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Domain walls (DWs) propagated along nanoscale magnetic wires by current or field pulses could potentially be used for data storage or logic applications, but the understanding of the DW dynamics, particularly under the influence of spin-polarized current, is incomplete. Measuring the velocity can give insights into the physics of the DW motion. Here we demonstrate DW velocity measurements in permalloy ( Ni 80 Fe 20 ) nanowires (1500 nm width and 20 nm thickness) using the techniques of X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) to image the magnetic contrast in the nanowires, and single shot Kerr microscopy, which allows for dynamic measurements. The magnetic imaging yields the average velocity as well as information on the DW spin structure, whereas the single shot method highlights the stochastic nature of the DW motion.

Here we report the synthesis, investigation as well as surface deposition of a truly axial symmetry Mn12-diphenylphosphinate (Mn12-phn) single molecule magnet. Out of 16 acetate ligands encapsulating the Mn12O12 core, 12 ligands were exchanged by diphenylphosphinate in this compound. Mn12-phn shows well-defined magnetic hysteresis curves indicating a very high crystal quality. A monolayer of Mn12-phn was chemically grafted on a functionalized Au(1 1 1) surface via ligand exchange reaction and studied by means of scanning tunneling microscopy and spectroscopy. Via distance–voltage spectroscopy we determine the real-space height of the Mn12-phn molecules with high accuracy. A large spread in the measured molecular heights obtained from the distance–voltage spectra indicates the absence of preferential orientation of Mn12-phn molecules with respect to the surface which we attribute to the equal anchoring probability of all diphenylphosphinate ligands in Mn12-phn while none of the four acetate ligands are exchanged. These results are compared with the experimental data obtained from a different Mn12 derivative containing 16 thiophenecarboxylate ligands. In general, we show that the substitution of the ligand shell may have a major impact on the surface orientation of the Mn12 clusters deposited on Au, i.e. on the orientation of the easy magnetization axis.

We investigated Zn0.94 Co0.06 O thin films on sapphire (0001) substrates with respect to their structural and magnetic properties. X-ray diffraction shows a c axis oriented growth and no secondary phases within its resolution. A clear improvement of the crystalline quality was obtained by post annealing under vacuum conditions. Further information about the local electronic structure is obtained by X-ray absorption spectroscopy at the Co L 2,3 and the O K edge. Magnetic properties were investigated with a superconducting quantum interference device (SQUID) and by X-ray magnetic circular dichroism at the Co L 2,3 edge. Both techniques yield mainly paramagnetic behavior of the samples. For low temperatures, an additional small ferromagnetic contribution was observed in SQUID measurements. Several possible origins of this ferromagnetic contribution are discussed.

Spin- and angle-resolved photoelectron spectroscopy was applied for studies of electronic and magnetic structures of Eu/Gd and Ce/Fe. Ferromagnetic coupling of 4f moments of Eu and Gd was found in the 1 ML Eu/Gd(0 0 0 1) system with high net Eu magnetization at low temperatures reflected by a spin polarization of ∼15% of the Eu 4f state. In case of the 1 ML Ce/Fe(1 1 0) system the antiparallel orientation of the Ce 4f spins with respect to the magnetization direction of the Fe substrate was observed. Very different shapes of the spin-up and spin-down Ce 4f spectral weights can be explained within periodic Anderson model by spin-dependent hybridization between Ce localized 4f and itinerant valence band states.

We report the direct transmission electron microscopy observation of spin structure transformations in nanoscale Permalloy zigzag wires due to Joule heating during the injection of current pulses. This heating is sufficient to overcome the energy barriers separating the different metastable domain wall spin structures. Due to the large energy barriers these are stable and observable at room temperature by off-axis electron holography and Fresnel imaging. The interaction between different domain walls is probed and the main pinning mechanism is determined to be the edge roughness. In addition to transformations, we also report on thermally assisted domain wall hopping between two pinning sites and structural changes that occur when the samples are subjected to even higher current pulses.

An experimental study of domain wall motion in Ni80Fe20 ring structures induced by current pulses as well as conventional magnetic fields is presented. Using constrictions we demonstrate that current-induced domain wall motion can be used to displace walls into parts of the structure where no pulsed currents are flowing. Measurements at variable temperatures between 2 and 300 K show that the fields necessary for wall motion decrease with increasing temperature, which can be explained by thermal activation. For the current-induced case we find, depending on the geometry and temperature range, that the current densities necessary for displacement can increase or decrease with rising temperature. This indicates that, in addition to thermal excitations, an intrinsic temperature dependence of the efficiency of the spin torque effect is present and leads to an increase in the critical current density with increasing temperature.

The surface structure and electronic properties of ultrathin MgO layers grown on epitaxial Fe(110) films were investigated at room temperature by means of electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and spin-resolved photoelectron spectroscopy. The spin polarization at the Fermi level (EF) of the Fe(110) film decreases sharply with increasing thickness of the MgO layer. This behavior arises from the formation of a thin FeO layer at the MgO(111)/Fe(110) interface, as revealed by structural and spectroscopic investigations. The strong attenuation of the intrinsic spin polarization is qualitatively attributed to the scattering of spin-polarized electrons at the unoccupied d-orbitals of Fe2+.

In this paper we present the switching phase diagram and the switching field trends of mesoscopic NiFe and Co rings. The type of switching (single, double, triple) is systematically identified from the hysteresis loop for different values of the important geometrical parameters (outer diameter, ring width and film thickness). The switching fields are determined and the evolution with varying geometry is studied for a wide range of parameters (outer diameter D : 90 nm– 4 μ m , ring width W : 10–90% of D and film thickness t : 2–42 nm). The trends observed are explained in terms of the different physical processes occurring during the transitions (e.g. domain wall depinning, propagation, nucleation). By adjusting the ring geometry, the switching fields can be tailored over a wide range of values spanning more than an order of magnitude.

The surface morphology of in situ oxidized 200 A thick Fe(1 1 0) films grown on Al 2 O 3 (1 1 −2 0) substrates using a Mo(1 1 0) buffer layer was investigated by low energy electron diffraction and scanning tunneling microscopy (STM). The epitaxially grown Fe(1 1 0) films were oxidized to magnetite, Fe 3 O 4 (1 1 1), in an oxygen atmosphere. For a low oxygen exposure of 6 L at room temperature a c(2 × 2)-O-reconstruction and for an intermediate oxygen exposure of 100 L followed by annealing at 600 K the formation of FeO(1 1 1) was observed. After high oxygen exposures (⩾1500 L) and annealing at 800–1000 K the formation of Fe 3 O 4 (1 1 1) films of high crystalline quality has been achieved. Atomically resolved STM images of the Fe 3 O 4 (1 1 1) surface show a hexagonal symmetry with a 6 A periodicity. The system was identified as a well-ordered epitaxial Fe 3 O 4 (1 1 1) layer on the Fe(1 1 0)/Mo(1 1 0)/Al 2 O 3 (1 1 −2 0) system.

The growth of ultrathin Mn films on single crystal Au(1 1 1) substrates was investigated over a wide range of coverages using scanning tunneling microscopy and low energy electron diffraction (LEED) at room temperature. The measurements show that for submonolayer coverages the Mn atoms nucleate at the elbows of the (22×√3) reconstruction of the Au(1 1 1) surface and are partly incorporated into the topmost layer of the Au(1 1 1) substrate forming a surface alloy. Further deposition leads to compact alloy islands with a thickness of (2.90 ± 0.10) A. A layer-by-layer growth of fcc manganese was observed up to 3 ML. A morphology transition from fcc Mn into the cubic bulk crystal structure of Mn proceeds between 4 and 6 ML of Mn with a corresponding change in the LEED pattern from a (1 × 1) structure for 0–4 ML of Mn to a (√3×√3)R30° structure for thicker Mn films.

The role of delocalization and hybridization in complex magnetic oxides has been investigated by magnetic oxygen K-edge absorption of circular polarized soft X-rays in epitaxial grown CrO2 as a function of the azimuthal angle at grazing incidence. Unusual strong variations in the typically small X-ray magnetic circular dichroism (XMCD) signal have been observed. Those angular dependencies of the typical CrO2 oxygen K-edge XMCD signal could be quantitatively interpreted in terms of an induced anisotropic Cr 3d orbital magnetism for different spectral regions of the unoccupied density of states. The results strongly suggest a delocalized non-ionic magnetic behavior of the conduction electrons.

Highly a-axis-textured CrO2 films have been deposited on Al2O3 (0001) substrates by chemical vapor deposition. CrO2 has been found to have highly a-axis (010)-oriented columnar growth on a Cr2O3 (0001) initial layer. The six-fold surface symmetry of the Cr2O3 initial layer leads to three equivalent in-plane orientations of the a-axis-oriented CrO2 unit cell. We report Cr L2,3 X-ray magnetic circular dichroism data along the surface normal and at 60° off-normal sample orientation. For a 60° sample alignment, a strong increase of the projected orbital moment could be observed for unoccupied majority t2g states using moment analysis. Therefore, the c axis is identified as the intrinsic magnetic easy axis of CrO2. In addition, a small spin moment and a very strong magnetic dipole term Tz have been found.

Growth morphology in Co/Cu/Co films deposited on stepped Cu(0 0 1) is investigated by reflection high-energy electron diffraction (RHEED) and magneto-optical Kerr effect. The interlayer Cu film thickness is varied from fractions of a monolayer to several monolayers in order to determine the influence of Cu island nucleation on subsequent Co growth. From the analysis of the oscillations in RHEED intensity and in the uniaxial step anisotropy a growth model is proposed which extends earlier scanning tunneling microscopy observations. The different role of Co atoms at island edges as compared to those at pre-existing steps is elucidated.

Magnetic force microscope (MFM) imaging in conjunction with longitudinal Kerr hysteresis loop measurements have been used to investigate the micromagnetic behavior of micron scale epitaxial (110) bcc Fe thin-film elements (50-nm thick) with rectangular, triangular, and needle-shaped ends and competing magnetic anisotropies. Thin-film elements of 2-mm width and 6-mm length and greater have been fabricated with their long axis oriented either parallel or perpendicular to the [001] in-plane magnetocrystalline easy axis. For elements with their long axis perpendicular to the [001] direction, the end shape is critical in determining domain nucleation, domain configurations, and magnetic hysteresis. The magnetization reversal mechanisms are revealed by direct field dependent MFM imaging. Magnetic vortex configurations within elements during reversal are seen to be affected by small changes in element corner shape. Similarly, small trapped domains and domain walls are observed to applied fields significantly larger than the coercive field and apparent magnetic saturation field, as determined by hysteresis loop measurements of arrays of elements. These are shown to have a dramatic effect on the character of the low-field magnetic hysteresis. Particles with long axis parallel to the [001] direction have large remanence and switching fields which also depend sensitively on end shape. The angular dependence of the switching field observed in these elements is contrasted to that of magnetization reversal by coherent rotation.

We probe the electron spin dynamics in ZnO and Zn1-x Cox O sol-gel films with time-resolved Faraday rotation spectroscopy. Dephasing times T 2 * on the order of nanoseconds are observed at room temperature due to charge-separated states. In ZnCoO the effective electron Lande g factor rises with increasing Co2+ concentration, providing the mean-field electron-Co2+ exchange energy N 0 α = +0.25 ± 0.02 eV.

Bi/Mn/Al/Bi/Mn multilayers with different Al‐interlayer thickness have been deposited on glass substrates at room temperature. The multilayers are not protected by a passivating layer, such as SiOx, in order to investigate the interplay between Al‐interlayer thickness and homogeneity, granularity, and topography before and after annealing. Due to the Al interlayer, the diffusion across the Bi/Mn/Al/Bi/Mn stack was reduced as demonstrated by x‐ray analysis, scanning electron microscopy, and Rutherford backscattering spectrometry. During annealing, two (MnxBi100−x)100−yAly layers are formed which are separated by an Al interlayer. After annealing, polar Kerr hysteresis loops as measured from the front side of the films show a superposition of two hysteresis loops, opposite in sign, with different coercive fields. The presence of different coercive fields is explained by different granularity of the top and bottom (MnxBi100−x)100−yAly layer. The coercive field of the top (MnxBi100−x)100−yAly layer reaches va...

We deposited (Mn/Bi)x bilayers (x=1,2,3) on clean glass substrates at room temperature and at 200 °C without using a protective SiOx layer in order to investigate the interaction between preparation conditions, homogeneity, granularity, and topography. During annealing at temperatures between 280 and 320 °C, no protective SiOx layer on top of the (Mn/Bi)x bilayers influences the formation of MnBi crystallites, i.e., the granularity. The coercive field of the resulting MnBi films is enhanced reaching values of up to 1.25 T. The large coercive fields indicate a single‐domain MnBi crystallite size of only 0.2–0.3 μm, which is favorable for a possible application as a magneto‐optic storage material.

Optical and magneto-optic properties at room temperature of Co/sub 1-x/(EuS)/sub x/ evaporated thin films are presented over a wide photon-energy range. Co/sub 1-x/(EuS)/sub x/ belongs to a new class of phase-separated magnetic materials, called macroscopic ferrimagnets, in which two magnetic phases couple antiferromagnetically across the phase boundary. Polar Kerr rotations of up to 2/spl deg/ are observed in Co rich samples (x/spl les/0.3) at photon energies of 4.5 eV. The high Kerr rotation coincides with a distinct decrease in reflectivity suggesting an optical enhancement effect. A model calculation indicates a new type of enhancement based on the relative index of refraction at the phase boundary between the Co matrix and the EuS precipitate particles.

Electronic and structural properties of epitaxial ultrathin aluminum oxide layers grown on epitaxial Fe(110) films were investigated at room temperature by means of spin-, angle- and energy-resolved photoemission spectroscopy. A spin polarization of −(15±5)% near the Fermi energy EF is found for 1 (ML) of α-Al2O3 on epitaxial Fe(110). This value results from the attenuation of −80% of Fe(110) by the oxide layer thickness and the spin-independent part of the scattering cross section.

We use time-resolved Faraday rotation spectroscopy to probe the electron spin dynamics in ZnO and magnetically doped Zn1-xCoxO sol-gel thin films. In undoped ZnO, we observe an anomalous temperature dependence of the ensemble spin dephasing time T2*, i.e., longer coherence times at higher temperatures, reaching T2* ∼ 1.2 ns at room temperature. Time-resolved transmission measurements suggest that this effect arises from hole trapping at grain surfaces. Deliberate addition of Co2+ to ZnO increases the effective electron Landé g factor, providing the first direct determination of the mean-field electron-Co2+ exchange energy in Zn1-xCoxO (N0alpha = +0.25 (0.02 eV). In Zn1-xCoxO, T2* also increases with increasing temperature,allowing spin precession to be observed even at room temperature.

We use time-resolved Faraday rotation spectroscopy to probe the electron spin dynamics in ZnO and magnetically doped Zn(1-x)Co(x)O sol-gel thin films. In undoped ZnO, we observe an anomalous temperature dependence of the ensemble spin dephasing time T(2), i.e., longer coherence times at higher temperatures, reaching T(2) ∼ 1.2 ns at room temperature. Time-resolved transmission measurements suggest that this effect arises from hole trapping at grain surfaces. Deliberate addition of Co(2+) to ZnO increases the effective electron Landé g factor, providing the first direct determination of the mean-field electron-Co(2+) exchange energy in Zn(1-x)Co(x)O (N(0)α = +0.25 ± 0.02 eV). In Zn(1-x)Co(x)O, T(2) also increases with increasing temperature, allowing spin precession to be observed even at room temperature.

We present a detailed study of the spin-dependent electronic structure of thin epitaxial magnetite films of different crystallographic orientations. Using spin- and angle-resolved photoelectron spectroscopy at room temperature, we determine for epitaxial Fe3O4(111) films a maximum spin polarization value of −(80 ± 5)% near EF. The spin-resolved photoelectron spectra for binding energies between 1.5 eV and EF show good agreement with the spin-split band structure from density functional theory (DFT) calculations which predict an overall energy gap in the spin-up electron bands in high symmetry directions, thus providing evidence for the half-metallic ferromagnetic state of Fe3O4 in the [111] direction. In the case of the Fe3O4(100) surface, both the spin-resolved photoelectron spectroscopy experiments and the DFT density of states give evidence for a half-metal to metal transition: the measured spin polarization of about − (55±10)%at EF and the theoretical value of −40% are significantly lower than the − 100% predicted by local spin density approximation (LSDA) calculations for the bulk magnetite crystal as well as the −(80 ± 5)% obtained for the Fe3O4(111) films. The experimental findings were corroborated by DFT calculations as due to a surface reconstruction leading to the electronic states in the majority-spin band gap and thus to the reduced spin polarization.(Some figures in this article are in colour only in the electronic version)

Magnetic domains and domain walls in epitaxial Fe3O4(100) elements (rings and wires) are imaged using magnetic force microscopy and photoemission electron microscopy. We show that the interplay between the four-fold magnetocrystalline anisotropy and the shape determines the equilibrium domain structure. Domain walls with a characteristic zig-zag structure are observed in Fe3O4(100) elements initially magnetized along one of the magnetocrystalline hard axes. We attribute the formation of zig-zag domain walls to the competition of the four-fold magnetocrystalline anisotropy, the exchange and dipolar coupling. A direct correlation between the wire width and the spin structure of zig-zag domain walls is found.

The magnetization reversal processes and magnetic domain states of microstructured epitaxial Fe(110) elements have been investigated by magnetic-force microscopy and longitudinal Kerr hysteresis loop measurements. The characteristic micromagnetic behavior, such as coercive and nucleation fields, can be tailored by taking advantage of the pronounced uniaxial anisotropy and by varying the shape of the elements. The magnetic domain states of rectangular and diamond-shaped elements with lateral dimensions of 1.5 µm × 0.5 µm have been investigated after magnetic saturation along the long and short axes of the elements. The observed flux-closure domain states have been compared with micromagnetic simulations.

Tunnel junctions of Co(10 nm)/AlOx (nominally 2 nm)/Co(20 nm) have been prepared by molecular beam epitaxy applying a shadow mask technique in conjunction with an UV light-assisted oxidation process of the AlOx barrier. The quality of the AlOx barrier has been proven by x-ray photoelectron spectroscopy and temperature dependent tunneling magnetoresistance (TMR) measurements. Optimum-oxidized tunnel junctions show a TMR of 20% at 285 K and up to 36% at 100 K. At 285 K the TMR values as a function of oxidation time are not symmetric about the optimum time. For underoxidized junctions the TMR is reduced more strongly than for overoxidized junctions. The temperature dependence of the junction’s resistance is a clear and reliable indicator whether pinholes (or imperfections) contribute to the conduction across the barrier.

Co(10 nm)/AlOx(nominally 2 nm)/Co(20 nm) tunnel junctions have been prepared under ultrahigh vacuum conditions applying a shadow mask technique. An ultraviolet light-assisted oxidation process of the AlOx barrier has been optimized by in situ x-ray photoelectron spectroscopy, in conjunction with temperature-dependent tunneling magnetoresistance measurements. Optimum-oxidized tunnel junctions show a magnetoresistance of 20% at 285 K, and up to 38% at 100 K. For under-oxidized samples, with a remaining Al layer between the Co bottom electrode and the AlOx barrier, the tunneling magnetoresistance decreases more rapidly with increasing temperature than observed for the over-oxidized samples. The resistance × area product of optimum-oxidized tunneling junctions exhibits a minimum, and increases for under- and over-oxidized samples.

We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be � ¼ 0:15 � 0:07, which is more than an order of magnitude larger than the damping constant � , pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.

We review our recent magnetotransport and micromagnetic studies of lithographically defined epitaxial thin film structures of bcc Fe and hcp Co with stripe domains. Micromagnetic structure and resistivity anisotropy are shown to be the predominant sources of low field magnetoresistance (MR) in these microstructures, with domain wall (DW) effects smaller but observable (DW-MR $\lesssim 1 %$). In Fe, at low temperature, in a regime in which fields have a significant effect on electron trajectories, a novel negative DW contribution to the resistivity is observed. In hcp Co microstructures, temperature dependent transport measurements for current perpendicular and parallel to walls show that any additional resistivity due to DW scattering is very small.

Regularly sized Ni nanoclusters (NCs) have been grown on a graphene Moiré on Rh(111). Using scanning tunneling microscopy, we determine that initial growth of Ni at 150 K leads to preferential nucleation of monodispersed NCs at specific sites of the Moiré superstructure. However, a defined long-range ordering of NCs with increasing coverage is not observed. Room temperature Ni deposition leads to the formation of flat triangular-shaped islands which are well-matched to the Moiré registry.

We have investigated the magnetoresistance of Permalloy (Ni80Fe20) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and SiO2 substrates. Growth and transport measurements were carried out under cryogenic conditions in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties are ascertained during growth. With increasing thickness the films exhibit a gradual transition from tunneling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains as well as the transition from superparamagnetic to ferromagnetic behavior of the film. Using an analysis based on a theoretical model of the island growth, we find that the observed evolution of the magnetoresistance in the tunneling regime originates from the changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunneling magnetoresistance and anisotropic magnetoresistance were found. We attribute this to an increasingly pronounced island growth and slower percolation process of Permalloy when comparing growth on SiO2, MgO and Al2O3 substrates. The different growth characteristics result in a markedly earlier onset of both tunneling magnetoresistance and anisotropic magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in a structure of the film containing two different contributions to the ferromagnetism which lead to two distinct coercive fields in the high thickness regime., 8 pages, 7 figures

The polar Kerr rotation and ellipticity spectra of epitaxially grown (001)-oriented half-metallic Sr2FeMoO6 thin films have been determined in the photon energy range from 1.2 to 4.9 eV. The Kerr rotation spectrum shows three maxima at E = 1.6, 4.0, and 4.65 eV. The maxima at 4.0 and 4.65 eV are consistent with spin-polarized band structure calculations for interband transitions from the O-2p to the minority-spin pi*Mo/Fe and majority-spin Mo-t2g bands, respectively. The overall maximum intrinsic Kerr rotation is ThetaK = 0.045° at a photon energy of 4.65 eV. The maximum of ThetaK at E = 1.6 eV coincides with a minimum in the reflectivity due to the plasma edge of Sr2FeMoO6 and, therefore, is not related to an interband transition.

We study the relationship between the damping $(\ensuremath{\alpha})$ and the nonadiabaticity of the spin transport $(\ensuremath{\beta})$ in permalloy nanowires. $\ensuremath{\alpha}$ is engineered by Ho doping, and from the characteristics of the current-induced domain-wall velocity, determined by high-resolution x-ray magnetic circular-dichroism photoemission electron microscopy, $\ensuremath{\beta}$ due to spin relaxation is measured. We find that $\ensuremath{\beta}$ scales with $\ensuremath{\alpha}$ and conclude that the spin relaxation that leads to nonadiabatic spin torque originates from the same underlying mechanism as the angular momentum dissipation that causes viscous damping.

Herein, different concepts for domain wall propagation based on currents and fields that could potentially be used in magnetic data storage devices based on domains and domain walls are reviewed. By direct imaging, we show that vortex and transverse walls can be displaced using currents due to the spin transfer torque effect. For the case of field-induced wall motion, particular attention is paid to the influence of localized fields and local heating on the depinning and propagation of domain walls. Using an Au nanowire adjacent to a permalloy structure with a domain wall, the depinning field of the wall, when current pulses are injected into the Au nanowire, was studied. The current pulse drastically modified the depinning field, which depended on the interplay between the externally applied field direction and polarity of the current, leading subsequently to an Oersted field and heating of the permalloy at the interface with the Au wire. Placing the domain wall at various distances from the Au wire and studying different wall propagation directions, the range of Joule heating and Oersted field was determined; both effects could be separated. Approaches beyond conventional field- and current-induced wall displacement are briefly discussed.

The possibility to use the Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) system as a substrate for future spin-polarized transport measurements on Mn12 single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn12 monolayers was studied. The results demonstrate that Mn12 can be deposited on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies.

In a combined theoretical and experimental study, we investigate the critical current densities for vortex domain walls in magnetic nanowires. We systematically determine the critical current densities for continuous motion of vortex walls as a function of the wire width for different wire thicknesses and we find that the critical current density increases monotonously with decreasing wire width. Theoretically we present a mechanism that predicts a threshold current density based on wall transformations and this leads to a scaling of the critical current density ${j}_{c}\ensuremath{\propto}1/\text{width}$. The origin of this scaling is found to be the different dependence of the spin torque energy and the vortex nucleation energy on the wire width and good agreement with the experimental observations is found.

The formation of self-assembled monolayers (SAMs) on gold by 2-(5-iodo-2-nitrophenyl) propoxycarbonyl (I-NPPOC)-protected thymidine with an attached mercaptohexyl succinate linker and the kinetics of photochemical release of the I-NPPOC group were monitored using X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) detection. In the XPS spectra, the iodine peaks allowed for specific and accurate monitoring of the presence and loss of I-NPPOC groups on the surface. In the SPR experiment, the overall signal change on photoillumination is in accord with a theoretical estimation of the density of I-NPPOC groups in a dense monolayer. The kinetics roughly follow a biexponential time dependence with two very different time constants, corresponding to photochemical quantum yields of 0.22 and 0.0032, respectively.

The magneto-optic properties of multilayered MnBi/Al films have been investigated and compared to pure MnBi films. The Al interlayers act as diffusion barrier during the MnBi formation process leading to a reduced MnBi particle size of approximately 50 nm. The smaller particle size changes the dominant magnetization reversal process from domain wall movement towards coherent rotation. This leads to a suppression of the anomalous increase of the high-temperature coercivity observed in pure MnBi. The comparison of the experimental polar Kerr spectra with theory elucidates the role of oxygen in MnBi films.

The magnetization reversal and magnetic domain configurations of 0.5-μm-wide epitaxial (110) Fe particles with rectangular and needle-shaped ends and competing magnetic anisotropies have been investigated. Magnetic force microscopy imaging and longitudinal Kerr hysteresis loop measurements in conjunction with micromagnetic simulations have been used to elucidate the basic micromagnetic behavior. End shape is shown to be a determining factor for the nucleation of magnetization reversal and the resulting magnetic domain configurations.

Using microwave currents, we excite resonances of geometrically confined pinned domain walls, detecting the resonance by the rectification of the microwave current. By applying magnetic fields, the resonance frequency of the domain wall oscillator can be tuned over a wide range. Increasing the power leads to a redshift due to the nonlinearity of the system. From this frequency shift, we directly deduce the quantitative shape of the potential, so that a complete characterization of the pinning potential is obtained.

Current induced domain wall (DW) depinning of a narrow DW in out-of-plane magnetized (Pt/Co)_{3}/Pt multilayer elements is studied by magnetotransport. We find that for conventional measurements Joule heating effects conceal the real spin torque efficiency and so we use a measurement scheme at a constant sample temperature to unambiguously extract the spin torque contribution. From the variation of the depinning magnetic field with the current pulse amplitude we directly deduce the large nonadiabaticity factor in this material and we find that its amplitude is consistent with a momentum transfer mechanism.

Individual ${\text{Mn}}_{12}$ single-molecule magnets have been investigated by means of scanning tunneling spectroscopy at room temperature. Current-voltage characteristics of a ${\text{Mn}}_{12}$ derivative are studied in detail and compared with simulations. A few-parameter scalar model for ballistic current flow through a single energy level is sufficient to describe the main features observed in scanning tunneling spectra of individual ${\text{Mn}}_{12}$ molecules and offers a deeper insight into the electronic transport properties of this class of single-molecule magnets. In addition, distance-voltage spectroscopy performed on individual ${\text{Mn}}_{12}$ molecules reveals a possibility to identify the orientation of the molecular easy axis. The results indicate a preferential orientation of the easy axis of the molecules nearly perpendicular to the surface.

Ferromagnetism at room temperature has been experimentally observed for several doped oxidic semiconductors (Co:TiO2, Mn:TiO2, Co:ZnO, Mn:ZnO, etc.) [1], but its origin is still under discussion. Therefore, Co(3 at%)-doped ZnO layers of approximately 150 nm thickness were grown on (001)Si and (001)sapphire substrates at 500°C by magnetron sputtering. For each substrate the Zn0.94Co0.06O layers were deposited under oxygen-poor and -rich conditions by using Ar and Ar/O2, respectively, for sputtering and post-annealing in O2 to ensure oxygen saturation for samples prepared under oxygen rich conditions. The magnetic properties of the Co-doped samples were investigated by SQUID (superconducting quantum interference device) magnetometry. Only samples that were grown under oxygen-poor conditions with no oxygen post-annealing on sapphire show a small ferromagnetic contribution to the paramagnetic background, while oxygen-rich ones and those on Si substrate do not, even at low temperatures. To correlate the magnetic behavior with both structural and chemical materials properties, transmission electron microscopy (TEM) including high-resolution and analytical TEM (HRTEM/AEM) have been applied.

International audience; Fully epitaxial Fe(110)/MgO(111)/Fe(110) magnetic tunnel junctions (MTJs) have been tested with respect to symmetry-enforced spin filtering. The Fe(110) electrodes exhibit Σ1↑ and Σ1↓ spin states, both crossing the Fermi level, but with a group velocity about 50% smaller for the minority states compared to the majority ones. These epitaxial but symmetry-mismatched MTJs yield tunneling magnetoresistance (TMR) values of 54% at 1.5 K and 28% at room temperature. The TMR value and the estimated tunneling spin polarization are consistent with a partial spin filtering due to the Σ1↑ states partially compensated by the Σ1↓ states.

Here we report the photoemission studies of intercalation process of Fe underneath graphene layer on Ni(111). The process of intercalation was monitored via XPS of corresponding core levels and UPS of the graphene-derived $\pi$ states in the valence band. \textit{fcc}-Fe films with thickness of 2-5 monolayers at the interface between graphene and Ni(111) form epitaxial magnetic layer passivated from the reactive environment, like for example oxygen gas., Comment: 8 pages, 3 figures

Different approaches to the deposition of Mn(12) single molecule magnets on the Au(111) surface and their characterization by a broad variety of techniques are investigated with respect to their suitability for a profound corroboration of the integrity of the Mn(12) core. In this context, the most recent improvements in the experimental approaches are presented and the latest results on the electronic properties of Mn(12) are linked to each other. The results confirm the high instability of Mn(12) single molecule magnets on surfaces and reveal the need for an amendment of the requirements to define the structural integrity of Mn(12) molecules on surfaces.

We report on the electronic properties of Mn12 molecules chemically grafted on the functionalized Au(111) surface studied by means of scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from Mn12 molecules showing a large region of low conductance around the Fermi energy. In agreement with the tunneling spectroscopy results the bias voltage variation upon scanning leads to apparent height changes of the Mn12 clusters. We discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn12 molecules.

By direct imaging we determine spin structure changes in Permalloy wires and disks due to spin transfer torque as well as the critical current densities for different domain wall types. Periodic domain wall transformations from transverse to vortex walls and vice versa are observed, and the transformation mechanism occurs by vortex core displacement perpendicular to the wire. The results imply that the nonadiabaticity parameter $\ensuremath{\beta}$ does not equal the damping $\ensuremath{\alpha}$, in agreement with recent theoretical predictions. The vortex core motion perpendicular to the current is further studied in disks revealing that the displacement in opposite directions can be attributed to different polarities of the vortex core.

The magnetization reversal in square lattice cobalt antidot arrays with the applied field at 45° to the antidot rows was investigated using Lorentz electron microscopy in the Fresnel mode. While the hysteresis loops from magneto-optical Kerr effect measurements only reflect the easy axis character of the reversal, several different reversal processes were identified in the Fresnel images depending on the field history. Details of this complex magnetization reversal were elucidated with micromagnetic simulations.

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20 nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500 nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1x10 12 A/m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in.