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We demonstrate using micromagnetic simulations and a theoretical model that a gradient in the saturation magnetization (${M}_{s}$) of a perpendicularly magnetized ferromagnetic film induces a nonreciprocal spin-wave propagation and, consequently, an asymmetric dispersion relation. The ${M}_{s}$ gradient adds a linear potential to the spin-wave equation of motion consistent with the presence of a force. We consider a transformation from an inertial reference frame in which the ${M}_{s}$ is constant to an accelerated reference frame where the resulting inertial force corresponds to the force from the ${M}_{s}$ gradient. As in the Doppler effect, the frequency shift leads to an asymmetric dispersion relation. We show that under certain circumstances, unidirectional propagation of spin waves can be achieved, which is essential for the design of magnonic circuits. Our results become more relevant in light of recent experimental works in which a suitable thermal landscape is used to dynamically modulate the saturation magnetization.

${\mathrm{Mn}}_{3}X$ compounds in which the magnetic $\mathrm{Mn}$ atoms form $AB$-stacked kagome lattices have received a tremendous amount of attention since the observation of the anomalous Hall effect in ${\mathrm{Mn}}_{3}\mathrm{Ge}$ and ${\mathrm{Mn}}_{3}\mathrm{Sn}$. Although the magnetic ground state has been known for some time to be an inverse triangular structure with an induced in-plane magnetic moment, there have been several controversies about the minimal magnetic Hamiltonian. We present a general symmetry-based model for these compounds that includes a previously unreported interplane Dzyaloshinskii-Moriya interaction, as well as anisotropic exchange interactions. The latter are shown to compete with the single-ion anisotropy which strongly affects the ground state configurations and elementary spin-wave excitations. Finally, we present the calculated elastic and inelastic neutron scattering intensities and point to experimental assessment of the types of magnetic anisotropy in these compounds that may be important.

The notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices., J. Phys. D, accepted for publication

The magnetic structure of the in-plane skyrmions in epitaxial MnSi/Si(111) thin films is probed in three dimensions by the combination of polarized neutron reflectometry (PNR) and small angle neutron scattering (SANS). We demonstrate that skyrmions exist in a region of the phase diagram above at temperature of 10 K. PNR shows the skyrmions are confined to the middle of the film due to the potential well formed by the surface twists. However, SANS shows that there is considerable disorder within the plane indicating that the magnetic structure is a 2D skyrmion glass.

We describe time-domain terahertz spectroscopy measurements of the frequency-dependent conductivity in MnSi thin films. We find that at low temperatures and frequencies, the scattering rate is proportional to both the square of the temperature and the square of the frequency, as predicted Fermi liquid theory. Unlike measurements on many other systems, the prefactors for each functional dependence are also consistent with the theory.

We introduce a new class of isolated three-dimensional skyrmion that can occur within the cone phase of chiral magnetic materials. These novel solitonic states consist of an axisymmetric core separated from the host phase by an asymmetric shell. These skyrmions attract one another. We derive regular solutions for isolated skyrmions arising in the cone phase of cubic helimagnets and investigate their bound states.

Axisymmetric solitonic states (chiral skyrmions) have been predicted theoretically more than two decades ago. However, until recently they have been observed in a form of skyrmionic condensates (hexagonal lattices and other mesophases). In this paper we report experimental and theoretical investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers two years ago (Science 341 , 636 (2013)). The results of spin-polarized scanning tunneling microscopy analyzed within the continuum and discrete models provide a consistent description of isolated skyrmions in thin layers. The existence region of chiral skyrmions is restricted by strip-out instabilities at low fields and a collapse at high fields. We demonstrate that the same equations describe axisymmetric localized states in all condensed matter systems with broken mirror symmetry, and thus our findings establish basic properties of isolated skyrmions common for chiral liquid crystals, different classes of noncentrosymmetric magnets, ferroelectrics, and multiferroics., Comment: 12 pages, 12 figures

Detailed X-ray photoelectron spectroscopy (XPS) depth profiling measurements were performed across the back n-layer/transparent conducting oxide (n/TCO) inter-faces for superstrate p-i-n solar cells to examine differences between amorphous silicon (a-Si:H) and microcrystalline silicon (mu c-Si:H) n-layer materials as well as TCO materials ZnO and ITO in the chemical, microstructural and diffusion properties of the back interfaces. No chemical reduction of TCO was found for all variations of n-layer/TCO interfaces. We found that n-a-Si:H interfaces better with ITO, while n-mu c-Si:H, with ZnO. A cross-comparison shows that the n-a-Si:H/ITO interface is superior to the n-mu c-Si:H/ZnO interface, as evidenced by the absence of oxygen segregation and less oxidized Si atoms observed near the interface together with much less diffusion of TCO into the n-layer. The results suggest that the n/TCO interface properties are correlated with the characteristics of both the n-layer and the TCO layer. Combined with the results reported on the device performance using similar back n/TCO contacts, we found the overall device performance may depend on both interface and bulk effects related to the back n/TCO contacts. (c) 2006 Elsevier B.V. All rights reserved.

Using the molecular beam epitaxy facilities of the Pacific Northwest Consortium Collaborative Access Team at the Advanced Photon Source, we have prepared and examined in situ the structures of iron films deposited on the 4 × 6-reconstructed surface of GaAs(0 0 1) with thicknesses ranging from 0.5 to 30 monolayers. We have employed the polarization-dependent X-ray absorption fine structure (XAFS) technique in total reflection mode to examine the iron environment and compare in-plane to out-of-plane structure in these films. The X-ray absorption near edge structure (XANES) does not indicate a change in the energy position of the iron K-absorption edge with thickness, but there are features that shift to lower energy just above the edge, and become less evident, with decreasing thickness owing to changes in bonding and increasing influence of substrate atoms on the iron. Near 4 monolayers, a transition from island to layer-by-layer growth modes is accompanied by the observation of a distortion of the iron to a body-centered tetragonal structure (as compared to bulk body-centered cubic iron) with a c/a ratio of 1.030(8), with no thickness dependence observed to 30 monolayers.

The magnetic states and switching processes are investigated in Co and NiFe rings with lateral dimensions from the micrometer range down to sub- 100 nm . Using non-intrusive imaging techniques we have directly observed the nanoscopic details of the magnetization configurations of epitaxial and polycrystalline mesoscopic ring structures with nm resolution. We have found head-to-head domain walls with different spin structures depending on ring width. Further, we can classify the geometry-dependent switching processes according to the number of transitions (single, double, triple) that a ring undergoes in a hysteresis cycle. In the case of triple switching we find a novel state with a complete vortex core present in the ring. It is found that the double switching can be retained in sub- 100 nm rings by tailoring the material and thickness.

We present Hall-effect measurements on MnSi/Si(111) epilayers and find an anomalous Hall contribution that is significantly smaller than that in bulk crystals, which enables the observation of an additional contribution to the anomalous signal previously overlooked in MnSi. Our measurements indicate the signal is not due to skyrmions in MnSi thin films, which are absent in out-of-plane fields, but rather are the result of scattering from the cone phase. The absence of magnetic contrast in the transmission electron microscopy (TEM) measurements is consistent with this interpretation [T. L. Monchesky et al., Phys. Rev. Lett. 112, 059701 (2014)]. We provide a method to model TEM images of skyrmion lattices to determine the conditions necessary for their observation in other B20 epilayers with an anisotropy that is favorable to their formation.

Polarized K-edge x-ray absorption fine structure spectroscopy (XAFS) studies were performed both in and ex situ on iron films deposited on GaAs substrates. Samples with 5 and 10 ML (monolayers) of iron deposited on sulfur-passivated and $(4\ifmmode\times\else\texttimes\fi{}6)$-reconstructed GaAs(001) surfaces, respectively, were studied ex situ (capped with 20 ML of gold) and compared with 9.3 ML on $\mathrm{GaAs}(001)\ensuremath{-}(4\ifmmode\times\else\texttimes\fi{}6)$ measured in situ. Analysis of XAFS spectra for both samples on $(4\ifmmode\times\else\texttimes\fi{}6)\ensuremath{-}\mathrm{GaAs}$ reveals a tetragonal distortion of the iron film relative to bulk body-centered-cubic iron. The distortion involves an in-plane contraction and an expansion perpendicular to the GaAs surface to give a $c/a$ ratio of 1.03(1), with a comparable to half the bulk lattice constant of GaAs. The sample on sulfur-passivated GaAs did not exhibit this distortion.

A procedure for growth of smooth As free Fe surfaces on (4\ifmmode\times\else\texttimes\fi{}6)-GaAs(100) is presented. Ferromagnetic resonance (FMR) revealed that the Fe films have anisotropies equal to bulk Fe, modified only by interface anisotropies. The Fe films served as templates for the growth of epitaxial Fe/Cu/Fe trilayers which were subsequently characterized by FMR, magneto-optical Kerr effect, and magnetoresistance. At room temperature, films coupled through a 13.2-ML Cu spacer exhibited 2.0% giant magnetoresistance and 0.3% anisotropic magnetoresistance. The results showed that the interlayer exchange coupling for a 13.2-ML Cu spacer could not be described by bilinear and biquadratic contributions alone. A different coupling, which varied as cosine cubed of the angle between the magnetizations of the Fe films, was required to explain the data (bicubic exchange coupling).

Fe whisker/Cr/Fe(0 0 1) samples can be prepared in a nearly perfect layer by layer growth. It will be shown that the initial reversal of the phase in the short wavelength oscillations of the interlayer exchange coupling is most likely caused by the presence of interface alloying at the Fe/Cr interface. The ability to grow nearly perfect layers of Mn between Cr and Fe layers allows one to investigate the role of a strong intrinsic antiferromagnet, Mn, on the exchange coupling through the weak antiferromagnetic spin density wave in Cr. It will be shown that the Mn layers do not affect the sign of the interlayer exchange coupling. The sign of the exchange coupling is given by the number (parity) of pure Cr atomic layers. These results are consistent with recent theoretical calculations that predict the BCT Mn on Fe is antiferromagnetic with fully compensated (0 0 1) atomic planes.

An electrical read-out mechanism for magnetic skyrmions that does not require spin-polarized currents could facilitate the use of these small magnetic states in memory devices.

In epitaxial MnSi/Si(111) films, the in-plane magnetization saturation is never reached due to the formation of specific surface chiral modulations with the propagation direction perpendicular to the film surfaces [Wilson et al. Phys. Rev. B 88, 214420 (2013)]. In this paper we show that the occurrence of such chiral surface twists is a general effect attributed to all bulk and con- fined magnetic crystals lacking inversion symmetry. We present experimental investigations of this phenomenon in nanolayers of MnSi/Si(111) supported by detailed theoretical analysis within the standard phenomenological model. In magnetic nanolayers with intrinsic or induced chirality, such surface induced instabilities become sizeable effects and play a crucial role in the formation of skyrmion lattices and other nontrivial chiral modulations.

We present an investigation of Mn {\delta}-doped layers in Si(001) grown by molecular beam epitaxy. We discovered that a Pb surfactant has significant effect on the structural and magnetic properties of the submonolayer of Mn, which depends on the Si capping layer growth temperature, T_Si, and the Mn coverage, {\theta}_Mn. The results presented in this paper identify three regions in the growth-phase-diagram characterized by distinct magnetic behaviors and crystal structures. In one region, X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM) experiments indicate that MnSi nanocrystallites form with B2-like crystal type structures. At the optimal growth conditions, T_Si = 200 C and {\theta}_Mn = 0.26 monolayer, a ferromagnetic phase develops with a Curie temperature T_C > 400 K and a saturation moment m_sat = 1.56 {\mu}_B/Mn, whereas T_C drops to zero for a control sample prepared without Pb. For T_Si > 200 C, MnSi-B20 type precipitates form with a T_C ~ 170 K. Rutherford backscattering spectroscopy shows that the increase in the remanent magnetization in these two phases is possibly correlated with an increase in the Mn substitutional fraction, which suggests that a Si_{1-x}Mn_x dilute magnetic semiconductor may be forming in the matrix between the precipitates. Density functional calculations show that Pb changes the pathway by which the Mn atoms access the Si substitutional sites, Mn_Si. While the Pb increases the formation energy of Mn_Si at the Si surface, it enables substitutional incorporation by lowering the formation energy of Si vacancies by 0.92 eV., Comment: 10 pages, 8 figures

This paper reports on magnetometry and magnetoresistance measurements of MnSi epilayers performed in out-of-plane magnetic fields. We present a theoretical analysis of the chiral modulations that arise in confined cubic helimagnets where the uniaxial anisotropy axis and magnetic field are both out-of-plane. In contrast to in-plane field measurements [Wilson et al., Phys. Rev. B 86, 144420 (2012)], the hard-axis uniaxial anisotropy in MnSi/Si(111) increases the energy of (111)-oriented skyrmions and in-plane helicoids relative to the cone phase, and it makes the cone phase the only stable magnetic texture below the saturation field. While induced uniaxial anisotropy is important in stabilizing skyrmion lattices and helicoids in other confined cubic helimagnets, the particular anisotropy in MnSi/Si(111) entirely suppresses these states in an out-of-plane magnetic field. However, it is predicted that isolated skyrmions with enlarged sizes exist in MnSi/Si(111) epilayers in a broad range of out-of-plane magnetic fields. These results reveal the importance of the symmetry of the anisotropies in bulk and confined cubic helimagnets in the formation of chiral modulations, and they provide additional evidence of the physical nature of the $A$-phase states in other B20 compounds.

A Comment on the Letter by Y. Li et al., Phys. Rev. Lett. 110, 117202 (2013). The authors of the Letter offer a Reply.

Magnetometry and magnetoresistance measurements in MnSi thin films and rigorous analytical solutions of the micromagnetic equations show that the field-induced unwinding of confined helicoids occurs via discrete steps. A comparison between the magnetometry data and theoretical results shows that finite size effects confine the wavelength and lead to a quantization of the number of turns in the helicoid. We demonstrate a prototypical spintronic device where the magnetic field can push or pull individual turns into a magnetic spring that can be read by electrical means.

We present an experimental and theoretical investigation of the influence of a uniaxial magnetocrystalline anisotropy on the magnetic textures that are formed in a chiral magnetic system. We show that the epitaxially induced tensile stress in MnSi thin films grown on Si(111) creates an easy-plane uniaxial anisotropy. The magnetoelastic shear stress coefficient is derived from SQUID magnetometry measurements in combination with transmission electron microscopy and x-ray diffraction data. Density functional calculations of the magnetoelastic coefficient support the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. Theoretical calculations based on a Dzyaloshinskii model that includes an easy-plane anisotropy predict a variety of modulations to the magnetic order that are not observed in bulk MnSi crystals. Evidence for these states is found in the magnetic hysteresis and polarized neutron reflectometry measurements.

Using scanning electron microscopy with polarization analysis, we observed the electron beam induced switching of the magnetic state of epitaxial single-crystal Fe(110) films grown on atomically flat cleaved GaAs(110). For low film thickness the magnetization lies along the [−110] in-plane direction, while above a thickness of 19 monolayers, the ground state magnetization configuration switches to the [001] in-plane direction. If Fe films are grown to a thickness greater than the critical thickness of the reorientation, the magnetization is caught in a metastable state, oriented along [−110]. We discovered that we can locally switch the metastable state to the stable [001] direction by irradiating the metastable magnetic state with a suitable electron current density. The reversal proceeds by the nucleation and growth of lancet-shaped domains that move in discrete jumps between pinning sites. Our results show that there is a permanent reduction of the strength of defect sites without a permanent change in...

A detailed investigation of the magnetization processes in epitaxial MnSi thin film reveals the existence of elliptically distorted skyrmion strings that lie in the plane of the film. We provide proof that the uniaxial anisotropy stabilizes this state over extended regions of the magnetic phase diagram. Theoretical analysis of an observed cascade of first-order phase transitions is based on rig- orous numerical calculations of competing chiral modulations, which shows the existence of helicoids, elliptic skyrmions, and cone phases.

The structural and magnetic properties of ultrathin Mn layers deposited onto Si(001) by molecular beam epitaxy at low temperature are reported. X-ray absorption fine structure studies reveal that the structure of the silicide layer that forms depends on the growth temperature of the capping layer. A capping layer grown at 200 \ifmmode^\circ\else\textdegree\fi{}C on 0.35-monolayer (ML) Mn results in a metastable MnSi phase with a B2-like (CsCl) structure, whereas a cap grown at room temperature on 0.5 ML followed by annealing at 200 \ifmmode^\circ\else\textdegree\fi{}C produces a lower coordinated MnSi phase with a B20-like structure. Increasing the Mn thickness from 0.5 to 4 monolayers does not trigger a structural transformation but drives the structure closer to MnSi-B20. The sample with B2-like structure has the largest Mn magnetic moment of 0.33 ${\ensuremath{\mu}}_{B}$/Mn at $T$ $=$ 2 K, and a Curie temperature ${T}_{C}$ above 250 K. MnSi-B20 layers showed lower moments and much lower ${T}_{C}$'s, in line with those reported for MnSi-B20 thin films.

The electron transport and magnetoresistance (MR) were investigated in high quality crystalline epitaxial Fe(001) and Au(001) films and exchange coupled Au/Fe/Au/Fe/GaAs(001) trilayer structures. Fits to the experimental data were based on the semiclassical Boltzmann equation, which incorporates the electronic properties obtained from first-principles local density functional calculations. The fits require a surprisingly high asymmetry for the spin dependent electron lifetimes in Fe, τ↓/τ↑=10 at room temperature. Despite the large atomic terraces at the Au/vacuum and Fe/GaAs interfaces the scattering at the outer interfaces was found to be diffuse. The origin of MR in Au/Fe/Au/Fe/GaAs(001) structures is due to electron channeling in the Au spacer layer. The measured MR is consistent with the diffusivity parameters s↑=0.55, s↓=0.77 at the metal–metal interfaces.

We have grownepitaxialsingle-crystal magnetotunnel junctions using Fe(001) substrates, MgO(001) spacers and Fe top electrodes. We have used scanning tunneling microscopy and atomic force microscopy to measure the tunneling characteristics as a function of position and demonstrated that local tunneling can be obtained such that the buried MgO can be characterized with nm resolution. Local I(V) curves revealed that most of the area had intrinsic tunneling properties corresponding to the proper MgO tunneling barrier. A small fraction of the scanned areas showed localized spikes in the tunneling current which are most likely caused by defects in the MgO.

We report on the correlation between the magnetic and structural properties of epitaxial MnSi (111) thin films grown by solid-phase epitaxy on Si (111) substrates. The Si (111) substrate, with a surface unit cell that is 3.0% larger than that of MnSi, causes an in-plane tensile strain in the film that is partially relaxed due to the presence of misfit dislocations located at the interface. However, the out-of-plane strain has a nonmonotonic dependence on thickness that is attributed to changes in the elastic constants of the film. The thickness dependence of the Curie temperature correlates strongly with strain and reaches a maximum of ${T}_{C}=43\text{ }\text{K}$, a value that is 46% greater than the bulk value of ${T}_{C}=29.5\text{ }\text{K}$. Although the films have a strong epitaxial relationship, $[1\overline{1}0]\text{MnSi}\ensuremath{\parallel}[11\overline{2}]\text{Si}$, there are inversion domains in the film due to the noncentrosymmetric crystal structure of MnSi. The presence of these domains implies that there are two magnetic chiralities, which likely contribute to the observed glassy magnetic response of the films.

The giant magnetoresistance of crystalline Fe/Cu/Fe(001) epitaxial structures characterized by scanning tunneling microscopy are presented. Fe/Cu/Fe(001) trilayers capped with Au are grown directly on GaAs(001) using a new procedure for producing pure Fe layers with As-free Fe surfaces on GaAs(001). The temperature dependence of the magnetoconductance and sheet resistance measured from 4 to 300 K is modeled by the Boltzmann equation assuming that the mean free paths in the crystalline epitaxial layers are equal to those in bulk materials. The results of the simple model suggest that the coefficient of the specular scattering at the Fe/GaAs interface is R=0.45, while the scattering at the outer Au interface is diffuse. Spin asymmetry scattering at the metallic interfaces is ΔT=|T↑−T↓|=0.34, Tavg=(T↑+T↓)/2=0.79. The sheet resistance was best modeled using a low temperature mean free path of 25 nm in the Fe layer.

Exchange coupling through spin density wave in Fe whisker/Cr/Fe(001) structures was studied by Brillouin light scattering (BLS) and longitudinal magneto-optical Kerr effect (MOKE) techniques. It will be shown that interface alloying at the Fe whisker/Cr interface profoundly affects the behavior of short wavelength oscillations. The first crossover to antiferromagnetic coupling occurs at 5 monolayers (ML), the phase of short-wavelength oscillations is reversed compared to that expected for the spin density wave in Cr(001), and the strength of coupling is significantly decreased from that obtained from first principle calculations. Using Cu and Ag atomic layers between the Cr(001) and Fe(001) films, heterogeneous interfaces showed that the exchange coupling in Cr(001) is strongly affected by electron multiple scattering. It appears that electron quantum well states in the Fe film play no important role in the strength of the exchange coupling when the Fe film is bounded on one side by Au, but they become im...

We observed three magnetic states of an ultrathin, atomically well-ordered Co film grown on a cleaved GaAs 110 substrate. For a Co thickness less than 3.4 monolayers ML , we find a ferromagnetically dead layer associated with the formation of interfacial Co2GaAs. For thicknesses greater than 4.1 ML, the Co film grows with a bcc structure that contains 6 at. % Ga. The films are ferromagnetic with an easy axis along the

Exchange coupling has been studied in Fe whisker/Cr/Fe(001) systems that were grown in a perfect layer by layer mode. The exchange coupling through Cr was found to be very sensitive to alloying at the Fe whisker/Cr(001) interface. It will be shown that the observed reversed phase of the short wavelength oscillations compared to those predicted by ab initio calculations can be caused by alloying at the Fe whisker/Cr(001) interface. In order to test this point, we have grown samples with the Cr/Fe(001) interface intentionally alloyed by codepositing the Cr and Fe atoms during the formation of the last Cr atomic layer. The strength of the exchange coupling has also been investigated in systems fabricated with heterogeneous spacers using bcc Cu(001) and fcc Ag(001). Cu and Ag layers have been inserted between the Cr spacer and the Fe(001) film. The strength of the antiferromagnetic coupling was found to be substantially increased due to the presence of Cu at the Cr/Fe(001) interface. It will be argued that the observed increase in the exchange coupling is caused by an increased asymmetry in spin dependent reflectivity at the Cr/Cu/Fe interface.

In situ resistance measurements of epitaxial Fe layers and Au/Fe bilayers were used to quantify the scattering in giant magnetoresistance GMR spin valve structures. The semiclassical Boltzmann transport equation, incorporating first-principles local density functional calculations, fitted the thickness dependence of the conductivity. Fits to the data indicate that Fe has a large spin asymmetry with bulk relaxation times ↓=3.0 10 14 s and ↑=2.510 15 s. These give a conductivity equal to that of bulk Fe. The interface scattering from the Fe/GaAs, the Fe/vacuum, and the Au/vacuum interfaces is purely diffuse. This is in contrast to the high electron reflection coefficients determined from kinematical calculations using scanning tunneling microscope images. Fits to conductivity measurements of Au/Fe/GaAs001 indicate that the Au films have the conductivity of bulk material modified only by interface scattering. The GMR of Au/Fe/Au/Fe/GaAs001 structures is 1.8% at room temperature and 2.9% at 10 K. The magnetoresistance is reduced by the presence of partial diffuse scattering at the inner interfaces, as indicated by the fits to both the GMR and the Au conductivity. The GMR in Fe/Au structures is intrinsically low due to a large electron band mismatch between Au and Fe band structures.

The magnetic nanostructure of epitaxial fcc Co/Cu(001) circular elements (∼1.7μm in diameter) has been imaged with scanning electron microscopy with polarization analysis. The disks are obtained by ultrahigh vacuum deposition of the metal films onto a prepatterned Si(001) substrate. The Si structures are 700 nm high, ensuring that the continuous background film and that of the circular structures are not physically connected. A closed flux configuration (a quadrant configuration) is observed for some of the disks, characteristic of systems with cubic anisotropy. The measured width of the 90° domain wall varies from 70′25 nm close to the vortex core, up to 150′25 nm at a normalized distance r/r d 0.625 from the vortex core (where r d is the domain wall length from the vortex core to the disk periphery), i.e., significantly exceeding the bulk domain wall width, and increasing further with increasing distance from the vortex core. Such a wide domain wall is a consequence of the geometrical constraints imposed by the element, thus defining a geometrically constrained domain wall. This view is supported by detailed micromagnetic simulations that also show that the domain wall width increases dramatically with radial position from the disk center.

By using the fully relativistic spin-polarized screened Korringa-Kohn-Rostoker method and the corresponding Kubo-Greenwood equation resistivities and the giant magnetoresistance are evaluated for Fe/Au/Fe/Au spin valves assuming both bcc-like Fe and fcc-Au leads. The theoretically obtained values for a spacer thickness of 7 monolayers of Au are in a rather good agreement with the corresponding experimental data. These data, in particular the temperature dependence of the resistivities reported in here, are analyzed with respect to the experimental setup and related to the theoretical models assumed. Since in the theoretical calculations interdiffused interfaces are taken into account by means of the inhomogeneous coherent potential approximation, the role of the interfaces in the magnetoresistance of this system can be discussed rigorously.

Using nonintrusive imaging techniques we have directly observed the nanoscopic details of the magnetization configurations of epitaxial and polycrystalline mesoscopic ring structures with a

Single-crystal epitaxial MgO thin films were grown directly onto high-quality Fe single crystal and Fe whisker substrates and covered with Fe/Au layers. Reflection high-energy electron diffraction and low-energy electron diffraction patterns and scanning tunneling microscopy images showed that the growth of MgO proceeded pseudomorphically in a nearly layer-by-layer mode up to six monolayers. A misfit dislocation network is formed for MgO layers thicker than six monolayers. The thin MgO films were characterized electrically by scanning tunneling spectroscopy. The tunneling barrier in MgO was found to depend on the MgO layer thickness, starting from 2.5 eV at two monolayer thickness to the expected full barrier of MgO of 3.6 eV at six monolayers. A small fraction of the scanned area showed randomly placed spikes in the tunneling conductance. Tunneling I-V curves at the defects showed a lower tunneling barrier than that in the majority of the MgO film. The total tunneling current integrated over areas of 1003100 nm 2 , however, was not dominated by spikes of higher conductance. These local defects in the MgO barrier were neither related to atomic steps on the Fe substrates nor to individual misfit dislocations. Magnetic anisotropies and exchange coupling in Fe/ MgO~001! and Fe/MgO/Fe~001! structures were studied using ferromagnetic resonance and Brillouin light scattering.

Smooth and As free Fe layers were grown directly on semi-insulating GaAs(001) templates with a pseudo 4 × 6 surface reconstruction. The STM studies revealed that the pseudo 4 × 6 reconstruction consists of 1 × 6 and 4 × 2 domains. The relative areas of the individual domains depend sensitively on the substrate temperature and that explains the large variations observed in the strength of the in plane uniaxial anisotropy. The temperature dependence of the magneto conductance and sheet resistance measured from 4 K to 300 K was modeled by the Boltzman's equation. Spin asymmetry at the metallic interfaces is |T↑ - T↓\ = 0.34 and (T↑ + T↓)/2 = 0.34 Single crystal epitaxial Magneto-Tunneling Junctions (MTJ) were grown using Fe whisker substrates, MgO oxide spacer and Au/Ni/Fe(001) top electrodes. The Rheed and Leed patterns show that the deposition of MgO proceeds pseudomorphically in a nearly layer by layer mode up to 6 atomic layers. The Fe whisker/MgO/Au/Ni/Fe(001) tunneling junctions were investigated using an Atomic Force Microscope (AFM) with a conducting tip. The AFM images showed low tunneling currents with an I-V characteristics corresponding to a tunneling barrier of 3.6 eV. This barrier height is in perfect agreement with the potential barrier in the crystalline MgO layer. AFM scans revealed some localized spikes in the tunneling current. It seems that the spikes originated in the oxide barrier. The total current over the scanned area is dominated by low current areas. The presence of spikes indicates that an AFM system using a conducting tip can be used to study the local tunneling I-V characteristics with nanometer lateral resolution.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

We investigated the effect of Pb surfactant on the growth mode and magnetism of dilute Si1−xMnx thin films grown by molecular beam epitaxy. We find that the nanocolumns that form in Pb-free samples are paramagnetic with a small saturation moment that drops with increasing Mn concentration. On the contrary, samples grown with 2 monolayers of Pb contain MnSi1.7 nanorods that lie in the plane of the films. Although the saturation moment of these samples also drops rapidly with increasing Mn concentration, it is as high as 0.65 μB/Mn. At the lowest Mn concentration (x = 0.5%), the samples grown with a Pb surfactant exhibit two ferromagnetic transitions at the Curie temperatures TC1≈40 K and TC2>400 K. While the transition at TC1 is consistent with the Curie temperature of bulk MnSi1.7 crystals, TC2 is possibly due to a second phase resulting from Mn in the Si matrix.

The magnetic nanostructure of epitaxial FCC Co/Cu (0 0 1) circular elements ( ∼1.7 μm in diameter) has been imaged with scanning electron microscopy with polarisation analysis. A closed flux configuration (quadrant configuration) is observed for most of the disks, characteristic of systems with cubic anisotropy. However, the measured width of the 90° domain wall varies with the distance from the vortex core, from 70±25 nm up to a value determined by the disk radius. Such a wide domain wall is a consequence of the geometrical constraints imposed by the element, thus defining a geometrically constrained domain wall, as confirmed by micromagnetic simulations.

Dilute magnetic semiconductors combine both magnetic ordering and semiconducting behaviour, leading to potential spintronic applications. Silicon containing dilute Mn impurities is a potential dilute magnetic semiconductor. We have grown Mn delta-doped films by deposition of 0.7 of a monolayer of Mn on Si(001) by molecular beam epitaxy and capping the film with Si. The magnetic properties are likely sensitive to the distribution of Mn on substitutional or interstitial sites and the formation of metallic precipitates. We have used polarization-dependent XAFS to examine the local structure. We compare to a thicker MnSi film grown on Si(111) and also examine the influence of lead on the manganese environment when used as a surfactant in the growth process.

Pt 1-x Ta x (0 ≤ x ≤ 1) thin-film libraries were prepared using combinatorial dc magnetron sputtering on a variety of substrates. For x 0.5 a nanocrystalline phase of Pt 1-x Ta x was observed. The bulk composition of the films obtained by electron microprobe before and after treatment in 0.5 M H 2 SO 4 at 80°C showed no detectable dissolution of Ta at any x in Pt 1-x Ta x . X-ray photoelectron spectroscopy of acid-treated samples revealed that for regions with high Ta content (x > 0.5), full oxidation of surface Ta had occurred. The samples, when used as oxygen-reduction electrocatalysts, also showed corrosion stability; however, the addition of Ta significantly lowered the activity towards oxygen reduction reaction for x > 0.1 in Pt 1-x Ta x .

The dissolution of Fe and Ni from Pt 1 - x M x (M = Fe, Ni; 0 0.6, the lattice constant expands indicating that transition metals dissolve also from the bulk. X-ray photoelectron spectroscopy results show complete removal of surface Ni (Fe) after acid treatment at 80°C for all compositions. The results of the acid treatments compare well to the composition changes that occur when a Pt 1 - x Fe x or Pt 1 - x Ni x combinatorial catalyst library is used in an operating PEM fuel cell.