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Nuclear resonant reflectivity (NRR) from an Fe60Al40 film was measured using synchrotron radiation at several grazing angles near the critical angle of total external reflection. Using laterally resolved measurements after irradiation with 20 keV Ne+ ions of gradually varying fluence of 0–3.0 × 1014 ions cm−2, the progressive creation of the ferromagnetic A2 phase with increasing ion fluence was confirmed. The observed depth selectivity of the method has been explained by application of the standing wave approach. From the time spectra of the nuclear resonant scattering in several reflection directions the depth profiles for different hyperfine fields were extracted. The results show that the highest magnetic hyperfine fields (∼18–23 T) are initially created in the central part of the film and partially at the bottom interface with the SiO2 substrate. The evolution of the ferromagnetic onset, commencing at a fixed depth within the film and propagating towards the interfaces, has been directly observed. At higher fluence (3.0 × 1014 ions cm−2) the depth distribution of the ferromagnetic fractions became more homogeneous across the film depth, in accordance with previous results.

We generated electro magnetic transients with a frequency content up to the terahertz regime from Co 2 Fe 0.4 Mn 0.6 Si Heusler alloy/heavy-metal bilayers by excitation with fs-laser pulses. We ascribe the generation process to the inverse spin Hall effect. We compared our results with ferromagnetic resonance measurements to investigate the efficiency of interfacial spin currents. We observed that the efficiency of the THz radiation can be described by the spin-orbit coupling, nonetheless the interface properties play an important role.

A new polyoxometalate compound consisting of the 39-tungsto-4-arsenate(III) unit with four incorporated V

Nuclear resonant reflectivity (NRR) from an Fe

In this contribution, we analyze the properties of two-phase magneto-electric (ME) composites. Such ME composite materials have raised scientific attention in the last decades due to many possible applications in a wide range of technical devices. Since the effective magneto-electric properties significantly depend on the microscopic morphology, we investigate in more detail the changes in the in-plane polarization due to an applied magnetic field. It was shown in previous works that it is possible to grow vertically aligned nanopillars of magnetostrictive cobalt ferrite in a piezoelectric barium titanate matrix by pulsed laser deposition. Based on x-ray linear dichroism, the displacements of titanate ions in the matrix material can be measured due to an applied magnetic field near the boundary of the interface between the matrix and the nanopillars. Here, we focus on (1–3) fiber-induced composites, based on previous experiments, where cobalt ferrite nanopillars are embedded in a barium titanate matrix. In the numerical simulations, we adjusted the boundary value problem to match the experimental setup and compare the results with previously made assumptions of the in-plane polarizations. A further focus is taken on the deformation behavior of the nanopillar over its whole height. Such considerations validate the assumption of the distortion of the nanopillars under an external magnetic field. Furthermore, we analyze the resulting magneto-electric coupling coefficient.

A series of unprecedented supramolecular associates of phenylarsonate-capped {MII PdII 12 O8 }-type (M=Co, Ni and Zn) polyoxopalladates with α-cyclodextrins (α-CD) was obtained and characterized in the solid state (single-crystal X-ray diffraction (XRD), FT-IR spectroscopy, elemental and thermogravimetric (TGA) analyses), in aqueous solution (1 H and 13 C NMR) and in the gas phase (ESI-MS). The non-covalent host-guest interactions between the organopolyoxoanions and α-CD rings alter the O8 coordination environment of a 3d transition metal ion (MII ) situated at the center of a cuboid polyoxododecapalladate shell. This synthetically controlled "chemical pressure" effectively induces axial distortion of the otherwise cubic polyoxopalladate environment between two trans-positioned α-CD moieties. Its effect on the magnetic properties and the electronic structure of the CoII derivative was assessed in a combined SQUID magnetometry, EPR, X-ray magnetic circular/linear dichroism (XMCD/XMLD), and X-ray absorption near-edge structure (XANES) spectroscopy study.

A new polyoxometalate compound consisting of the 39-tungsto-4-arsenate(III) unit with four incorporated VIV ions, isolated as (NH4)22[(VIVO)2(VIVO(H2O))(AsIIIWVI9O33)2(AsIIIWVI8.5VIV0.5(OH)O32)2(WVIO2)4]·48H2O (NH4-As4W39(V4)), was synthesized and fully characterized. SQUID magnetometry shows three weakly coupled VIV centers with an antiferromagnetic exchange interaction and one isolated VIV ion as a spin-1/2 Curie paramagnet. UV-vis spectroscopy indicates that the As4W39(V4) structure remains intact in aqueous solution for at least 24 h. To enable the deposition of As4W39(V4) from solution on gold surfaces, its trihexyltetradecylphosphonium salt, THTDP-As4W39(V4), was prepared. The IR spectra of both congeners reveal the structural identity of As4W39(V4) independent of the countercations. The X-ray absorption near-edge structure data confirm the presence of VIV centers in a distorted square-pyramidal coordination geometry in NH4-As4W39(V4) and THTDP-As4W39(V4). X-ray photoelectron spectroscopy of the latter, deposited on Au(111), shows that the 4 V and 35 W centers preserve their IV+ and VI+ oxidation states, while the remaining 4 W ions are reduced to IV+.

We generated pulses of electromagnetic radiation with a frequency content up to three terahertz (THz) by optical excitation of Co2Fe0.4Mn0.6Si Heusler alloy/heavy metal bilayers (CFMS/HM) using fs-laser pulses. We attribute the generation process to the conversion of a spin current, generated by the illumination with a fs-laser pulse, to a charge current via the inverse spin Hall effect. We compared the THz emission efficiency in CFMS/Pt and CFMS/Ta bilayers due to their high spin-orbit coupling of Pt and Ta. Surprisingly, our data reveal that CFMS/Pt shows substantially larger THz amplitudes compared to CFMS/Ta. Both bilayers exhibit a tunability of the THz amplitude by external magnetic field both at 300 K and 100 K. Ferromagnetic resonance measurements demonstrate that CFMS/Ta has a high effective spin mixing conductance, describing the efficiency of interfacial spin transport. We observe that the efficiency of the THz radiation cannot be solely described by the spin-orbit coupling strength and the spin diffusion length in the HM material plays an important role.

Loss mechanisms in fluid heating of cobalt ferrite (CFO) nanoparticles and CFO-Pd heterodimer colloidal suspensions are investigated as a function of particle size, fluid concentration and magnetic field amplitude. The specific absorption rate (SAR) is found to vary with increasing particle size due to a change in dominant heating mechanism from susceptibility to hysteresis and frictional loss. The maximum SAR is obtained for particle diameters of 11-15 nm as a result of synergistic contributions of susceptibility loss, including Néel and Brownian relaxation and especially hysteresis loss, thereby validating the applicability of linear response theory to superparamagnetic CFO nanoparticles. Our results show that the ferrofluid concentration and magnetic field amplitude alter interparticle interactions and associated heating efficiency. The SAR of the CFO nanoparticles could be maximized by adjusting the synthesis parameters. Despite the paramagnetic properties of individual palladium nanoparticles, CFO-Pd heterodimer suspensions were observed to have surprisingly improved magnetization as well as SAR values, when compared with CFO ferrofluids. This difference is attributed to interfacial interactions between the magnetic moments of paramagnetic Pd and superparamagnetic/ferrimagnetic CFO. SAR values measured from CFO-Pd heterodimer suspensions were found to be 47-52 W gFerrite-1, which is up to a factor of two higher than the SAR values of commercially available ferrofluids, demonstrating their potential as efficient heat mediators. Our results provide insight into the utilization of CFO-Pd heterodimer suspensions as potential nanoplatforms for diagnostic and therapeutic biomedical applications, e.g., in cancer hyperthermia, cryopreserved tissue warming, thermoablative therapy, drug delivery and bioimaging.

Loss mechanisms in fluid heating of cobalt ferrite (CFO) nanoparticles and CFO–Pd heterodimer colloidal suspensions are investigated as a function of particle size, fluid concentration and magnetic field amplitude. The specific absorption rate (SAR) is found to vary with increasing particle size due to a change in dominant heating mechanism from susceptibility to hysteresis and frictional loss. The maximum SAR is obtained for particle diameters of 11–15 nm as a result of synergistic contributions of susceptibility loss, including Néel and Brownian relaxation and especially hysteresis loss, thereby validating the applicability of linear response theory to superparamagnetic CFO nanoparticles. Our results show that the ferrofluid concentration and magnetic field amplitude alter interparticle interactions and associated heating efficiency. The SAR of the CFO nanoparticles could be maximized by adjusting the synthesis parameters. Despite the paramagnetic properties of individual palladium nanoparticles, CFO–Pd heterodimer suspensions were observed to have surprisingly improved magnetization as well as SAR values, when compared with CFO ferrofluids. This difference is attributed to interfacial interactions between the magnetic moments of paramagnetic Pd and superparamagnetic/ferrimagnetic CFO. SAR values measured from CFO–Pd heterodimer suspensions were found to be 47–52 W gFerrite−1, which is up to a factor of two higher than the SAR values of commercially available ferrofluids, demonstrating their potential as efficient heat mediators. Our results provide insight into the utilization of CFO–Pd heterodimer suspensions as potential nanoplatforms for diagnostic and therapeutic biomedical applications, e.g., in cancer hyperthermia, cryopreserved tissue warming, thermoablative therapy, drug delivery and bioimaging.

Loss mechanisms in fluid heating of cobalt ferrite (CFO) nanoparticles and CFO-Pd heterodimer colloidal suspensions are investigated as a function of particle size, fluid concentration and magnetic field amplitude. The specific absorption rate (SAR) is found to vary with increasing particle size due to a change in dominant heating mechanism from susceptibility to hysteresis and frictional loss. The maximum SAR is obtained for particle diameters of 11-15 nm as a result of synergistic contributions of susceptibility loss, including Neel and Brownian relaxation and especially hysteresis loss, thereby validating the applicability of linear response theory to superparamagnetic CFO nanoparticles. Our results show that the ferrofluid concentration and magnetic field amplitude alter interparticle interactions and associated heating efficiency. The SAR of the CFO nanoparticles could be maximized by adjusting the synthesis parameters. Despite the paramagnetic properties of individual palladium nanoparticles, CFO-Pd heterodimer suspensions were observed to have surprisingly improved magnetization as well as SAR values, when compared with CFO ferrofluids. This difference is attributed to interfacial interactions between the magnetic moments of paramagnetic Pd and superparamagnetic/ferrimagnetic CFO. SAR values measured from CFO-Pd heterodimer suspensions were found to be 47-52 W gFerrite-1, which is up to a factor of two higher than the SAR values of commercially available ferrofluids, demonstrating their potential as efficient heat mediators. Our results provide insight into the utilization of CFO-Pd heterodimer suspensions as potential nanoplatforms for diagnostic and therapeutic biomedical applications, e.g., in cancer hyperthermia, cryopreserved tissue warming, thermoablative therapy, drug delivery and bioimaging.

Time-dependent and constituent-specific spectral changes in soft near edge X-ray spectroscopy (XAS) of an [Fe/MgO]$_8$ metal/insulator heterostructure upon laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining X-ray absorption spectroscopy with ultrafast electron diffraction measurements and ab initio calculations demonstrate that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near edge XAS to phonons is demonstrated., 10 pages, 6 figures

The structural properties of cobalt ferrite nanoparticles have significant effects on their magnetic behavior. In the current study, we thoroughly scrutinized and optimized the structural characteristics of CoFe2O4 nanoparticles and the correlation with their magnetic properties as a function of the synthesis parameters using Rietveld structure refinement. Nanoparticles were synthesized using co-precipitation method based on design of experiments and then characterized using X-ray diffraction, vibrating sample magnetometry, transmission electron microscopy, and energy dispersive X-ray spectroscopy analyses. Based on response surface methodology studies, we identified factors that had an effect on the structural and magnetic features. In order to reach maximum magnetization, the cations distribution was optimized, and the pH amount and reaction temperature were identified as the most influential factors. We observed that the initial cation ratio of Co2+/Fe3+ sharply affected the cations distribution, which was subsequently involved in the different structural characteristics and magnetization of nanoparticles. This can be attributed to the hybrid structure formation and magnetic exchange interactions of cations. Finally, the maximum magnetization was achieved at the optimum cations distribution of (Co0.32 Fe0.68)(Co0.70Fe1.30)O4, where the difference between distributed cobalt cations in tetrahedral and octahedral sites was minimum.

The performance of magnetic nanoparticles in different applications is severely depended on their size characteristics, so the study of effective parameters on these properties can play significant roles in qualifications of nanoparticles. In present work, some important factors on size features of CoFe 2 O 4 superparamagnetic nanoparticles include the mixing order of synthesis components, the utilized reduction agents, stabilization process, and chelating mechanisms were investigated. Moreover, in order to optimize several influential factors such as the temperature, pH, and cation ratio of reaction, the experimental design was done by using central composite design method of response surface methodology. The simultaneous effects on the particles size and their size distribution were investigated by different methods i.e. dynamic light scattering, X-ray diffraction, Fourier transform inferred spectroscopy, vibration sample magnetometer, and transmission electron microscopy. Results demonstrated the mixing order of reduction agent to salt solution and also the employing of NH 4 OH as a reduction agent could cause to significant decreasing of particles size and size distribution. Furthermore, the nitric acid could stabilize and chelate nanoparticles more appropriate than citric acid. Based on the optimization results, the quadratic polynomial models were fitted on the responses which could predict their amounts, while temperature, pH, and their interactions had higher effectiveness. In addition, the optimum amounts of particle size (14 nm) and size distribution (4.61 nm) were achieved while temperature, pH, and cation ratio amounts are equal to 89.82 °C, 11, and 0.52, respectively.

We demonstrate the emergence and control of magnetic phases between magnetite (Fe

We demonstrate the emergence and control of magnetic phases between magnetite (Fe3O4), a ferrimagnetic halfmetal, and SrTiO3, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition (TV) was detected to persist from bulk-like down to ultrathin Fe3O4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe2+ ions with decreasing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe2O3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γ-Fe2O3 that finally leads to stoichiometric and ferrimagnetic Fe3O4/Sr...

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L$_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L$_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics., 12 pages, 10 figures

V2O3 doped with 1.1% Cr is investigated at its isostructural correlation-driven metal-insulator transition near room temperature in its paramagnetic state with X-ray magnetic circular dichroism (XMCD) spectroscopy in external magnetic fields. A relative XMCD amplitude of about 2 permille is observed at the L-2,L-3 absorption edges of vanadium as expected for magnetic moment per mass values of the order of 1 J T-1 kg(-1) from magnetometry and the literature. Across the metal-insulator transition, the vanadium XMCD spectral shape significantly changes. According to atomic multiplet simulations, these changes are due to a changing orbital occupation indicating a changing phase composition. According to estimates used in this study, the dipole moment of the spin density distribution 7⟨Tz⟩ in the bulk increases such that the effective vanadium spin moment increases by a few percent with temperature in the two-phase region. Thereby, it partially compensates for the decrease in the relative XMCD amplitude due to a decreasing alignment of the paramagnetic moments. After a few minor temperature cycles, the sample is in a two-phase state in which the XMCD and X-ray linear dichroism spectra hardly depend on the temperature, and the specific electrical resistance is intermediate, showing only a weak sign of the metal-insulator transition. Early access: The article have been peer-reviewed and accepted for publication. The article content has been finalized, but it has not been assigned to an issue yet.

Polyoxopalladates hosting 3d, 4d or 4f central ions form a new class of molecular materials with well-defined, tunable morphology and ions coordination to be considered as model systems for basic research. We investigated the isotropic X-ray absorption near-edge structure (XANES), X-ray magnetic circular dichroism (XMCD), and X-ray magnetic linear dichroism (XMLD) to monitor the influence of symmetry changes on electronic and magnetic properties. After a brief introductory review about recent results on 3d and 4d transition metal ions, we (i) show that a change of the coordination symmetry from cubic eightfold (tetrahedral) to sixfold (octahedral) reduces the magnetic moment of Co2+ significantly and (ii) investigate whether axial distortions can modify the 4f states of trivalent lanthanide ions. While for Gd3+ there is no effect visible within experimental uncertainties, changes were observed for Dy3+ and Ho3+. The results are compared to atomic multiplet calculations including the intra-atomic spin-dipole term 〈 T z 〉 . Furthermore, several experimental pitfalls are discussed that may lead to misinterpretation of X-ray absorption data but can be excluded in the present study.

Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ thin films along the order-disorder $(B2\ensuremath{\rightarrow}A2)$ phase transition initiated by 20-keV ${\mathrm{Ne}}^{+}$ ion irradiation with fluences of $(0.75--6)\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}\text{ions}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and $\ensuremath{\sim}1%$ of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at $\ensuremath{\sim}60$ eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ model structures for the ordered $(B2)$ and the disordered $(A2)$ phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied.

Thin films of Fe-rich transition metal aluminide Fe 60 Al 40 (at. %) are very promising for implementation in modern technology due to a possibility of having tailored magnetic properties. Being weakly ferromagnetic at room temperature in the chemically ordered B2 phase, Fe 60 Al 40 films can be reversibly transformed into the ferromagnetic chemically disordered A2 phase with much higher magnetization in a well controlled way by means of ion irradiation. Prior studies point on important influence of created chemical disorder on static and dynamic magnetic properties of Fe 60 Al 40 films while achievements in magnetic patterning and studies of magnetization reversal have shown their perspective for further use in spin-transport devices [1–3]. Detailed studies performed on a local scale can clarify hidden mechanisms of disorder induced ferromagnetism phenomena in Fe 60 Al 40 films via understanding the influence of the local surrounding and features of Fe-Al hybridization on the magnetic properties. In our work, element-specific X-ray absorption spectroscopy (EXAFS, XANES, and XMCD) in hard-and soft energy ranges together with synchrotron-based XRD (SR-XRD) have been applied to probe the local rearrangements and related magnetic and electronic properties of Fe and Al atoms in bare Fe 60 Al 40 thin films of 40 nm thickness through the order-disorder $( \mathrm {B}2 \rightarrow {A}2$) phase transition initiated by 20keV Ne+ ion irradiation with low fluences ~1014 ions cm-2). Extended X-ray absorption fine structure (EXAFS) spectra recorded at the Fe and Al K edges at room temperature (RT) and low temperature of 5K (LT) and SR-XRD have shown significant changes in the local environment of Fe and Al absorbers before and after the irradiation. In the course of the transition a number of Fe-Fe nearest-neighbors grew from 3.47(7) up to 5.0(1) for the ordered B2 and the fully disordered A2 phases, correspondingly, and ~1% of the unit cell volume expansion was found. Distinct changes of Fe and Al coordination due to disordering resulted in increased Fe 3d spin and 4p orbital polarizations and characteristic changes in electronic structure of Al atoms as was demonstrated by RT XMCD at the Fe L 2,3 and Fe K edges as well as LT XANES at the Al K edge, respectively [4, 5]. A unique possibility to probe the magnetism of 3d states by hard X-rays has been realized by recording the XMCD signal at ~60 eV above the Fe K edge where so-called magnetic multi-electronic excitations (MEE, secondary processes) are present [6, 7]. The analysis of MEE peak amplitude and its integrated intensity has revealed similar tendencies in their changes with fluence as for 3d effective spin and 3d orbital magnetic moments obtained from XMCD spectra at the Fe L 2,3 edges, respectively. Moreover, this analysis points towards increased localization of Fe 3d states in A2 phases created by fluencies of (0.75-6) × 1014 ions cm-2. Element-specific hysteresis loops (ESHL) recorded by XMCD either at the Fe K or L 3 absorption edges have confirmed the preferential in-plane magnetic anisotropy of irradiated films; the variations in coercive fields depending on temperature and irradiation fluence have been checked by ESHL at the Fe L 3 edge. The specific shoulder related to hybridization effects between Fe and Al has been uncovered with the help of in-situ hydrogen plasma treatment. It has been also illustrated that the reduction of the top oxide layer leads to increase or decrease of Fe 3d spin magnetic moments in the surface region depending on the time of treatment. This suggests that the use of a capping layer could either protect the films from continuous uncontrolled oxidation or further increase the macroscopic magnetization of films that is much more favorable for technological applications. A theory support was provided by self-consistent DFT calculations using the VASP program package applied to relaxed model systems. A clear variation of local configurations in B2 and A2 phases followed by sizable changes in Fe and Al magnetic moments has been found. The work was partially funded by Helmholtz Association (Young Investigator's Group “Borderline Magnetism”, VH-NG-1031). The authors thank the ESRF (ID12 beamline) and HZB-BESSY II (UE46_PGM-1 and KMC-2 beamlines) for provision of synchrotron radiation facilities and allocation of synchrotron radiation; Swedish National Infrastructure for Computing (SNIC) is acknowledged for providing high performance supercomputing time.

In this article we report on the synthesis and multiferroic properties of cobalt ferrite (CoFe2O4)–barium titanate (BaTiO3) biphasic composites. The initial composite nanopowder was synthesized by a combination of co-precipitation and organosol methods. A ceramic sample with (3–0) connectivity, i.e. BaTiO3 grains in a CoFe2O4 matrix was obtained by a combination of spark plasma sintering and annealing. In order to understand the correlations between morphology, electric properties, and magnetization, we present a detailed study at different preparation steps and compare it to the properties of a conventionally sintered sample with the traditional (0–3) connectivity, i.e. CoFe2O4 grains in a BaTiO3 matrix. We observe that the (3–0) sample shows improved magnetic properties in comparison to the conventionally sintered composite of the same composition. In spite of relatively large leakage current for the (3–0) sample compared to the traditional (0–3) one, it exhibits a converse magnetoelectric effect that follows the Hdc dependence of the piezomagnetic coefficient. The magnetic field-dependence of electric polarization at the surface was investigated utilizing X-ray absorption spectroscopy and its associated linear and circular dichroisms.

Magnetoelectric coupling is the material based coupling between electric and magnetic fields without recurrence to electrodynamics. It can arise in intrinsic multiferroics as well as in composites. Intrinsic multiferroics rely on atomistic coupling mechanisms, or coupled crystallographic order parameters, and even more complex mechanisms. They typically require operating temperatures much below T = 0°C in order to exhibit their coupling effects. Room temperature applications are thus excluded. Consequently, composites have been designed to circumvent this limitation. They rely on field coupling between magnetostrictive and piezoelectric materials or in more advanced scenarios on quantum coupling in between both phases. This overview will describe experimental techniques and their particular limitations in accessing these coupling phenomena at different scales. Strain coupling is the dominant coupling mechanism at the macroscale as well as down to the micrometer. At the nanoscale more subtle effects can arise and some care has to be taken when investigating local coupling at interfaces using scanning probe techniques, e. g. due to semiconductor effects, field screening, or gradient and surface effects. At the smallest length scale atomic or molecular coupling can be tested using X-ray dichroism or probe atoms like 57Fe in Mossbauer spectroscopy. We display a selection of measuring techniques at the different scales and outline possible pitfalls for experimentalists as well as theoreticians when using material parameters extracted from such experimental work. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We create an interface of graphene with a metallic and magnetic support that leaves its electronic structure largely intact. This is achieved by exposing epitaxial graphene on ferromagnetic thin films of Co and Ni to vapor of the rare earth metal Eu at elevated temperatures, resulting in the intercalation of an Eu monolayer in between graphene and its substrate. The system is atomically well defined, with the Eu monolayer forming a (√3×√3)R30∘ superstructure with respect to the graphene lattice. Thereby, we avoid the strong hybridization with the (Ni,Co) substrate 3d states that otherwise drastically modify the electronic structure of graphene. This picture is suggested by our x-ray absorption spectroscopy measurements which show that after Eu intercalation the empty 2p states of C atoms resemble more the ones measured for graphite in contrast to graphene directly bound to 3d ferromagnetic substrates. We use x-ray magnetic circular dichroism at the Co and Ni L2,3 and Eu M4,5 as an element-specific probe to investigate magnetism in these systems. An antiferromagnetic coupling between Eu and Co/Ni moments is found, which is so strong that a magnetic moment of the Eu layer can be detected at room temperature. Density functional theory calculations confirm the antiferromagnetic coupling and provide an atomic insight into the magnetic coupling mechanism.

Novel methods of switching magnetism with electric fields and vice versa, and aiming at magnetoelectric (ME) data processing are reported. First, the patented MERAM@ uses the electric field control of exchange bias via an epitaxial Cr2O3 layer and exchange coupling to a Pt/Co/Pt trilayer. It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic switching of the electric polarization by a transverse magnetic field in a 3-1 composite of a vertically poled BaTiO3 thick film embedding CoFe2O4 nanopillars produces a regular surface polarization pattern with rectangular symmetry. Its possible use for data processing is discussed.

The influence of local chemical composition, structure and surface modifications on the magnetic properties of FexPt1 − x nanoparticles around the equiatomic composition have been studied by means of X-ray absorption spectroscopy (XAS). In particular, the extended X-ray absorption fine structure (EXAFS) and the X-ray magnetic circular dichroism (XMCD) have been employed to investigate the local structure around Fe and Pt atoms and to determine spin and orbital magnetic moments of the two atomic species, respectively. The detailed atomistic picture deduced from these experimental results was compared to recent calculations within the density functional theory that gave the possibility to state design rules for the preparation of nanoparticles with desired magnetic properties. In this paper, a short survey about these findings is given focusing on the experimental work. Wavelet transform (WT) of extended X-ray absorption fine structure at the Pt L3 absorption edge of FePt nanoparticles in the as-prepared state.

X-ray absorption spectroscopy facilitated by state-of-the-art synchrotron radiation technology is presented as a powerful tool to study nanoscale systems, in particular revealing their static element-specific magnetic and electronic properties on a microscopic level. A survey is given on the properties of nanoparticles, nanocomposites and thin films covering a broad range of possible applications. It ranges from the ageing effects of iron oxide nanoparticles in dispersion for biomedical applications to the characterisation on a microscopic level of nanoscale systems for data storage devices. In this respect, new concepts for electrically addressable magnetic data storage devices are highlighted by characterising the coupling in a BaTiO(3)/CoFe(2)O(4) nanocomposite as prototypical model system. But classical magnetically addressable devices are also discussed on the basis of tailoring the magnetic properties of self-assembled ensembles of FePt nanoparticles for data storage and the high-moment material Fe/Cr/Gd for write heads. For the latter cases, the importance is emphasised of combining experimental approaches in x-ray absorption spectroscopy with density functional theory to gain a more fundamental understanding.

The intra-atomic magnetic dipole moment - frequently called < T-z > term - plays an important role in the determination of spin magnetic moments by x-ray absorption spectroscopy for systems with nonspherical spin density distributions. In this work, we present the dipole moment as a sensitive monitor to changes in the electronic structure in the vicinity of a phase transiton. In particular, we studied the dipole moment at the Fe2+ and Fe3+ sites of magnetite as an indicator for the Verwey transition by a combination of x-ray magnetic circular dichroism and density functional theory. Our experimental results prove that there exists a local change in the electronic structure at temperatures above the Verwey transition correlated to the known spin reorientation. Furthermore, it is shown that measurement of the dipole moment is a powerful tool to observe this transition in small magnetite nanoparticles for which it is usually screened by blocking effects in classical magnetometry.

The coupling of submonolayer coverages of Fe-phthalocyanine molecules on bare and oxygen-covered ferromagnetic Co(001) films was studied by x-ray-absorption spectroscopy, especially the x-ray magnetic circular dichroism, in combination with density functional theory. We observe that the magnetic moments of the paramagnetic molecules are aligned even at room temperature, resulting from a magnetic coupling to the substrate. While the magnetization of the Fe ions directly adsorbed on the Co surface is parallel to the magnetization of the Co film, the introduction of an oxygen interlayer leads to an antiparallel alignment. As confirmed by theory, the coupling strength is larger for the system FePc/Co than for FePc/O/Co, causing a stronger temperature dependence of the Fe magnetization for the latter system. Furthermore, the calculations reveal that the coupling mechanism changes due to the O layer from mostly direct exchange to Co of the bare surface to a 180${}^{\ensuremath{\circ}}$ antiferromagnetic superexchange via the O atoms. Finally, by comparing the experimental x-ray-absorption spectra at the N $K$ edge with the corresponding calculations, the contribution of the individual orbitals has been determined and the two inequivalent N atoms of the molecules could be distinguished.

The structural and magnetic properties of Fe octaethylporphyrin (OEP) molecules on Cu(001) have been investigated by means of density functional theory (DFT) methods and X-ray absorption spectroscopy. The molecules have been adsorbed on the bare metal surface and on an oxygen-covered surface, which shows a $\sqrt{2}\times2\sqrt{2}R45^{\circ}$ reconstruction. In order to allow for a direct comparison between magnetic moments obtained from sum-rule analysis and DFT we calculate the dipolar term $7< T_z>$, which is also important in view of the magnetic anisotropy of the molecule. The measured X-ray magnetic circular dichroism shows a strong dependence on the photon incidence angle, which we could relate to a huge value of $7< T_z>$, e.g. on Cu(001) $7< T_z>$ amounts to -2.07\,\mbo{} for normal incidence leading to a reduction of the effective spin moment $m_s + 7< T_z>$. Calculations have also been performed to study the influence of possible ligands such as Cl and O atoms on the magnetic properties of the molecule and the interaction between molecule and surface, because the experimental spectra display a clear dependence on the ligand, which is used to stabilize the molecule in the gas phase. Both types of ligands weaken the hybridization between surface and porphyrin molecule and change the magnetic spin state of the molecule, but the changes in the X-ray absorption are clearly related to residual Cl ligands., 17 figures, full article

Abrupt room temperature switching (T(c) = 295 K with a 5 K hysteresis) was achieved in a neutral Fe(II) complex based on a 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine ligand. Structural characterization and spin crossover study (via SQUID magnetometry, photoexcitation and X-ray absorption spectroscopy) in the solid state are described.

The effectiveness of amine-borane as reducing agent for the synthesis of iron nanoparticles has been investigated. Large (2-4 nm) Fe nanoparticles were obtained from [Fe{N(SiMe3)2}2]. Inclusion of boron in the nanoparticles is clearly evidenced by extended X-ray absorption fine structure spectroscopy and Mossbauer spectrometry. Furthermore, the reactivity of amine-borane and amino-borane complexes in the presence of pure Fe nanoparticles has been investigated. Dihydrogen evolution was observed in both cases, which suggests the potential of Fe nanoparticles to promote the release of dihydrogen from amine-borane and amino-borane moieties.

Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail.