XMCD of a single layer of single molecule magnets

A detailed report on the X-ray Magnetic Circular Dichroism (XMCD) investigation of monolayers of Mn12-based single molecule magnets (SMMs) deposited on gold Au(111) is presented. A semi-quantitative analysis of data is provided in order to extract chemical and magnetic information on Mn ions, by comparison with XMCD on bulk samples. This work points that XMCD is a key-tool for the characterization of SMMs-based nanostructured systems. XMCD surface sensitivity and element-specificity will play a fundamental role in the identification of good candidates for SMMs based devices.     

Bulk and Surface Effects in X-Ray Magnetic Circular Dichroism of Iron Clusters

General trends in x-ray magnetic circular dichroism (XMCD) spectra of iron clusters of different sizes are theoretically investigated using the fully-relativistic spin-polarized multiple-scattering formalism. Purely geometrical effects of clustering and cluster-size effects are explored separately from effects of various scattering potentials and/or treatment of the surface barrier. We found that a high portion of non-bulk atoms makes the XMCD spectra of iron clusters to appear significantly different from bulk spectrum, even for quite large clusters. If the vacuum surrounding the cluster is taken into account, XMCD spectra of iron clusters change essentially (in comparison with clusters cut out of the bulk or with clusters embedded in a sea of free electrons).     

Strong anisotropy of projected 3d moments in epitaxial CrO2 films

Soft x-ray magnetic circular dichroism (XMCD) spectra have been investigated for different crystallographic projections of CrO2. Strong anisotropic orbital Cr 3d contributions and a change of sign of the XMCD signal is observed and attributed to t(2g) majority states near the Fermi level. Additionally, moment analysis exhibits anisotropic behavior in the projected spin contributions of CrO2 assigned to a strong magnetic dipole term T(z), consistent with an intrinsic magnetic easy axis behavior along the CrO2 [001] axis. A reduced projected isotropic Cr 3d spin moment has been interpreted in terms of hybridization with oxygen.     

Spin-flop ordering from frustrated ferro- and antiferromagnetic interactions: a combined theoretical and experimental study of a Mn/Fe(100) monolayer

The occurrence of a noncollinear magnetic structure at a Mn monolayer grown epitaxially on Fe(100) is predicted theoretically, using spinor density-functional theory, and observed experimentally, using x-ray magnetic circular dichroism (XMCD) and linear dichroism (XMLD) spectroscopies. The combined use of XMCD and XMLD at the Mn-absorption edge allows us to assess the existence of ferromagnetic and antiferromagnetic order at the interface, and also to determine the moment orientations with element specificity. The experimental results thus obtained are in excellent agreement with the magnetic structure determined theoretically.     

Direct observation of ferromagnetic spin polarization in gold nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.     

Mn distribution and spin polarization in Ga(1-x)Mn x As

Using the density functional full-potential linearized augmented plane wave approach, the x-ray absorption and magnetic circular dichroism (XMCD) spectra of Ga(1-x)Mn x As are calculated. Significantly, XMCD of Mn is highly sensitive to the change in environment, and thus can be utilized to characterize impurity distribution. The nature of Mn-induced spin polarization on Ga and As sites, vital for the carrier mediated magnetic ordering, is discussed in light of computational and experimental results.     

Epitaxy and magnetic properties of surfactant-mediated growth of bcc cobalt

High resolution core level photoemission spectroscopy, photoelectron diffraction, and x-ray magnetic circular dicroism (XMCD) have been used to characterize the structural and magnetic properties of bcc-cobalt films grown on GaAs(110) substrates by using Sb as a surfactant. We have unambiguously disentangled the surfactant role played by the Sb which improves the crystallinity and reduces the lattice distortion of the metallic films as well as changes the interdiffusion process at the interface compared to the Co/GaAs(110) system. As a consequence of these combined effects, an improvement on the magnetic response of the grown Co thin films has been observed by XMCD measurements.     

X-ray circular magnetic dichroism in the presence of strong spin fluctuations

The use of X-ray magnetic circular dichroism (XMCD) spectra for determininghe t magnitude of atomic magnetic moments in compounds of rare-earth and transition elements is discussed. The standard sum rule approach often yields a magnitude of moments that is often smaller than values obtained from magnetic measurements. We attribute this to strong spin fluctuations in the surface layers in which XMCD signals form. A way of determining the values of local magnetic moments in the presence of strong fluctuations is proposed and tested.     

Nature of magnetic coupling between Mn ions in As-grown Ga1-xMnxAs studied by X-ray magnetic circular dichroism

The magnetic properties of as-grown Ga1-xMnxAs have been investigated by the systematic measurements of temperature and magnetic field dependent soft x-ray magnetic circular dichroism (XMCD). The intrinsic XMCD intensity at high temperatures obeys the Curie-Weiss law, but a residual spin magnetic moment appears already around 100 K, significantly above the Curie temperature (T_{C}), suggesting that short-range ferromagnetic correlations are developed above T_{C}. The present results also suggest that the antiferromagnetic interaction between the substitutional and interstitial Mn (Mn_{int}) ions exists and that the amount of the Mn_{int} affects T_{C}.     

X-ray absorption and magnetic circular dichroism characterization of Mo1–xFexO2 (x = 0–0.05) thin films grown by pulsed laser ablation

The electronic and magnetic properties of well characterized Mo_1???xFe_xO₂ (x = 0–0.5) thin films that show ferromagnetism at room temperature (RT) have been investigated by the means of near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments at the O K-, Fe L-, and Mo M-edges. The NEXAFS spectra at O K- and Mo M_3,2 -edges show a strong hybridization of O 2p-4d Mo orbitals, and Mo ions change their symmetry with the substitution of Fe ions into MoO₂ matrix. The Fe 2p NEXAFS/XMCD spectra exhibit multiple absorption peaks and an appreciable XMCD signal that persists even at RT. These results demonstrate that Fe is in a mixed valence state of Fe^2?+?–Fe^3?+?, substituting at the Mo site and that the Fe^2?+?/3?+? ions are ferromagnetically polarized.     

A new ultra‐high‐vacuum variable temperature and high‐magnetic‐field X‐ray magnetic circular dichroism facility at LNLS

X‐ray magnetic circular dichroism (XMCD) is one of the most powerful tools for investigating the magnetic properties of different types of materials that display ferromagnetic behavior. Compared with other magnetic‐sensitive techniques, XMCD has the advantage of being element specific and is capable of separating the spin and magnetic moment contributions associated with each element in the sample. In samples involving, for example, buried atoms, clusters on surfaces or at interfaces, ultrathin films, nanoparticles and nanostructures, three experimental conditions must be present to perform state‐of‐the‐art XMCD measurements: high magnetic fields, low temperatures and an ultra‐high‐vacuum environment. This paper describes a new apparatus that can be easily installed at different X‐ray and UV beamlines at the Brazilian Synchrotron Light Laboratory (LNLS). The apparatus combines the three characteristics described above and different methods to measure the absorption signal. It also permits in situ sample preparation and transfer to another chamber for measurement by conventional surface science techniques such as low‐energy electron diffraction (LEED), reflection high‐energy electron diffraction (RHEED), X‐ray photoelectron spectroscopy (XPS) and X‐ray photoelectron diffraction (XPD). Examples are given of XMCD measurements performed with this set‐up on different materials.     

X-ray magnetic circular dichroism measurements in Ni up to 200 GPa: resistant ferromagnetism

The structural stability of fcc Ni over a very large pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by simple compression. K-edge x-ray magnetic circular dichroism (XMCD) shows that fcc Ni is ferromagnetic up to 200 GPa, contradicting recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Density functional theory calculations point out that the pressure evolution of the K-edge XMCD closely follows that of the p projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa.     

XMCD study of the local magnetic and structural properties of microcrystalline NdFeB-based alloys

X-ray Magnetic Circular Dichroism (XMCD) technique was used to investigate local magnetic properties of microcrystalline Nd_10.4Zr_4.0Fe_79.2B_6.4 samples, oriented along either easy or hard magnetization direction. The Nd L _2,3 and Fe K edge XMCD spectra were measured at room temperature under a magnetic field of T. A very strong dependence of XMCD spectra on the sample orientation has been observed at the NdL _2,3-edges, whereas the Fe K-edge XMCD spectra are found to be practically isotropic. This result indicates that magnetic anisotropy of NdFeB-based alloys originates from the Nd sublattice. In addition, element selective magnetization curves have been recorded by measuring the intensity of XMCD signals as a function of an applied magnetic field up to T. To find a correlation between local and macroscopic magnetic properties of studied samples we compared these data with magnetization curves, measured by vibrating sample magnetometer up to T. Results are important for understanding the origin of high-coercivity state in NdFeB-based intermetallic compounds.     

ODE: a new beam line for high-pressure XAS and XMCD studies at SOLEIL

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) under pressure are probes of local order and microscopic magnetic properties. XMCD is a selective probe that has access to a large variety of elements. The dispersive extended X-ray absorption fine structure (EXAFS) station at SOLEIL (ODE beam line) provides the possibility to perform numerous pressure XAS and XMCD experiments with an excellent statistic. The main advantages of dispersive XAFS are the focusing optics, the short acquisition time (few μs) and great stability during the measurements due to the absence of any mechanical movement. These advantages allow the study of small samples, 70 μm at SOLEIL, which is mandatory in the case of high-pressure studies. We present the new ODE beam line at SOLEIL and its first high-pressure XMCD results.     

Negative tunneling magnetoresistance by canted magnetization in MgO/NiO tunnel barriers

The influence of the insertion of an ultrathin NiO layer between the MgO barrier and the ferromagnetic electrodes in magnetic tunnel junctions has been investigated from measurements of the tunneling magnetoresistance and via x-ray magnetic circular dichroism (XMCD). The magnetoresistance shows a high asymmetry with respect to bias voltage, giving rise to a negative value of up to -16% at 2.8 K. We attribute this effect to the formation of noncollinear spin structures at the interface of the NiO layer as inferred from XMCD measurements. The magnetic moments of the interface Ni atoms tilt from their easy axis due to exchange coupling with the neighboring ferromagnetic electrode, and the tilting angle decreases with increasing NiO thickness. The experimental observations are further supported by noncollinear spin density functional calculations.     

Spontaneous and Induced Magnetization Reversal in Thin GaMnSb Films

The time, temperature, and magnetic field dependences of the magnetic moment of thin GaMnSb films containing MnSb clusters are measured using a SQUID (superconducting quantum interference device) magnetometer. It is found that magnetic field-induced magnetization reversal and thermally activated spontaneous magnetization reversal in thin GaMnSb films are interrelated as follows: the maximum of the magnetic-field dependence of the magnetic viscosity coincides with the coercive force for the samples.     

Thickness-dependent magnetic domain structures of Co ultra-thin film investigated by scanning transmission X-ray microscopy

Thickness-dependent magnetic domain structure of ultrathin Co wedge films (0.3 nm–1.0 nm) sandwiched by Pt layers was investigated by scanning transmission x-ray microscopy (STXM) employing X-ray magnetic circular dichroism (XMCD), utilizing elliptically polarized soft x-rays and electromagnetic fields, with a spatial resolution of 50 nm. The magnetic domain images measured at the Co L₃ edge showed the evolution of the magnetic domain structures from maze-like form to the bubble-like form as the perpendicular magnetic field was applied. The asymmetric domain expansion of a 500 nm-scale bubble domain was also measured when the in-plane and perpendicular external magnetic field were applied simultaneously.     

Spin-dependent Fabry-Pérot interference from a Cu thin film grown on fcc Co(001)

Spin-dependent electron reflection from a Cu thin film grown on Co/Cu(001) was investigated using spin-polarized low-energy electron microscopy (SPLEEM). Fabry-Pe rot type interference was observed and is explained using the phase accumulation model. SPLEEM images of the Cu overlayer reveal magnetic domains in the Co underlayer, with the domain contrast oscillating with electron energy and Cu film thickness. This behavior is attributed to the spin-dependent electron reflectivity at the Cu/Co interface which leads to spin-dependent Fabry-Pe rot electron interference in the Cu film.     

Correlation between ferromagnetic state and thermally stable layer of Fe on the W(001) surface

The variations of electronic and magnetic properties of ultrathin Fe overlayers on a W(001) surface as a function of Fe film thickness (1.0–4.0 ML) has been investigated using X-ray magnetic circular dichroism (XMCD) in conjunction with ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). We found that the ferromagnetic property of Fe film started to build up over 2.0 ML, as we confirmed the spin and angular moment contribution to the magnetic moment using XMCD experiments. We also confirmed that a thermally stable layer is over 2.0 ML of Fe film as we change the annealing temperature taken after Fe deposition at 300 K and at 400 K using UPS. We will systematically demonstrate that the occurrence of ferromagnetic property of Fe film on a W(001) surface is closely correlated to a thermally stable layer of Fe film on a W(001) surface.     

第一性原理研究闪锌矿结构MnSb和MnBi半金属性和磁性的稳定性

采用基于第一性原理的全势能线性缀加平面波方法对闪锌矿结构MnSb和MnBi的电子结构进行自旋极化计算。闪锌矿结构MnSb和MnBi处于晶格平衡时都是半金属性的,并且它们自旋向下电子能带带隙分别是1.32eV 和1.27eV。闪锌矿结构MnSb和MnBi的自旋总磁矩都为4.00μB/formula,总磁矩主要来源于Mn的原子磁矩,Sb和Bi的原子磁矩对总磁矩的贡献很小而且为负值,它们具有明显的铁磁性特征. 使晶体晶格在±10%的范围内发生各向同性形变,对闪锌矿结构MnSb和MnBi的电子结构进行计算. 计算结果表明,当晶格各向同性形变分别为-1 % ~ 10 %和-4 % ~10 %时,闪锌矿结构MnSb和MnBi仍然保持半金属铁磁性,并且总磁矩都稳定于4.00μB/formula.