cu₃Au(100) by means of photoemission using magnetic circular dichroism in the angular distribution. We clearly observe a well-defined three-dimensional band dispersion in 15 monolayer films. On the basis of relativistic selection rules, this dispersion can be related to specific electronic states. Since the spin - orbit coupling (ΔE_so) is smaller for Ni than for Pd, the alloy will assume an average value. We find that this average spin - orbit coupling clearly shows up in the electronic structure and correlates directly to the composition of the NiPd alloy film. The results are discussed in terms of a simple qualitative model. [copyright] 2001 American Institute of Physics

[en] We have studied the effect of the dipolar magnetic coupling (also known as Neel coupling or 'orange-peel' coupling) in tunneling magnetoresistive (TMR) elements. With an in situ scanning tunneling microscope we directly accessed the roughness of the films and found a close correspondence between the values for the coupling fields determined by the magneto-optical Kerr effect and the ones computed on the basis of the measured morphology parameters. We confirm an increase of the dipole coupling between the magnetic layers with decreasing barrier thickness as predicted by the model. Deviations from the theoretical predictions are observed for the case of thinner soft magnetic layers, which can be explained by reduced magnetization in very thin films. We demonstrate the importance of dipolar coupling for understanding the magnetic behavior of TMR elements by comparing TMR curves for optimized and nonoptimized structures. [copyright] 2001 American Institute of Physics

[en] Co/Cu multilayers were prepared by dc magnetron sputtering, varying the individual layer thicknesses t_Cu≅t_Co in the range of t=0.6 - 50 nm. Additionally, the ratio t_Co/t_Cu was varied as t_Co/t_Cu=0.4 - 4. Giant magnetoresistance (GMR) and saturation ferromagnetic (FM) resistivity for the first three antiferromagnetic (AFM) coupling maxima were measured as a function of temperature in the range of T=4.2 - 300 K, with the GMR values ranging up to 115%. For equidistant Co and Cu layers the saturation resistivity at T=4.2 K matches the size effect dependence in single thin films. Even when changing the single layer thickness or the Co/Cu thickness ratio by an order of magnitude a uniform normalized temperature dependence ρ(T,t)/ρ(T₀,t) is found and no shunting effects are observed for t_Co,t_Cu<10 nm. The results favor the following GMR model: The resistivity in the FM (aligned) state is dominated by (hybridized) majority spin electrons, the states of which at similar s like regions of the Fermi surfaces of face-centered-cubic majority Co and Cu allow them to transmit the Co/Cu interfaces with a large mean free path λ. The temperature dependence of λ is governed by the transmittance and it is therefore uniform. Diffuse scattering at crystalline defects at the interfaces reduces λ in accordance with the size effect in single layers. The transition to the AFM state reduces the large λ to the dimensions of Cu - Co↑, ↓ - Cu trilayers, that can be understood in terms of a temperature independent size effect, too. [copyright] 2001 American Institute of Physics

[en] The magnitude of the giant magnetoresistance (GMR) observed in multilayers is known to change irreversibly at elevated temperatures. To improve the thermal stability of devices, a fundamental understanding of the GMR and its correlation to the temperature-induced structural and morphological changes in a given system is mandatory. We therefore investigated the structural and magnetic properties of sputtered Co/Cu multilayers in the pre- and postannealed states (temperature regime for annealing up to 750 degreeC) by in situ x-ray diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect (MOKE). We were able to identify a sequence of distinct structural changes each of which sets in above a characteristic critical temperature. These critical temperatures depend strongly on the thickness of the individual layers. The structural alterations observed range from interfacial sharpening through texture reorientations up to the formation of a granular state, and are associated with distinct in/decreases of the GMR signal. Using FMR and MOKE we determined in-plane magnetic anisotropies and interlayer coupling for as-grown and annealed samples. [copyright] 2001 American Institute of Physics

[en] The study of magnetodynamics using stroboscopic time-resolved x-ray photoemission electron microscopy (TR-XPEEM) involves an intrinsic timescale provided by the pulse structure of the synchrotron radiation. In the usual multi-bunch operation mode, the time span between two subsequent light pulses is too short to allow a relaxation of the system into the ground state before the next pump-probe cycle starts. Using a deflection gating mechanism described in this paper we are able to pick the photoemission signal resulting from selected light pulses. Thus, PEEM measurements can be carried out in a flexible timing scheme with longer delays between two light pulses. Using this technique, the magnetodynamics of both Permalloy and iron structures have been investigated. The differences in the dynamic response on a short magnetic field pulse are discussed with respect to the magnetocrystalline anisotropy.

[en] Individual carbon nanotubes are filled with fullerene molecules directly on the substrate. Two different oxidation techniques for opening the tubes prior to the filling, annealing in air, and acid treatment, are compared. High-resolution transmission electron microscopy images indicate that both methods induce defects on the sidewalls of the nanotubes. In the case of acid treatment, the inner walls can be damaged without affecting the outer walls, while the inner walls are opened along with the outer ones by heating in air. The effect of acid treatment on the tubes is much stronger than known from bulk samples. In contrast to previous studies, we find amorphous carbon inside the nanotubes after oxidation, and an additional high-temperature annealing step is needed to remove these plugs in order to open the tubes for filling.

[en] Co/Cu/Co lateral spin valves (LSV), with Co being the topmost layer, are in situ prepared and measured under ultrahigh vacuum conditions. The clean process yields a non-local spin signal of 0.9 mΩ. Scanning electron microscopy with polarization analysis (SEMPA) reveals domain structures in both magnetic electrodes that depend on the LSV dimensions. The spin signal correlates to SEMPA images as well as the anisotropic magnetoresistance of both Co magnets, revealing a strong impact of multi-domain states on the spin signal.

[en] Growth and magnetic characterization of thin films of Co₂Cr_0.6Fe_0.4Al and Co₂MnSi full-Heusler compounds are investigated. Thin films were deposited by magnetron sputtering at room temperature directly onto oxidized Si wafers. These Heusler films are magnetically very soft and ferromagnetic with Curie temperatures well above room temperature. Polycrystalline Co₂Cr_0.6Fe_0.4Al Heusler films combined with MgO barriers and CoFe counter electrodes are structured to magnetic tunnel junctions and yield almost 50% magnetoresistance at room temperature. The magnetoresistance shows a strong bias dependence with the maximum occurring at a voltage drop well above 1 V. This special feature is accompanied by only a moderate temperature dependence of the tunnel magnetoresistance

[en] This work presents a systematic characterization including x-ray diffractometry, SQUID magnetometry and x-ray absorption spectroscopy on all-oxide ferromagnetic/ferroelectric heterosystem BaTiO₃/La_0.7Sr_0.3MnO₃/SrTiO₃(001) fabricated by pulsed-laser deposition. The key aspect of this study is to provide accurate information about differences in electronic and structural properties in LSMO thin films as a function of the oxygen pressure during BTO growth. X-ray absorption spectroscopy experiments at the Mn L _3,2 edge have revealed the conservation of ${\mathrm{M}\mathrm{n}}^{3+}/{\mathrm{M}\mathrm{n}}^{4+}$ mixed valency configuration of the LSMO films near the interface while tuning the oxygen pressure of overlayer BTO growth. The existing ${\mathrm{M}\mathrm{n}}^{3+}$ ions favor an in-plane e _g orbital ordering, reducing the in-plane strain and promoting room temperature ferromagntism in the LSMO film. Furthermore, diffraction experiments showed that the out-of plane lattice parameter of BTO reduces with increasing oxygen pressure consistent with the x-ray linear dichrosim at Ti edge showing less tetragonal symmetry, although the chemical environment of the Ti ions was not changed notably. We demonstrated a way to control magnetic properties and orbital ordering in LSMO thin films while optimizing the ferroelectric properties of BTO overlayer films, which are promising results in terms of magnetoelectric applications using functional heterosystems. (letter)

[en] Full text: Spin damping in thin ferromagnetic films has recently gained attraction due to its importance for the switching of fast spinelectronic devices. Here we present a FMR study of Fe films with thicknesses between 5nm and 50nm prepared by MBE on top of hexagonal GaN(0001). XRD and LEED results suggest the growth in crystallographic Fe(110) domains with three different orientations. The magnetic properties have been investigated by in-plane angle-dependent FMR at frequencies from 4.5GHz until 15.5GHz. All samples show a weak hexagonal in-plane anisotropy with the hard axis oriented parallel to the GaN [11-20] directions. The anisotropy field strength of 8mT is found to be independent of thickness and the driving frequency. The origin of the anisotropy is not clear as interface effects can be ruled out by the thickness dependence and first order cubic and uniaxial anisotropies will cancel out when averaged over the three orientations. The FMR linewidth versus frequency curves are linear with no zero-frequency offset indicating a good homogeneity of the magnetic properties over the sample area. However the effective damping parameter $/alpha$ shows a pronounced anisotropy and thickness dependence, with enhanced damping along the hard-axis and for thicker layers. We suggest that additional damping can be explained by two-magnon scattering at defects which are due to the triple domain formation