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We report a comprehensive study of the temperature evolution of in-plane (IP) and out-of-plane (OOP) effective magnetic anisotropies in compensated ferrimagnetic ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Gd}}_{x}$ alloy films by employing direct current magnetometry and radiofrequency (RF) transverse susceptibility (TS) measurements. We suggest that our ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Gd}}_{x}$ system is chemically inhomogeneous and phase segregates into Fe- and Gd-enriched regions. Our IP and OOP magnetometry results indicate that the system undergoes a temperature-driven transformation from an IP-spin-configuration-dominated state to an OOP-spin-configuration-dominated state below a certain temperature (spin reorientation temperature). A two-step reversal behavior emerges in the OOP $M$($H$) loop near compensation, which we attribute to the sequential magnetization reversals of Fe- and Gd-enriched domains. Field-induced spin-flop transitions were also observed near the compensation. Our RF TS measurements indicate that the effective magnetic anisotropy for the OOP configuration dominates over that for the IP configuration below a certain spin reorientation temperature. Both IP and OOP anisotropy fields determined from our TS measurement exhibit a minimum around the compensation temperature, which has been explained in the framework of the Stoner-Wohlfarth model.

We have used a combination of x-ray magnetic circular dichroism (XMCD) spectroscopy and polarized small-angle neutron scattering (P-SANS) to investigate the distribution of magnetization in heterogenous magnetic nanoparticles (NPs) consisting of a metallic Fe core/Fe oxide shell (CS NPS) or Fe core/partial void layer/oxide shell (CVS NPs). Fe ${L}_{2,3}$ XMCD spectra were analyzed with a combination of experimental metallic Fe XMCD spectra and calculated $L$-edge spectra for the Fe cations in the oxide shell. Analyses of the temperature-dependent spectra indicate a weak variation of the relative contribution of the metallic and oxide contributions for the CS NPs, and a somewhat larger contribution from the metallic Fe core near the blocking temperature ${T}_{B}$ of the CVS NP ensemble. The P-SANS data also indicate a larger variation in the magnetization of the CVS NPs near ${T}_{B}$. Modeling of the spin-dependent neutron scattering reveals large variations in the radial magnetization distribution, with a region of reversed magnetization adjacent to the metallic core. Interfacial roughness may play a role in the development of this magnetization profile.

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging and comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. The domains show evidence of inhibited growth after nucleation, resulting in a quasi-2nd order temperature behavior.

Strained materials can exhibit drastically modified physical properties in comparison to their fully relaxed analogues. We report on the x-ray absorption spectra (XAS) and magnetic circular dichroism (XMCD) of a strained NiFe2O4 inverse spinel film grown on a symmetry matched single crystal MgGa2O4 substrate. The Ni XAS spectra exhibit a sizable difference in the white line intensity for measurements with the x-ray electric field parallel to the film plane (normal incidence) vs when the electric field is at an angle (off-normal). A considerable difference is also observed in the Fe L 2,3 XMCD spectrum. Modeling of the XAS and XMCD spectra indicate that the modified energy ordering of the cation 3d states in the strained film leads to a preferential filling of 3d states with out-of-plane character. In addition, the results point to the utility of x-ray spectroscopy in identifying orbital populations even with elliptically polarized x-rays.

The electronic and atomic structures of amorphous transparent tin oxides have been investigated by a combination of X-ray spectroscopy and atomistic calculations. Crystalline SnO is a promising p-type transparent oxide semiconductor due to a complex lone-pair hybridization that affords both optical transparency despite a small electronic band gap and spherical s-orbital character at the valence band edge. We find that both of these desirable properties (transparency and s-orbital valence band character) are retained upon amorphization despite the disruption of the layered lone-pair states by structural disorder. We explain the anomalously large band gap widening necessary to maintain transparency in terms of lone-pair stabilization via atomic clustering. Our understanding of this mechanism suggests that continuous hole conduction pathways along extended lone pair clusters should be possible under certain stoichiometries. Moreover, these findings should be applicable to other lone-pair active semiconductors.

We report on x-ray and neutron scattering studies that reveal the structure of interfaces of C60 layers with adjacent transition metal layers, in this instance, Cu. Such interfaces produce room-temperature long-range spin order that is not described by conventional theories of metallic magnetism. We use a combination of hard x-ray reflectivity and neutron scattering to investigate the interfacial structure of two C60/Cu layered samples: a superlattice with multiple C60/Cu repeats and a simpler tri-layer structure. For both structures, we develop a consistent structural model for the two scattering techniques, which details the critical interfacial roughness between the layers. We find that while x-ray reflectivity provides a strong contrast between the C60 and Cu layers, the similar neutron scattering length density of the two materials severely reduces the neutron scattering contrast. Our results can be used to design material systems that permit studies of the magnetism of the C60/transition metal interfaces with spin-sensitive scattering probes such as polarized neutron reflectometry.

Incorporating Fe atoms into the lattice is shown to significantly alter electronic and magnetic properties of $\mathrm{Ti}{\mathrm{O}}_{2}$ nanotubes synthesized by electrochemical anodization of Ti-Fe alloy sheets. The effects of Fe incorporation on the nanotube morphology, crystallinity, crystal structure, magnetic behavior and electronic structure were investigated with crystallographic and magnetic probes, including synchrotron-based spectroscopy. Results indicate that the iron cations predominately adopt the $\mathrm{F}{\mathrm{e}}^{3+}$ configuration, leading to a large increase of the electronic density of states at the Fermi energy. This increase is anticipated to provide enhanced catalytic action, for instance, in the degradation of water and of air pollutants. These results provide insight for tailoring the functionality of these nanostructures for energy-related applications.

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically &lt, 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions.

The nature of the metal-insulator transition in thin films and superlattices of LaNiO3 only a few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate–film interface quality may also affect the observable properties of ultrathin films. Here we report results obtained for near-ideal LaNiO3 films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO3 substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO2 termination). Electronic structure measurements using X-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal-insulator transition., The electronic behaviour of complex oxides such as LaNiO3 depends on many intrinsic and extrinsic factors, making it challenging to identify microscopic mechanisms. Here the authors demonstrate the influence of oxygen vacancies on the thickness-dependent metal-insulator transition of LaNiO3 films.

Preventing stray magnetic interactions is important to improving technology that relies upon manipulating an electron's charge and spin. To this end, the authors investigate a binary intermetallic that has an internal magnetic field, yet does not affect the spin of surrounding components. As-grown epitaxial thin films of Mn${}_{3}$Al on GaAs(001) present the the ideal D0${}_{3}$ Heusler phase. This compound should be quite interesting for optimizing tomorrow's spintronic devices.

The magnetic structure of Fe-doped CoFe2O4 (Co1−xFe2+xO4) grown on MgO (0 0 1) and SrTiO3 (0 0 1) substrates is studied with superconducting quantum interference device magnetometry and soft x-ray magnetic spectroscopies. X-ray and electron diffraction show that the choice of substrate has large effects on the strain, crystal structure and surface morphology of Co1−xFe2+xO4 thin films. Samples grown on MgO have small, coherent strains and surfaces that are nearly atomically flat, whereas films grown on SrTiO3 have large tensile strains and surfaces terminated with islands, which indicate the presence of a large density of misfit dislocations. These differences in structural properties correlate with the large differences seen in the magnetic structure; samples grown on SrTiO3 have larger magnetic moments and increased anisotropies compared to those grown on MgO. Most strikingly, the large magnetic spin and orbital moments found in the films grown on SrTiO3 suggest a suppression of anti-phase boundary formation, which we attribute to the large compressive lattice mismatch and the formation of misfit dislocations during the film growth in order to relieve the epitaxial strain. This results in the films grown on SrTiO3 having magnetic properties that are more similar to bulk Co1−xFe2+xO4 than those grown on MgO, demonstrating that epitaxial strain can result in large changes in the magnetic structure of Co1−xFe2+xO4.

Tensile strain along the ${c}_{R}$ axis in epitaxial ${\mathrm{VO}}_{2}$ films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the ${\mathrm{TiO}}_{2}$ substrate and identify the phases of ${\mathrm{VO}}_{2}$ as a function of temperature in epitaxial ${\mathrm{VO}}_{2}/{\mathrm{TiO}}_{2}$ thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the ${c}_{R}$ axis is insufficient for describing the transition pathway of ${\mathrm{VO}}_{2}$ epitaxial films and that a strain induced increase of electron correlation effects must be considered.

Ferromagnetic resonance (FMR) spectroscopy, x-ray magnetic circular dichroism (XMCD) spectroscopy and magnetic transmission soft x-ray microscopy (MTXM) experiments have been performed to gain insight into the magnetic anisotropy and domain structure of ultrathin Co | Ni multilayer films with a thin permalloy layer underneath. MTXM images with a spatial resolution better than 25 nm were obtained at the Co L3 edge down to an equivalent thickness of Co of only 1 nm, which establishes a new lower boundary on the sensitivity limit of MTXM. Domain sizes are shown to be strong functions of the anisotropy and thickness of the film.

Cobalt nanoparticles have been embedded either within magnetic thin films of CuNi or Ni, or at their interface with an Al capping layer. Soft X-ray magnetic circular dichroism (XMCD) was used to measure element-specific hysteresis loops, allowing discrimination between the nanoparticles and the host film. The hysteresis shows coupling between the nanoparticles and the films and a reduction in coercivity is observed when positioning the nanoparticles within the film, as opposed to at its interface with the cap. Sum rule analysis of the XMCD spectra makes it possible to determine the orbital to spin moment ratios for Co and Ni in the samples. The difference in the calculated ratios is consistent with the differences in the coercivities, the films appearing more homogenous due to hybridization of the 3d bands of Co and Ni when the nanoparticles are embedded deeper within the films, creating less pinning sites and hence a lower coercivity.

We have studied the effects of the initial stages of the annealing on magnetic tunnel junctions with MgO barriers and CoFeB electrodes. We report changes in the resistance–voltage characteristics and tunneling magnetoresistance for patterned transport junctions, and correlate these with the observed changes in the structural and magnetic interface morphologies determined by soft X-ray resonant magnetic scattering from sheet films from the same wafer. An important feature of our experiment was that all measurements were carried out within the soft X-ray diffractometer on samples from the same wafer subjected to simultaneous annealing cycles, so that our magnetotransport and scattering data are directly comparable. The as-grown junction showed a tunneling magnetoresistance ratio of 5.5%, and a specific barrier resistance of 85.6 k Ω μ m 2 . A 200 ∘ C anneal for 1 h resulted in a small rise in barrier resistance and magnetoresistance coupled with a smoothing of the magnetic interfaces, consistent with the healing of barrier defects and removal of tunneling hot-spots. A subsequent 300 ∘ C anneal for a further hour resulted in further smoothing, and a rise in the magnetoresistance ratio to 72%, and a much weaker dependence of the parallel state resistance upon voltage bias, indicating the development of ( 0 0 1 ) crystallographic texture in the electrodes. Annealing to 325 ∘ C yielded a further decrease in magnetic interface width (the quadrature sum of roughness and intermixing length scales). The reduction in interface width for Co species occurred at higher temperatures than for Fe throughout the experiments.

Ferromagnetic nanoparticles in the 10-14 nm size range are examined for their size and interaction dependent magnetic properties. From X-ray magnetic circular dichroism the orbital-to-spin magnetic moment ratio is determined and found to decrease significantly with particle size. This is in accordance with previous complementary studies on smaller particles and highlights the difficulty of fitting to a simple core-shell model. Vibrating sample magnetometry experiments on samples with more than 1000 particles per square micron show a wide distribution of blocking temperatures from 50 to greater than 650 K. This is attributed to the dipole-dipole magnetic coupling forces between particles. The blocking temperatures show an unexpected negative correlation with increasing particle density.

Discovering new antiferromagnetic compounds is at the forefront of developing future spintronic devices without fringing magnetic fields. The antiferromagnetic gapless semiconducting D03 phase of V3Al was successfully synthesized via arc-melting and annealing. The antiferromagnetic properties were established through synchrotron measurements of the atom-specific magnetic moments, where the magnetic dichroism reveals large and oppositely-oriented moments on individual V atoms. Density functional theory calculations confirmed the stability of a type G antiferromagnetism involving only two-third of the V atoms, while the remaining V atoms are nonmagnetic. Magnetization, x-ray diffraction and transport measurements also support the antiferromagnetism. This archetypal gapless semiconductor may be considered as a cornerstone for future spintronic devices containing antiferromagnetic elements., Comment: Accepted to Physics Review B on 02/23/15

Epitaxial ZnO films of ∼450 nm thicknesses were grown by MOCVD on r-sapphire and doped by implantation of 200 keV Mn ions to a dose of 5 × 1016 ions/cm2. The structural, chemical, and magnetic properties of the films were investigated with X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), X-ray absorption spectroscopy (XAS) and SQUID magnetometry. XRD and RBS show both Mn-doped ZnO and pure ZnO epitaxial layers in the as-implanted film, which is ferromagnetic at 5 K but nonmagnetic at room temperature. For the as-implanted materials, only Mn2+ ions are observed with XAS. Post-implantation annealing partially recovers the lattice damage and redistributes Mn into the entire ZnO film; in addition, Mn2+ ions are converted to a mixture of Mn3+ and Mn4+, and ferromagnetism is now observed above 300 K. Our results show that ion implantation is a viable route for achieving room temperature ferromagnetism in epitaxial ZnO films. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

O1s and Mn2p near-edge X-ray absorption spectroscopy on La1-xSr1+xMnO4 (0 ≤x ≤0.5) single crystals shows that Sr doping does not only provide holes to the system but also induces a continuous transfer of electrons from out-of-plane d3z2-r2 to in-plane d3x2-r2/d3y2-r2 orbitals. Furthermore, a non-vanishing electron occupation of in-plane dx2-y2 and out-of-plane d3z2-r2 orbitals is observed up to relatively high doping contents. These findings demonstrate that the energy difference between all these orbital types has to be very small and manifest that the orbital degree of freedom is determined not just by crystal field effects but also by orbital coupling. Moreover, the doping-dependent transfer of spectral weight observed in the current data identifies La1-xSr1+xMnO4 as a charge-transfer insulator.

Bulk-sensitive O 1s near-edge X-ray absorption spectroscopy on Ba1 − xKxBiO3 and BaPb1 − yBiyO3 single crystals shows for undoped BaBiO3 a distinct charge-density wave derived band slightly above the Fermi level; upon K doping it is effectively reduced and competes with bipolaronic hole states whose spectral weight increases with K content. For Pb doping, on the other hand, a wide Pb-O band is formed near EF while the intensity of the bipolaronic hole band remains unaffected. The data suggest a crucial role of hole bipolarons for bismuthate superconductivity.

Characterization of the inactive clusters formed by high dose implantation silicon are one of the crucial topics in the semiconductor industry. Analytical techniques, which could provide quantitative information on the detailed description of the complexes that are formed at dopant concentrations above solid solubility levels, are valuable to post-implantation researchers. Previously, X-ray absorption fine-structure spectroscopy (XAFS) studies of the As and Sb implanted Si wafers revealed not only that of the chemical nature of clusters, but also the arsenic case, the ratio of the precipitated versus the substitutional form of the As in the system [1] , [2] . In this study, we have used F K-edge and Ge K-edge (for the Ge pre-amorphized wafers) XAFS in order to probe the nature and of the clusters in B, BF2 and BF3 implanted Si wafers. Implants with various doses and implant energies were subjected to various annealing techniques such as laser annealing, spike annealing, solid state phase epitaxy, and flash annealing and the evolution of the spectral features of the clusters were followed upon annealing. The theoretical multiple scattering XAFS calculations were performed in order to correlate the spectral features in the XAFS data with the near-neighbor configuration around the main absorbing atom. The identification and the relative weight of the clusters after different annealing conditions will be presented in order to provide insight to the optimum annealing conditions in these systems.

Mixed oxide interfaces are critical for delivering active components of demanding catalytic processes such as the photocatalytic splitting of water. We have studied CeOx–TiO2 catalysts with low ceria loadings of 1, 3, and 6 wt % that were prepared with wet impregnation methods to favor a strong interaction between CeOx and TiO2. In these materials the interfaces between CeOx–TiO2 have been sequentially loaded (1%, 3%, and 6%), with and without Pt (0.5 wt %). The structure and properties of the catalysts were characterized using several X-ray and electron based techniques including XRD, XPS, UPS, NEXAFS, UV–vis, and HR-STEM/STEM-EELS to unravel the local morphology, bulk structure, surface states, and electronic structure. The combination of all these techniques allows us to analyze in a systematic way the complete structural and electronic properties that prevail at the CeOx–TiO2 interface. Fluorite structured nanocrystallites of ceria on anatase-structured titania were identified by both XRD and NEXAFS. A sequential increase of the CeOx loading led to the formation of clusters, then plates, and finally nanoparticles in a hierarchical manner on the TiO2 support. The electronic structures of these catalysts indicate that the interaction between TiO2 and CeO2 is closely related to the local morphology of nanostructured CeO2. Ce3+ cations were detected at the surface of CeO2 and at the interface of the two oxides. In addition, the titania is perturbed by the interaction with ceria and also with Pt. The photocatalytic activity for the splitting of H2O using UV light was measured for these materials and correlated with our understanding of the electronic and structural properties. Optimal catalytic performance and photoresponse results were found for the 1 wt % CeOx–TiO2 catalyst where low dimensional geometry of the ceria provided ideal electronic and geometrical properties. The structural and electronic properties of the interface were critical for the photocatalytic performance of this mixed-oxide nanocatalyst system., The research carried out in this manuscript was performed at Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under Contract DE-SC0012704. L.B. also acknowledges financial support from the JAE-CSIC grant program

Using a controllable wet chemical approach, the polyol process, we developed a cobalt carbide nanomagnet consisting of an assembly of Co2C and Co3C nanoparticles as an alternative to rare earth permanent magnets (PMs). The thermodynamically stable mixed phase cobalt carbide nanoparticles are shown to be acicular in morphology. Their exchange-coupled magnetic interaction possessing high maximum energy product of 20.7 kJ m(-3) and room temperature coercivity (2.9 kOe) has been confirmed through vibrating sample magnetometer (VSM) and first order reversal curves (FORCs). These metastable carbide nanoparticles offer improved magnetic properties compared to their pure bulk form. An understanding of the formation mechanism, using in situ time-resolved X-ray spectroscopy (TR-XAS), and the correlation between phase contributions to the properties are described in detail. Our strategy presents a controllable route to preparing the cobalt carbide nanomagnets, which could be potentially useful in permanent magnet clean energy applications. Additionally, the in situ apparatus offers a promising way to directly explore the effects of reaction variables for high-temperature wet chemical reactions.

The electronic and structural properties of thin LaNiO₃ films grown by using molecular beam epitaxy are studied as a function of the net ionic charge of the surface terminating layer. We demonstrate that electronic transport in nickelate heterostructures can be manipulated through changes in the surface termination due to a strong coupling of the surface electrostatic properties to the structural properties of the Ni—O bonds that govern electronic conduction. We observe experimentally and from first-principles theory an asymmetric response of the structural properties of the films to the sign of the surface charge, which results from a strong interplay between electrostatic and mechanical boundary conditions governing the system. The structural response results in ionic buckling in the near-surface NiO₂ planes for films terminated with negatively charged NiO₂ and bulklike NiO₂ planes for films terminated with positively charged LaO planes. The ability to modify transport properties by the deposition of a single atomic layer can be used as a guiding principle for nanoscale device fabrication.

The magnetic, transport, and electronic properties of ultrathin epitaxial ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) films at near half doping ($x=0.47$, 0.50, and 0.55), grown under different misfit strains on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$(001) and ${\mathrm{La}}_{0.18}{\mathrm{Sr}}_{0.82}{\mathrm{Al}}_{0.59}{\mathrm{Ta}}_{0.41}{\mathrm{O}}_{3}$(001) (LSAT) substrates, are investigated. We find that all films exhibit metallic behavior below the magnetic critical temperature, while the magnetic properties change markedly with both doping and strain. However, while increased doping favors antiferromagnetic ordering, strain is the driving mechanism for the change in the magnetic properties, where with increasing tensile strain the magnetic ground state changes from ferromagnetic to antiferromagnetic at a critical lattice misfit threshold of about \ensuremath{-}1%. The bulk magnetometry data are confirmed by x-ray magnetic circular dichroism spectroscopy, while x-ray magnetic linear dichroism measured at room temperature demonstrates a progressive change in the orbital occupancy with increasing misfit strain from out of plane to in-plane, leading to a preferred antiferromagnetic metallic state at larger tensile strains.

A series of manganese ferrite thin film samples were prepared by alternating target laser ablation deposition and conventional pulsed laser deposition techniques on (111) MgO substrates. By extended X-ray absorption fine structure (EXAFS) analysis, we have discovered that the cation distribution was sensitive to the processing oxygen pressure and the preparation technique. Correspondingly, the saturation magnetization and uniaxial anisotropy fields change. Saturation magnetization was found to decrease while the percentage of Mn ions on the octahedral site increased, as a function of oxygen processing pressure. The highest magnetization (~4.5 kG) and anisotropy field (~0.5 kOe) corresponded to the sample grown at the lowest oxygen pressure

The internal phase profile of electromagnetic radiation determines many functional properties of metal, oxide or semiconductor heterostructures. In magnetic heterostructures, emerging spin electronic phenomena depend strongly upon the phase profile of the magnetic field H at gigahertz frequencies. Here we demonstrate nanometre-scale, layer-resolved detection of electromagnetic phase through the radio frequency magnetic field H(rf) in magnetic heterostructures. Time-resolved X-ray magnetic circular dichroism reveals the local phase of the radio frequency magnetic field acting on individual magnetizations M(i) through the susceptibility as M = χH(rf). An unexpectedly large phase variation, ~40°, is detected across spin-valve trilayers driven at 3 GHz. The results have implications for the identification of novel effects in spintronics and suggest general possibilities for electromagnetic-phase profile measurement in heterostructures.

Fe ions of dose 5×1016cm−2 were implanted at 200keV into ZnO nanotips at room temperature. Scanning electron microscopy images show that the implanted ZnO tips are much sharper than the as-grown ones. The sharpening effect of the ZnO nanotips is attributed to the interplay between ion sputtering and ion-induced surface diffusion. X-ray diffraction indicates that ZnO crystallinity was preserved during implantation. Photoluminescence measurements show development of a broad blue band and severe UV quenching upon ion bombardment, and partial recovery after annealing. X-ray absorption spectroscopy is consistent with Fe ions that are substitutional for Zn in the ZnO wurtzite lattice.Fe ions of dose 5×1016cm−2 were implanted at 200keV into ZnO nanotips at room temperature. Scanning electron microscopy images show that the implanted ZnO tips are much sharper than the as-grown ones. The sharpening effect of the ZnO nanotips is attributed to the interplay between ion sputtering and ion-induced surface diffusion. X-ray diffraction indicates that ZnO crystallinity was preserved during implantation. Photoluminescence measurements show development of a broad blue band and severe UV quenching upon ion bombardment, and partial recovery after annealing. X-ray absorption spectroscopy is consistent with Fe ions that are substitutional for Zn in the ZnO wurtzite lattice.

The magnetic properties of iron-doped cobalt ferrite (Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$) (001) thin films grown epitaxially on MgO (001) substrates are investigated by superconducting quantum interference device magnetometry and soft x-ray magnetic linear and circular dichroisms. All Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$ (0.01 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 0.63) samples have out-of-plane magnetic easy axes and large coercive fields, unlike Fe${}_{3}$O${}_{4}$, due to a large Co${}^{2+}$ orbital moment. The magnetic moments for those samples are significantly reduced from their bulk values; however, as $x$ increases, the magnetic moments tend nearer to their bulk values and increase more rapidly as $x$ approaches 1. This reduction in magnetic moment is attributed to spin canting among the Co${}^{2+}$ cations, owing to a small in-plane tensile strain in the film and to an increased antiferromagnetic alignment among all the cations caused by a partially inverse spinel cubic structure and the likely presence of antiphase boundaries. Our results show that small changes in stoichiometry can lead to significant changes in the magnetic moment of Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$, especially at large values of $x$.

Chromium-doped gallium sesquiselenide, Cr:Ga${}_{2}$Se${}_{3}$, is a member of a new class of dilute magnetic semiconductors exploiting intrinsic vacancies in the host material. The correlation among room-temperature ferromagnetism, surface morphology, electronic structure, chromium concentration, and local chemical and structural environments in Cr:Ga${}_{2}$Se${}_{3}$ films grown epitaxially on silicon is investigated with magnetometry, scanning tunneling microscopy, photoemission spectroscopy, and x-ray absorption spectroscopy. Inclusion of a few percent chromium in Ga${}_{2}$Se${}_{3}$ results in laminar, semiconducting films that are ferromagnetic at room temperature with a magnetic moment $\ensuremath{\geqslant}4{\ensuremath{\mu}}_{B}/\text{Cr}$. The intrinsic-vacancy structure of defected-zinc-blende $\ensuremath{\beta}\ensuremath{-}$Ga${}_{2}$Se${}_{3}$ enables Cr incorporation in a locally octahedral site without disrupting long-range order, determined by x-ray absorption spectroscopy, as well as strong overlap between Cr $3d$ states and the Se $4p$ states lining the intrinsic-vacancy rows, observed with photoemission. The highest magnetic moment per Cr is observed near the solubility limit of roughly one Cr per three vacancies. At higher Cr concentrations, islanded, metallic films result, with a magnetic moment that depends strongly on surface morphology. The effective valence is Cr${}^{3+}$ in laminar films, with introduction of Cr${}^{0}$ upon islanding. A mechanism is proposed for laminar films whereby ordered intrinsic vacancies mediate ferromagnetism.

The electronic, magnetic and transport properties of iron-doped cobalt ferrite (Co{sub 1-x}Fe{sub 2+x}O{sub 4}) thin films grown epitaxially on MgO (001) substrates are investigated by soft x-ray absorption and photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, superconducting quantum interference device magnetometry, and resistivity measurements. The crystal structure for Co{sub 1-x}Fe{sub 2+x}O{sub 4} is determined to be nearly inverse spinel, with the degree of inversion increasing for increased doping until it becomes fully inverse spinel for Fe{sub 3}O{sub 4}. The doped iron cations have a valency of 2+ and reside solely on octahedral sites, which allows for conduction owing to hopping between Fe{sup 2+} and Fe{sup 3+} octahedral cations. The addition of Fe{sup 2+} cations increases the electron density of states near the Fermi energy, shifting the Fermi level from 0.75 to 0 eV with respect to the top of the valence band, as the doping increases from x = 0.01 to 1. This change in electronic structure results in a change in resistivity by over two orders of magnitude. In contrast, the magnetic properties of CoFe{sub 2}O{sub 4} thin films, characterized by a significantly reduced saturation magnetization compared to the bulk and large magnetic anisotropies, are affected less significantly by doping inmore » the range from 0 to 0.63. These results show that Co{sub 1-x}Fe{sub 2+x}O{sub 4} has tunable electronic properties while maintaining magnetic properties similar to CoFe{sub 2}O{sub 4}.« less

Using the surface, interface, and element specificity of x-ray resonant magnetic scattering in combination with x-ray magnetic circular dichroism, we have spatially resolved the magnetic spin polarization, and the associated interface proximity effect, in a Mn-based high-susceptibility material close to a ferromagnetic Co layer. We have measured the magnetic polarization of Mn and Cu 3d electrons in paramagnetic CuMn alloy layers in [Co/Cu(x)/CuMn/Cu(x)](20) multilayer samples with varying copper layer thicknesses from x=0 to 25 angstrom. The size of the Mn and Cu L-2,L-3 edge dichroism shows a decrease in the Mn-induced polarization for increasing copper thickness indicating the dominant interfacial nature of the Cu and Mn spin polarization. The Mn polarization is much higher than that of Cu. Evidently, the Mn moment is a useful probe of the local spin density. Mn atoms appear to be coupled antiferromagnetically with the Co layer below x=10 angstrom and ferromagnetically coupled above. In contrast, the interfacial Cu atoms remain ferromagnetically aligned to the Co layer for all thicknesses studied.

The nanoscale magnetic structure of FeRh epilayers has been studied by polarized neutron reflectometry. Epitaxial films with a nominal thickness of 500 angstrom were grown on MgO (001) substrates via molecular-beam epitaxy and capped with 20 angstrom of MgO. The FeRh films show a clear transition from the antiferromagnetic (AF) state to the ferromagnetic (FM) state with increasing temperature. Surprisingly the films possess a FM moment even at a temperature 80 K below the AF-FM transition temperature of the film. We have quantified the magnitude and spatial extent of this FM moment, which is confined to within similar to 60-80 angstrom of the FeRh near the top and bottom interfaces. These interfacial FM layers account for the unusual effects previously observed in films with thickness

This research was funded by the Engineering and Physical Sciences Research Council, (EP/P50385X/1), the European Research Council (ERC-2009-AdG 247276 NOVOX). The work at Texas A&M was funded by the U.S. National Science Foundation (DMR-1401266). The work at Los Alamos was supported by the U.S. Department of Energy through the LANL/LDRD program and was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

We demonstrate an isolated magnetic interface anisotropy in amorphous CoFeB films on (Al)GaAs(001), similar to that in epitaxial films but without a magnetocrystalline anisotropy term. The direction of the easy axis corresponds to that due to the interfacial interaction proposed for epitaxial films. We show that the anisotropy is determined by the relative orbital component of the atomic magnetic moments. Charge transfer is ruled out as the origin of the interface anisotropy, and it is postulated that the spin-orbit interaction in the semiconductor is crucial in determining the magnetic anisotropy.

To furnish insight into correlations of electronic and local structure and photoactivity, arrays of short and long TiO2 nanotubes were synthesized by electrochemical anodization of Ti foil, followed by thermal treatment in O2 (oxidizing), Ar (inert), and H2 (reducing) environments. The physical and electronic structures of these nanotubes were probed with x-ray diffraction, scanning electron microscopy, and synchrotron-based x-ray absorption spectroscopy, and correlated with their photocatalytic properties. The photocatalytic activity of the nanotubes was evaluated by monitoring the degradation of methyl orange under UV-VIS light irradiation. Results show that upon annealing at 350 °C all as-anodized amorphous TiO2 nanotube samples partially transform to the anatase structure, with variations in the degree of crystallinity and in the concentration of local defects near the nanotubes' surface (∼5 nm) depending on the annealing conditions. Degradation of methyl orange was not detectable for the as-anodized ...

We demonstrate a layer- and time-resolved measurement of ferromagnetic resonance (FMR) in a Ni81Fe19 / Cu / Co93Zr7 trilayer structure. Time-resolved x-ray magnetic circular dichroism has been developed in transmission, with resonant field excitation at a FMR frequency of 2.3 GHz. Small-angle (to 0.2 degree), time-domain magnetization precession could be observed directly, and resolved to individual layers through elemental contrast at Ni, Fe, and Co edges. The phase sensitivity allowed direct measurement of relative phase lags in the precession oscillations of individual elements and layers. A weak ferromagnetic coupling, difficult to ascertain in conventional FMR measurements, is revealed in the phase and amplitude response of individual layers across resonance., 22 pages, 6 figures submitted to Physical Review B

We demonstrate spectral shift and broadening of 120fs, 800nm pulses in ZnTe with the presence of ~1ps, ~40uJ coherent terahertz pulses. This spectral modulation is attributed to cross-phase modulation induced by the strong THz field.

International audience; We have carried out resonant magnetic x-ray scattering from Co/ Ru multilayers with weak antiferromagnetic coupling. We have measured hysteresis loops at different points in reciprocal space ͑specular or diffuse, integer or half-order peaks͒, which reveal a rich variety of different shapes. These arise from different degrees of mixing of the scattering arising from the field dependence of the order parameters describing the degree of ferromagnetic, antiferromagnetic correlations in the sample, with the off-specular measurements giving information about their lateral extent. We make a comparison with macroscopic measurements that give information about the degree of different forms of magnetic order averaged over the whole sample in real space.

We report the results of soft x-ray resonant magnetic scattering experiments conducted on Co∕Ru magnetic multilayers patterned into a reflection grating. Sample magnetization dependent scans of the in-plane wave vector transfer were performed to probe the relationship between the lateral physical and magnetic structures. Different Fourier components were found to dominate the sum and difference of these spectra, corresponding to different lateral wave forms for the structure-structure and structure-magnetism correlations.

We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps., see http://magnet.ap.columbia.edu/pubs for related work

We have investigated the magnetic anisotropy and interlayer coupling in trilayer films of permendur(100 \AA)/Ru/permalloy(100 \AA), with the thickness of the Ru spacer varying from 0 to 200 \AA. Wh ...

Nanoislands of ??-FeRh were formed by the deposition of a 10?nm film onto (0?0?1)-MgO and observed by atomic force microscopy. This island-like architecture results in stabilized ferromagnetic (FM) ordering at low temperatures as noted by asymmetry in the x-ray diffraction peaks, a magnetostructural transition with a large magnetic background signal, and broad thermal hysteresis. The combination of structural and magnetic results suggest that the crystallographic arrangement of the FeRh nanoislands consists of a gradient of lattice parameters where there is a compressed inner region of lower lattice parameter values (attributed to antiferromagnetic ordering) which gradually relaxes outward to a region of larger lattice parameters (attributed to a retained FM ordering). The impact of this configuration on magnetostructural transformation is discussed in the context of classical nucleation theory.

The distribution of cations in Fe-doped cobalt ferrite (Co1−xFe2+xO4) is investigated as a function of epitaxial strain through x-ray absorption measurements of samples grown on SrTiO3, MgO, and CoCr2O4-buffered MgAl2O4 (001). In agreement with recent theoretical calculations, compressive (tensile) strain results in the films having a larger (smaller) degree of cation inversion for iron doping levels up to x = 0.62. Measurements of the resistivity further conclude that the degree of cation inversion has a direct effect on the size of the bandgap for stoichiometric CoFe2O4, an effect that is reduced as the iron doping level is increased.

The polar discontinuity which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La0.7Sr0.3MnO3 on Nb-doped SrTiO3(0 0 1) deposited via pulsed laser deposition (Lee et al 2010 Phys. Rev. Lett. 105 257204). Here, we report on a similar effect for La0.7Sr0.3MnO3 grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO3 (STO) substrates. The reversed magnetic state occurred in both samples, while enriched Mn3+ regions (intermediate layer) are slightly different. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: ∼125 K for the Nb : STO substrate and ∼240 K for the undoped STO substrate. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films.

It is common to find a thin (∼0.5 nm) layer of Mg deposited prior to the MgO tunnel barrier in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions, due to the improved device performance that results. However, despite their common usage, the reasons why such layers are effective are unclear. We use structures that model the lower electrode of such devices to show that a suitably thick Mg insert layer enhances the crystal quality of both MgO and CoFe(B), permits interfacial oxides to reduce back to a metallic ferromagnetic state, and hence improves magnetic switching of the CoFe(B) electrode, properties which are inextricably linked to device performance.

We have studied the effect of annealing on the interface magnetization in a CoFeB/MgO structure which models the lower electrode in a magnetic tunnel junction device. We find that MgO deposition causes Fe to diffuse toward the CoFeB/MgO interface, where it preferentially bonds with oxygen to form a Fe-O-rich interfacial region with reduced magnetization. After annealing at 375 °C the compositional inhomogeneity remains; Fe is reduced back to a ferromagnetic metallic state and the full interfacial magnetization is regained.

Surface structural effects accompanying the antiferromagnetic-ferromagnetic magnetostructral transition of epitaxial FeRh thin films were investigated by grazing incidence x-ray scattering. Measurement of the film lattice parameters and variation of x-ray incident angles allow observation of the transition character on scales ranging from a few nm to the total through-thickness of the film. Out-of-plane lattice measurements confirm that the ferromagnetic phase nucleates from the surface during the heating process and is retained at the surface below the transition temperature during the cooling process. These results suggest that surface strain relief fosters nucleation of the ferromagnetic phase.