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The interfacial Dzyaloshinskii-Moriya interaction (DMI) is intimately related to the prospect of superior domain-wall dynamics and the formation of magnetic skyrmions. Although some experimental efforts have been recently proposed to quantify these interactions and the underlying physics, it is still far from trivial to address the interfacial DMI. Inspired by the reported tilt of the magnetization of the side edge of a thin film structure, we here present a quasi-static, straightforward measurement tool. By using laterally asymmetric triangular-shaped microstructures, it is demonstrated that interfacial DMI combined with an in-plane magnetic field yields a unique and significant shift in magnetic hysteresis. By systematic variation of the shape of the triangular objects combined with a droplet model for domain nucleation, a robust value for the strength and sign of interfacial DMI is obtained. This method gives immediate and quantitative access to DMI, enabling a much faster exploration of new DMI systems for future nanotechnology., Comment: 4 figures, 1 table

We have observed a longitudinal nonlinear (second-harmonic) Kerr rotation up to 22\ifmmode^\circ\else\textdegree\fi{} for an Fe(2 nm)/Cr(2 nm) multilayer on an Si${\mathrm{O}}_{2}$ substrate that shows a linear Kerr rotation of only a few hundreths of a degree. The latter value excludes any linearly induced mechanism and confirms the interface sensitivity of this nonlinear Kerr technique. The enormous enhancement is shown to be in good agreement with recent predictions, based on spin dependent band structure calculations. The local optical field effects are shown to play an important role for the additional enhancements for such multilayers on a substrate.

Perpendicularly magnetized materials have attracted significant interest owing to their high anisotropy, which gives rise to extremely narrow, nanosized domain walls. As a result, the recently studied current-induced domain wall motion (CIDWM) in these materials promises to enable a new class of data, memory and logic devices1, 2, 3, 4, 5. Here we propose the spin Hall effect as an alternative mechanism for CIDWM. We are able to carefully tune the net spin Hall current in depinning experiments on Pt/Co/Pt nanowires, offering unique control over CIDWM. Furthermore, we determine that the depinning efficiency is intimately related to the internal structure of the domain wall, which we control by the application of small fields along the nanowire. This manifestation of CIDWM offers an attractive degree of freedom for manipulating domain wall motion by charge currents, and sheds light on the existence of contradicting reports on CIDWM in perpendicularly magnetized materials6, 7, 8, 9, 10, 11.

Domain wall motion in materials exhibiting perpendicular magnetic anisotropy has been the subject of intensive research because of its large potential for future spintronic devices. Recently, it has been shown that perpendicular anisotropy of thin films can be influenced by electric fields. Voltage-controlled magnetic switching has already been realized, which is envisioned to lead to low-power logic and memory devices. Here we demonstrate a radically new application of this effect, namely control of domain wall motion by electric fields. We show that an applied voltage perpendicular to a Co or CoB wire can significantly increase or decrease domain wall velocities. Velocity modification over an order of magnitude is demonstrated (from 0.4 to 4 μm s−1), providing a first step towards electrical control of domain wall devices. This opens up possibilities of real-time and local control of domain wall motion by electric fields at extremely low power cost. The motion of domain walls in magnetic materials characterized by a perpendicular axis of magnetization is a promising means of controlling information in future memory and logic devices. Schellekenset al. show the velocity of domain walls in such systems can be controlled by using an applied electric field.

Novel nanofabrication methods and the discovery of an efficient manipulation of local magnetization based on spin polarized currents has generated a tremendous interest in the field of spintronics. The search for materials allowing for fast domain wall dynamics requires fundamental research into the effects involved (Oersted fields, adiabatic and non-adiabatic spin torque, Joule heating) and possibilities for a quantitative comparison. Theoretical descriptions reveal a material and geometry dependence of the non-adiabaticity factor beta, which governs the domain wall velocity. Here, we present two independent approaches for determining beta : (i) measuring the dependence of the dwell times for which a domain wall stays in a metastable pinning state on the injected current and (ii) the current-field equivalence approach. The comparison of the deduced beta values highlights the problems of using one-dimensional models to describe two-dimensional dynamics and allows us to ascertain the reliability, robustness and limits of the approaches used.

Despite the relevance of current-induced magnetic domain wall (DW) motion for new spintronics applications, the exact details of the current-domain wall interaction are not yet understood. A property intimately related to this interaction is the intrinsic DW resistivity. Here, we investigate experimentally how the resistivity inside a DW depends on the wall width D, which is tuned using focused ion beam irradiation of Pt/Co/Pt strips. We observe the nucleation of individual DWs with Kerr microscopy, and measure resistance changes in real-time. A 1/D^2 dependence of DW resistivity is found, compatible with Levy-Zhang theory. Also quantitative agreement with theory is found by taking full account of the current flowing through each individual layer inside the multilayer stack., Submitted to Physical Review Letters 4 pages, 3 figures

We explore the magnetoelectroluminescence (MEL) of organic light-emitting diodes by evaluating the magnetic-field dependent fraction of singlet excitons formed. We use two- and multisite polaron-hopping models with spin mixing by hyperfine fields and different singlet and triplet exciton formation rates k(S) and k(T). A huge MEL is predicted when exciton formation is in competition with spin mixing and when k(T) is significantly larger than k(S). This competition also leads to a low-field structure in the MEL that is in agreement with recent experiments.

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.

The stochastic Liouville equation is applied to the field of organic magnetoresistance to perform detailed microscopic calculations on the different proposed models. By adapting this equation, the influence of a magnetic field on the current in bipolaron, electron-hole pair, and triplet models is calculated. The simplicity and wide applicability of the stochastic Liouville equation makes it a powerful tool for interpreting experimental results on magnetoresistance measurements in organic semiconductors. New insights are gained on the influence of hopping rates and disorder on the magnetoresistance.

The dependence of organic magnetoresistance (OMAR) on the orientation of the magnetic field has been investigated. In contrast with previous claims, a finite and systematic change in magnitude is observed when the orientation of the field is changed with respect to the sample. It is demonstrated that, to explain these effects, spin-spin interactions have to be included in the models previously suggested for OMAR. Dipole coupling and exchange coupling are introduced in combination with either an anisotropy of the orientation of the spin pairs or an anisotropy in the hyperfine fields.

We investigate the non-adiabaticity of current-induced domain wall motion by time resolved analysis of thermally activated domain wall motion between two metastable states within a Co/Pt multilayer wire with a strong uniaxial perpendicular anisotropy. By measuring the dwell times for which the domain wall remains in one state, we deduce the non-adiabaticity factor beta using two independent approaches: (i) the dependence of the dwell times on the injected current and (ii) the current-field equivalency. The comparison of the results allows us to gauge their reliability and the observed differences highlight the importance of the two dimensional (2D) nature of the domain wall. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663215]

We systematically study the changes in the local atomic environments of Co in CoFe-Al alloys as a function of Al content by means of nuclear magnetic resonance. We find that a Co2 FeAl Heusler type structure is formed on a local scale. The observed formation of a highly spin-polarized Heusler compound may explain the improved magnetotransport properties in CoFe-Al based current-perpendicular-to-the-plane spin-valves.

A surprisingly large "organic magnetoresistance" (OMAR) has been found in both polymers and small molecule organic semiconductors at relatively small applied magnetic fields and at room temperature. We review recent highlights in OMAR research and discuss some of the models that have been proposed to explain the effect. In all models spin dephasing due to hyperfine fields plays an essential role. In particular we focus on the characteristic magnetic field dependence, which is generally fitted with either a Lorentzian or a so-called non-Lorentzian function. The shape is determined by both the hyperfine fields and an additional broadening due to microscopic mechanisms, as described in the models. Within the present work, a new empirical function is introduced that captures the two effects separately and converges to the earlier introduced lineshapes in specific limits. Recently it has been demonstrated that an additional feature can be observed at ultra-small magnetic fields. This effect can be easily incorporated in our empirical approach by explicitly treating the limit in which hopping of carriers is no longer slow compared to spin precession in the hyperfine fields. Our approach is used to analyze several theoretical and experimental results. It is shown that experimentally observed trends can be well-understood and important parameters can be obtained from experimental data without prior knowledge about which model applies.

New developments in the nascent field of organic spintronics are discussed. Two classes of phenomena can be discerned. In hybrid organic spin valves (OSVs), an organic semiconducting film is sandwiched between two ferromagnetic (FM) thin films, aiming at magnetoresistive effects as a function of the relative alignment of the respective magnetization directions. Alternatively, organic magnetoresistance (OMAR) is achieved without any FM components, and is an intrinsic property of the organic semiconductor material. Some of the exciting characteristics of OMAR, in both electrical conductance and photoconductance, are presented. A systematic, combined experimental–theoretical study of sign changes between positive and negative magnetoresistance is shown to provide important insight about the underlying mechanisms of OMAR. A simple explanation of experimental observations is obtained by combining a ‘spin-blocking’ mechanism, an essential ingredient in the recently proposed bipolaron model, with specific features of the device physics of space charge limited current devices in the bipolar regime. Finally, we discuss possible links between the physics relevant for OMAR and that for OSVs. More specifically, weak hyperfine fields from the hydrogen atoms in organic materials are thought to be crucial for a proper understanding of both types of phenomena.

We studied the optical-second-harmonic generation from thin C60 films, using a combination of frequency-, rotational-, angular-, and film-thickness-dependent measurements. We present a method to resolve the phase of chi(2) by exploiting the interference between the C60 overlayer contribution and the anisotropic Si(111) substrate contributions. We find a strong and sharp resonance at 2HBARomega = 3.60 eV. The linewidth is found to be surprisingly small as compared to linear ellipsometry and third-harmonic-generation experiments. The intensity as a function of film thickness agrees with a model considering interference effects between equal second-harmonic contributions- from the inner C60/Si interface and the outer C60 surface. The possibility of nonlocal bulk contributions is discussed.

Spin pumping and spin-to-charge conversion in hybrid metal–organic devices reveal the physical mechanisms at work in semiconducting polymers.

Real-time in situ optical measurements were performed during the oxidation of ultrathin Al layers with a view to study oxidation kinetics and provide process control. The optical technique combines high temporal and spatial resolution with submonolayer sensitivity. In situ x-ray photoelectron spectroscopy measurements on the same samples are in good agreement, confirming the accuracy and sensitivity of the technique. For all thicknesses studied, the initial oxidation is extremely rapid, with quasilogarithmic behavior over the whole time scale. This technique could be utilized to study a wide variety of in situ reactive processes.

10 páginas, 8 figuras, 1 tabla.-- et al., 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 H2O 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., Funding by the Spanish Ministry of Science through project no. MAT2008- 06567-C02, including FEDER funding, and the Aragon Regional Government grant no. E26 are acknowledged.

We demonstrate that a part of interface at a subsurface nanocavity in Cu(110) can efficiently induce electron scattering back to the surface even if it is inclined with respect to the surface, if the condition for electron diffraction is fulfilled. This backscattering induces oscillations of electron local density of states at the surface versus electron energy. In agreement with our model calculations, the diffraction is assigned to a specific atomic structure at the interface, and is found to be significantly enhanced by focussing of electron waves for propagation along the [110] direction.

We report on low temperature current induced domain wall depinning experiments on (Co/Pt) multilayer nanowires with perpendicular magnetization. Using a special experimental scheme, we are able to extract the different contributions of the Oersted field and spin torque from the dependence of the depinning field on the injected current for selected magnetization configurations. The spin torque contribution is found to be dominant with a small contribution of the Oersted field leading to a nonadiabaticity factor beta in line with previous measurements. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3405712]

A general rule is derived, relating the order of a nonlinear optical process to the highest possible symmetry which can be resolved in a rotational analysis. We show that with an Nth order optical technique, rotational anisotropy can be observed only up to (N + L)-fold rotational symmetry, where L is the multipole order of the nonlinear process. We discuss the connection with rotational (non)linear optical surface studies as carried out on cubic materials, as well as implications for noncrystalline point-group symmetries.

We present a theory for spin diffusion in disordered organic semiconductors, based on incoherent hopping of a charge carrier and coherent precession of its spin in an effective magnetic field, composed of the random hyperfine field of hydrogen nuclei and an applied magnetic field. From Monte Carlo simulations and an analysis of the waiting-time distribution of the carrier we predict a surprisingly weak temperature dependence, but a considerable magnetic-field dependence of the spin-diffusion length. We show that both predictions are in agreement with experiments on organic spin valves.

We report a correlation between the spin polarization of the tunneling electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF. We show that probing the s and d-bands individually provides clear and crucial evidence for such a correlation to exist through s-d hybridization, and demonstrate the tuneability of the electronic and magnetic properties of CoFeB alloys., Accepted for publication in Physical Review Letters. Letter (4 pages) and Supplementary material (4 pages)

By combining experiments with simple model calculations, we obtain new insight in spin transport through hybrid, $\mathrm{CoFeB}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(1.5\text{ }\text{ }\mathrm{nm})/\mathrm{\text{tris}}(8\mathrm{\text{-hydroxyquinoline}})\mathrm{\text{aluminium}}$ $({\mathrm{Alq}}_{3})/\mathrm{Co}$ spin valves. We have measured the characteristic changes in the $I\mathrm{\text{\ensuremath{-}}}V$ behavior as well as the intrinsic loss of magnetoresistance at the onset of multiple-step tunneling. In the regime of multiple-step tunneling, under the condition of low hopping rates, spin precession in the presence of hyperfine coupling is conjectured to be the relevant source of spin relaxation. A quantitative analysis leads to the prediction of a symmetric magnetoresistance around zero magnetic field in addition to the hysteretic magnetoresistance curves, which are indeed observed in our experiments.

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.

Apparent c(2×2) superstructures within the narrow beams of an interference pattern spreading in the 100 directions at the surface of Cu(001) are observed by scanning tunneling microscopy. These features are induced by electron scattering from Ar- and Ne-filled subsurface nanocavities. The beams originate from electron anisotropy resulting in focusing of bulk electrons. We developed a model providing a good agreement between simulations and experiments. Particularly, a simple explanation of the angular distribution for the interference pattern and the period in the superstructure is found. © 2009 The American Physical Society.

Argon-filled nanocavities embedded in a single crystal of copper near the surface reflect electrons and induce a quantum well (QW) between the nanocavity and the atomically flat Cu(001) surface. The spatial variation of conductance at the surface above the nanocavity was studied by scanning tunnelling microscopy and/or spectroscopy. Interference features were observed over several nanometers at some locations on the surface. In the [100] and [010] directions, the interference fringes propagate over longer distances up to tens of nanometers. In addition to these spatially resolved features, the conductance reveals an oscillatory behavior as a function of energy of injected electrons. A model taking into account the specific shape of the nanocavity, as well as the band structure of copper, allows us to simulate the spatial variation of the conductance in close agreement with the experiment. The modeling demonstrates that not only the specific shape of the subsurface nanocavity reflecting electrons is crucial to explain the observed pattern, but also the anisotropy of the band structure and the phenomenon of focusing of hot electrons connected to it. Our approach opens up opportunities to examine buried nano-objects with scanning tunneling microscopy, and also to study how the anisotropy of a crystal influences the spatial variation of QW properties.

The recently proposed bipolaron model for large "organic magnetoresistance" (OMAR) at room temperature is extended to an analytically solvable two-site scheme. It is shown that even this extremely simplified approach reproduces some of the key features of OMAR, viz., the possibility to have both positive and negative magnetoresistance, as well as its universal line shapes. Specific behavior and limiting cases are discussed. Extensions of the model, to guide future experiments and numerical Monte Carlo studies, are suggested.

In this work, we show that an electric field can be used to tune the g-factor in (In,Ga)As quantum dots and that one can even change its sign. We measure the degree of circular polarization of photoluminescence of quantum dots in a magnetic field. The quantum dots are grown in the center of a GaAs p–i–n structure and they experience, therefore, a built-in electric field. This electric field can be compensated by photo-created charge carriers. The circular polarization changes as the excitation density is increased, and eventually changes its sign, which is interpreted as a change of sign of the exciton g-factor. This sign change is associated with the electric field, since the Stark shift in the photoluminescence spectrum occurs in the same excitation density range as the polarization sign change. Furthermore, we extract a longitudinal spin relaxation time T1 of 10.1 ns. A time-resolved Kerr rotation spectroscopy measurement yields an in-plane g-factor for the electron in the range 0.3–0.4 and a decoherence time T2 in the range 500–800 ps.

Surface structures and related electronic properties of flat Co nanoislands supported on Cu(1 1 1) are studied before and after adsorbing different doses of molecular hydrogen at 10 K by low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) at 5 K. It is found that the adsorption process on Co consists in two steps. First, H 2 dissociatively chemisorbs on Co forming an atomic H adlayer. Subsequently, molecules impinging on that H-terminated Co surface are physisorbed. When adsorbing a low dose of hydrogen on the Co/Cu(1 1 1) system, STM analysis shows that the chemisorbed hydrogen adlayer on Co forms a p( 2 × 2 ) superstructure. On Cu surface however, no superstructure is observed. When adsorbing a high dose of H 2 , hydrogen-induced superstructures are observed on both Co and Cu surfaces. These superstructures are ascribed to the presence of physisorbed molecules as revealed by the STS spectra. A ( 3 × 3 ) superstructure is observed on Co and a mixture of ( 2 × 2 ) and ( 3 × 3 ) is identified on Cu. A model describing the adsorption sites of molecules is proposed: H 2 molecules occupy on-top sites when arranged as a ( 2 × 2 ) phase and they occupy on-top and bridge sites when arranged as a ( 3 × 3 ) phase. Besides, STM-induced desorption is used to desorb atomic hydrogen from selected Co nanoisland surface. The desorption is visualized in terms of a disappearance of the p( 2 × 2 ) superstructure.

The availability of femtosecond pulsed-laser sources in combination with the development of time-resolved and sensitive magneto-optical schemes, have paved the way toward the exploration of the dynamics of spin systems at ultimate time scales. This chapter focuses on the exploration of subpicosecond processes in itinerant ferromagnet systems, which after a decade of research has now become an exciting and mature field of research. Both the potential and limitations of the experimental techniques, mostly concentrating on all-optical approaches, and the subtle relation with processes being probed are being explained. The main part, though, is devoted to a review of experimental efforts and the physical insight that has emerged for the complementary spin-dependent processes being investigated: populational dynamics (including state-filling effects such as dichroic bleaching and direct transfer of angular momentum from the laser field), loss of magnetic order after laser heating, laser-induced modification of anisotropy (including dynamics of exchange-coupled systems)–triggering precessional phenomena, and the very recently discovered possibility to generate magnetic order at ultrafast timescales by driving metamagnetic phase transitions. Keywords: magnetization dynamics; magneto-optics; time-resolved spectroscopies; demagnetization; angular momentum transfer; ultrafast phase transitions

We study the transition between positive and negative organic magnetoresistance (OMAR) in tris-(8 hydroxyquinoline) aluminium (${\mathrm{Alq}}_{3}$), in order to identify the elementary mechanisms governing this phenomenon. We show how the sign of OMAR changes as function of the applied voltage and temperature. The transition from negative to positive magnetoresistance (MR) is found to be accompanied by an increase in slope of $\mathrm{log} (I)$ versus $\mathrm{log} (V)$. ac admittance measurements show this transition coincides with the onset of minority charge (hole) injection in the device. All these observations are consistent with two simultaneous contributions with opposite sign of MR, which may be assigned to holes and electrons having different magnetic field responses.

We present a mechanism for the recently discovered magnetoresistance in disordered $\ensuremath{\pi}$-conjugated materials, based on hopping of polarons and bipolaron formation, in the presence of the random hyperfine fields of the hydrogen nuclei and an external magnetic field. Within a simple model we describe the magnetic field dependence of the bipolaron density. Monte Carlo simulations including on-site and longer-range Coulomb repulsion show how this leads to positive and negative magnetoresistance. Depending on the branching ratio between bipolaron formation or dissociation and hopping rates, two different line shapes in excellent agreement with experiment are obtained.

Magnetization-induced optical second harmonic generation (MSHG) from exchange-biased $\mathrm{Mn}∕\mathrm{Co}$ thin films shows monolayer period oscillations at the $\mathrm{Mn}∕\mathrm{Co}$ interface as a function of Co thickness. Similar oscillations are found in the exchange bias $({H}_{E})$ and the coercivity $({H}_{C})$ in both the interface sensitive MSHG and the bulk sensitive magneto-optical Kerr effect, indicating that magnetic reversal in the Co bulk and at the $\mathrm{Mn}∕\mathrm{Co}$ interface is collinear. Assuming a linear relationship between the MSHG asymmetry and the magnetic moment, our results suggest that there is an enhancement of the interface net magnetic moment at the full monolayer regions.

We present measurements of tunnelling spin polarization (TSP) of CoFeB alloys in Al/AlOx/CoFeB tunnel junctions using superconducting tunnelling spectroscopy. We observe that though Co72Fe20B8 (at.%) layers crystallize in the fcc structure, in junctions annealed above 300 °C the measured TSP degrades, indicating that in these junctions the TSP of amorphous CoFeB is higher than its crystalline counterpart.

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 ${\ensuremath{\tau}}_{M}$ and electron-phonon equilibration time ${\ensuremath{\tau}}_{E}$ 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%.

Co2Mn1-xFexSi Heusler alloys crystallize in the L21 structure. This structure type requires random distribution of Mn and Fe in case of the mixed alloys. The spin echo nuclear magnetic resonance (NMR) technique probes the direct local environments of the active atoms and is thus able to resolve next neighboring shells providing a unique tool to verify the random distribution of Mn and Fe in Co2Mn1-xFexSi. Exemplarily, the half-metallic quaternary Heusler alloy Co2Mn0.5Fe0.5Si was investigated by means of NMR. The 55Mn NMR measurements unambiguously demonstrate the random distribution of Mn and Fe on the 4b Wyckoff position in Co2Mn0.5Fe0.5Si.

We show that over-oxidation of sub-nm thin Al2O3 barriers of magnetic tunnel junctions can be observed in real time using in situ differential ellipsometry measurements. The change in ellipsometry signal of Al layers grown on CoFe films is proportional to the amount of oxidized metallic material. As a result, the derivative of this signal is a direct measure of the oxidation rate. Further analysis of this oxidation rate allows us to determine the onset of the CoFe oxidation. We found the onset to be proportional to the deposited Al layer thickness. The amount of CoO determined from in situ x-ray photoelectron spectroscopy data on identical samples was found to be proportional to that obtained from ellipsometry.

We discuss our approach to realizing all-electrical spin injection and detection in GaAs via ferromagnetic metals and oxide tunnel barriers. A critical requirement is suppression of the formed Schottky barrier, which causes conductivity mismatching and prevents detection in reverse bias. However, initial I-V measurements of engineered GaAs/AlOx/CoFe tunnel contacts still show too high resistance area products in comparison to the semiconductor conductance, due to a large Schottky barrier contribution. To this end, optimized doping levels of the tunnel contacts and semiconductor spin transport region are calculated. Secondly, control over the magnetic properties of injector and detector electrode is established by designing both electrodes to have different switching fields, due to different widths. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-resolved pump-probe measurements of the magnetic phase change from paramagnetic to ferromagnetic and back in FeRh thin films are presented. Data are compared with simulations of laser-induced magnetization dynamics using the Landau-Lifshitz-Gilbert equation with a time dependent magnetization density, responding to the evolution of the thermal profile throughout the film. We demonstrate that the observed magneto-optical transients should be interpreted in terms of an interplay between the local magnetic moment, nonlocal domain growth or alignment, and magnetization precession as launched by the varying demagnetizing fields. Kerr rotation and reflectivity are shown to provide a complementary view on the phase transition.

The spin dynamic response to femtosecond laser excitation in epitaxial Co/fct-Mn??001?? exchange-coupled bilayers is studied. Damped oscillations in the polar Kerr rotation transients, representing a precessional motion of the magnetization vector, are observed for an atomically flat Co/Mn interface, when a magnetic field is applied perpendicular to the exchange bias direction. The absence of a measurable ultrafast demagnetization in the first picosecond after laser excitation proves that the precession is launched by a laser-induced subpicosecond quenching of the exchange coupling. The exact value of the damping parameter varies within the range 0.05??????0.2, depending on the detailed structure of the sample.

Magnetic storage and logic devices based on magnetic domain wall motion rely on the precise and synchronous displacement of multiple domain walls. The conventional approach using magnetic fields does not allow for the synchronous motion of multiple domains. As an alternative method, synchronous current-induced domain wall motion was studied, but the required high-current densities prevent widespread use in devices. Here we demonstrate a radically different approach: we use out-of-plane magnetic field pulses to move in-plane domains, thus combining field-induced magnetization dynamics with the ability to move neighbouring domain walls in the same direction. Micromagnetic simulations suggest that synchronous permanent displacement of multiple magnetic walls can be achieved by using transverse domain walls with identical chirality combined with regular pinning sites and an asymmetric pulse. By performing scanning transmission X-ray microscopy, we are able to experimentally demonstrate in-plane magnetized domain wall motion due to out-of-plane magnetic field pulses., Magnetic domain walls could form the basis for information technology with high storage density, but require comparatively high current densities to be moved by spin torque. Here, the authors demonstrate a radically different approach with perpendicular magnetic field pulses moving domain walls synchronously.

Atomically ordered Al2O3 obtained by thermal oxidation of the Ni3Al(001) surface was studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). LEED analysis has shown that oxidation at 800 and 1100 K forms two different structural phases of Al2O3. Detailed STM studies of both phases also reveal remarkable differences in surface pattern structure and arrangement of the domains. We have shown that well-ordered Al2O3 layers are, in principle, full of dislocations. However, the dislocations, which are responsible for surface superstructures, are well ordered, forming either stripes in the low-temperature phase, or patches in the high-temperature phase. Interacting with the single-crystalline substrate, the nanopatches form their own lattice of superarrangement, introducing linelike or more complex superdislocations.

We report on the detection of localized spin modes in a multilayered spintronic device by means of time-resolved scanning Kerr microscopy. Measurements on this 13×9 µ m2 exchange-biased magnetic tunnel junction element at different applied bias fields indicate a strong effect of the stray field from the pinned CoFe layer on the magnetization dynamics in the free NiFe layer. This view is supported by micromagnetic simulations, which also show that the dynamics can be attributed to the specific shape of the internal magnetic field in the element. © 2005 American Institute of Physics.

In situ differential ellipsometry measurements are used to investigate the plasma oxidation dynamics of subnanometer Al layers in real time. An analysis of the measured oxidation rate allows us to monitor the conversion from Al to Al2O3 in real time and mark the onset of the CoFe electrode oxidation. In situ x-ray photoelectron spectroscopy (XPS) measurements of identical samples that were plasma oxidized for 3 s are used to confirm the ellipsometry results. A linear relationship was found between the amount of Al2O3 determined from the XPS data and the amount of oxidized Al derived from the ellipsometry data. From this, we conclude that the conversion from Al to Al2O3 of subnanometer thin barriers for magnetic tunnel junctions can be observed by using the derivative of the ellipsometer signal.

Polycrystalline silicon on glass substrates was grown by a method based on the creation of nucleation sites using laser crystallization of amorphous silicon followed by thermal annealing at temperatures below 600 °C. Annealing induces the crystallization of the material around the seeds, eventually leading to coalescence of adjacent domains before spontaneous nucleation sets in. Micro‐Raman spectroscopy shows that the seeds experience a tensile stress, which causes a radial birefringence in the surrounding amorphous silicon, detected by optical anisotropy measurements. We conjecture that this stress facilitates the crystallization of the material around the seed upon thermal annealing.

We present a technique, microscopic reflectance difference spectroscopy (μRDS), for the measurement of optical anisotropy with sub‐micron resolution. The technique is applied to the determination of the in‐plane anisotropy of GaAs/AlAs multiple quantum well structures in a phase resolved way, both below and above the fundamental gap. Confinement and local field effects are discussed, and a comparison is made with microscopic calculations based on a tight‐binding Hamiltonian for the electronic states.

We performed pump-probe magneto-optical Kerr spectroscopy to explore the role of excitonic effects on the electron and hole spin relaxation in Cd1˛xMnxTe-based quantum well structures. Particularly, important information is obtained from detailed interpretation of the temperature and wavelength dependence of the observed transient magneto-optics in specially engineered structures. In manganese-rich wells the electron spin relaxation is found to be strongly enhanced in the exciton due to the increased mass of the bound electron-hole pair, in agreement with earlier predictions. An even stronger enhancement of the relaxation time is found for the hole spin, which is explained by the additional mixing of heavy-hole and light-hole states in the bound state. In structures where Mn is confined to the barrier region, a different behavior is observed, indicative for a competition of various contributions.