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.     

Effect of Nitrogenation and Hydrogenation on the Magnetic Properties and Structure of the Sm<Subscript>2</Subscript>Fe<Subscript>17</Subscript> Alloy: Analysis of XMCD Data

Changes in the local magnetic and structural properties of Sm₂Fe₁₇ alloys at nitrogenation and hydrogenation of samples have been studied by the X-ray magnetic circular dichroism (XMCD) technique at the Fe K absorption edge and Sm L₃ absorption edge using synchrotron radiation. The results have been discussed in comparison with X-ray diffraction data and macroscopic vibration magnetometry measurements. The observed changes in XMCD spectra indicate a noticeable effect of nitrogenation on the local magnetic properties of sublattices of both iron and samarium, whereas hydrogenation of samples makes a small effect. The mentioned effects have been analyzed and discussed in terms of the effect of nitrogen (N) and hydrogen (H) interstitial atoms on Sm 5d and Fe 4p electronic states. The effect of nitrogenation is larger than the effect of hydrogenation because the volume expansion of the crystal lattice of initial Sm₂Fe₁₇ in the case of nitrogenation is larger than that in the case of hydrogenation. The studied local magnetization curves for samarium and iron sublattices in magnetic fields up to 17 T also indicate a strong increase in the magnetocrystalline anisotropy at nitrogenation.     

Electronic and Magnetic States of Pr and Mn in the Pr<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> Films Studied by XANES and XMCD Spectroscopy

The spectral dependences of X-ray absorption near-edge spectroscopy (XANES) and X-ray magnetic circular dichroism (XMCD) and the field dependences of XMCD near the K edge of Mn and the L_2,3 edges of Pr in the Pr_0.8Sr_0.2MnO₃ and Pr_0.6Sr_0.4MnO₃ films at T = 90 K are studied. The spectral dependences point to a mixed valence state of Mn and Pr in the films. It is found that, as compared to XANES, XMCD is more sensitive to the valence state of Pr⁴⁺. The field dependences of XMCD point to ferromagnetic behavior of Mn ions and the Van Vleck paramagnetism of Pr ions, which makes a significant contribution to the total magnetization of the films. It is shown that as the Sr concentration increases, the XMCD intensity at the K edge of Mn increases, which indicates a growth of the total magnetic moment of the film due to an increase in the 4p–3d hybridization.     

X-ray detected magnetic resonance at the Fe K-edge in YIG: Forced precession of magnetically polarized orbital components

X-ray detected magnetic resonance (XDMR) has been measured for the first time on exciting the Fe K-edge in a high-quality yttrium iron garnet film epitaxially grown on a gadolinium gallium garnet substrate. This challenging experiment required resonant pumping of yttrium iron garnet at high microwave power, i.e., in the foldover regime. X-ray magnetic circular dichroism (XMCD) was used to probe the change in the longitudinal component of the magnetization M _Z induced by the precession of magnetic moments located at the iron sites. Since XMCD at the Fe K-edge refers mostly to the equilibrium contribution of magnetically polarized 4p orbital components, XDMR at the Fe K-edge should reflect the precessional dynamics of the latter orbital moments. From the measured precession angle, we show that there is no dynamical quenching of the polarized orbital components at the iron sites in yttrium iron garnet.     

Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

Field dependence of spin and orbital moments of Fe in L10 FePt magnetic thin films

The field dependence of spin and orbital magnetic moments of Fe in L1₀ FePt magnetic thin films was investigated using X-ray magnetic circular dichroism (XMCD). The spin and orbital moments were calculated using the sum rules; it was found that the spin and orbital moment of Fe in L1₀ FePt films are ∼2.5 and 0.2 μ_B, respectively. The relative XMCD asymmetry at Fe L₃ peak on the dependence of applied field suggested that the majority magnetic moment of L1₀ FePt films resulted from Fe.     

Investigation of a Spin Transition in a LaCoO<Subscript>3</Subscript> Single Crystal by the Method of X-Ray Magnetic Circular Dichroism at the Cobalt <Emphasis Type="Italic">K</Emphasis>- and <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript>-Edges

Spin transitions of cobalt ions in LaCoO₃ single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L_2,3-edges of Co³⁺ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L_2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.     

Analysis of the structure and magnetic properties of an interface in multilayered (Fe/Si) N nanostructures with the surface-sensitive XMCD method

The structural and magnetic properties of (Fe/Si) _N nanostructures obtained by successive deposition on the SiO₂/Si(100) surface at a temperature of the substrate of 300 K have been studied. The thicknesses of all Fe and Si layers have been determined by transmission electron microscopy measurements. The magnetic properties have been studied by the X-ray magnetic circular dichroism (XMCD) method near the Fe L _{3, 2} absorption edges. The orbital (m _l ) and spin (m _S ) contributions to the total magnetic moment of iron have been separated. The thicknesses of magnetic and nonmagnetic iron silicide on the Si/Fe and Fe/Si interfaces have been determined with the surface sensitivity of the XMCD method and the model of the interface between the nonmagnetic and weakened magnetic phases.     

Relativistic multiple scattering theory of K-edge X-ray magnetic circular dichroism

We discuss the angular dependent K-edge X-ray magnetic circular dichroism (XMCD) spectra based on the non-relativistic Green’s function expansion of the relativistic 4×4 Green’s function developed by Gesztesy et al. [Ann. Inst. Henri Poincaré 40 (1984) 159]. For the core functions we use the Dirac equation solutions whereas the relativistic effects for photoelectrons are automatically taken into account in the Gesztesy expansion. Analyses of the angular anisotropy provides us useful information on local symmetry violation around X-ray absorbing atoms even though we include relativistic effects. We explicitly show three different types of the symmetry which give rise to the suppression of the sin β-dependence in XMCD spectra. We also present explicit formulas of XMCD for randomly oriented and spatially fixed systems. Discussion on Debye–Waller factors is given in the present theoretical framework. Some illustrative calculations are also shown to understand the relativistic effects on the XMCD. The results are given for the Gd L₁-edge and Fe K-edge XMCD.     

Magnetic twisted state of Fe/Tb multilayers

The information of the Fe and Tb magnetic moments in [Fe(12 nm)/Tb(15 nm)]₂₅ multilayer was got separately with X-ray magnetic circular dichroism (XMCD) measurements at various temperature. The Tb magnetic moments become to twist with increasing the applied magnetic field, as follows. (1) When the applied field H is less than the coercive force H_C, Fe and Tb magnetic moments align anti-parallel, Fe moments being parallel to the magnetic field. This would be due to the ordinary exchange coupling between Fe and Tb magnetic moments. (2) H>H_C, a twisted magnetic structure appears when the sample temperature is low, particularly lower than 150 K. This magnetic phase could come from the competition among the exchange coupling, the Zeeman energy and the anisotropic energy.     

Local magnetic ordering of Fe impurities in Pd2MnSn

Mössbauer effect of Fe⁵⁷ embedded as very dilute substitutional impurities in Pd₂MnSn was studied. The impurities are seen to replace the three elements in the alloy. Although the Curie temperature of the alloy is 189K, well below the room temperature, the Mössbauer spectrum recorded at room temperature consisted of two distinct 6-finger magnetic hyperfine spectra and a single unsplit line. One of the 6-finger patterns which corresponds to an internal magnetic field ofH _int=?375 kOe is inferred to arise due to local magnetic coupling of the localized magnetic moments of Fe impurities at the Pd sites with those of the 4 Mn first nearest neighbours of the Fe impurities. The other 6-finger pattern which corresponds to an internal magnetic field ofH _int=?335 kOe is inferred to arise due to the local magnetic coupling of the localized magnetic moments of the Fe impurities at the Sn sites with those of the 6 Mn second nearest neighboours of the Fe impurities. The difference in the internal magnetic fields observed at the Pd and Sn sites in the alloy could be understood qualitatively, on the basis of RKKY theory, as arising due to the different conduction electron polarization contributions to the net internal magnetic field at the Fe impurity sites. The results of the measurements suggest that the localized magnetic moments of Fe⁵⁷ impurities at Pd and Sn sites are antiferromagnetically coupled with the moments of their neighbouring Mn atoms.     

Electronic and magnetic properties of CeAl2 nanoparticles

We presented the X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) studies of heavy fermion compound CeAl₂ bulk and 8 nm nanoparticles, performed at the Ce M_4,5- and L₃- absorption edges. XMCD and XAS revealed that Ce in bulk CeAl₂ exhibits localized 4f¹ character with magnetic ordering. The Ce in nanoparticles, on the other hand, shows a small amount delocalized 4f⁰ character with non-magnetic Kondo behavior. By applying general sum rules, an estimation of the orbital and spin contribution to those Ce 4f moments can be obtained. Our results also demonstrated that the magnetic behavior in CeAl₂ is very sensitive to the degree of localization of the 4f electrons.     

Element selective X-ray magnetic circular and linear dichroisms in ferrimagnetic yttrium iron garnet films

X-ray magnetic circular dichroism (XMCD) was used to probe the existence of induced magnetic moments in yttrium iron garnet (YIG) films in which yttrium is partly substituted with lanthanum, lutetium or bismuth. Spin polarization of the 4d states of yttrium and of the 5d states of lanthanum or lutetium was clearly demonstrated. Angular momentum resolved d-DOS of yttrium and lanthanun was shown to be split by the crystal field, the two resolved substructures having opposite magnetic polarization. The existence of a weak orbital moment involving the 6p states of bismuth was definitely established with the detection of a small XMCD signal at the Bi M₁-edge. Difference spectra also enhanced the visibility of subtle changes in the Fe K-edge XMCD spectra of YIG and {Y, Bi}IG films. Weak natural X-ray linear dichroism signatures were systematically observed with all iron garnet films and with a bulk YIG single crystal cut parallel to the (1 1 1) plane: this proved that, at room temperature, the crystal cannot satisfy all requirements of perfect cubic symmetry (space group: ), crystal distortions preserving at best trigonal symmetry ( or R3m). For the first time, a very weak X-ray magnetic linear dichroism (XMLD) was also measured in the iron K-edge pre-peak of YIG and revealed the presence of a tiny electric quadrupole moment in the ground-state charge distribution of iron atoms. Band-structure calculations carried out with fully relativistic LMTO-LSDA methods support our interpretation that ferrimagnetically coupled spins at the iron sites induce a spin polarization of the yttrium d-DOS and reproduce the observed crystal field splitting of the XMCD signal.     

Structural and magnetic characterization of soft-magnetic FeCo alloy nanoparticles

Soft-magnetic FeCo alloy nanoparticles with diameters less than 100 nm are prepared by ball milling. X-ray photoemission spectroscopy (XPS) and X-ray magnetic circular dichroism (XMCD) are used to characterize these particles. While the XPS spectrum from the as-prepared sample clearly shows Co photoemission peaks, no sign of Fe is observed in the same spectrum. However, Fe photoemission peaks appear after 1 h of Ar ion sputtering. A quantitative analysis of the XPS spectra shows an increase of Fe concentration versus sputtering time until the Fe:Co ratio of the bulk alloy is reached. In addition, the narrow scan Fe and Co 2p XPS spectra show that Co is more oxidized than Fe. All these measurements indicate that the nanoparticles have a Co shell and an Fe-rich core. They further demonstrate the usefulness of XPS combined with depth-profiling via sputtering to obtain element- and chemically-sensitive structural information on nanoparticles. XMCD as an element-specific magnetic analysis tool further reveals that Fe and Co are ferromagnetically coupled in these particles. The information obtained is useful for establishing a structure–property relation for the studied material that is expected to have applications as a soft magnetic material at high temperatures.     

X-ray magnetic circular dichroism at IrL2,3 edges in Fe100−x Ir x and Co100−x Ir x alloys: Magnetism of 5d electronic states

The formation of induced 5d magnetic moment on Ir in Fe_100−x Ir _x (x=3, 10 and 17) and Co_100−x Ir _x (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L_2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)μB to −0.030(1)μB in Fe-Ir alloys and −0.067(2)μB to 0.024(1)μB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.     

Transferred hyperfine fields at 119Sn in Fe1−xMxSn [M≡Mn, Co and Ni]

The transferred hyperfine fields at ¹¹⁹Sn, using Mössbauer spectroscopy are reported for the hexagonal B-35 compounds with a general formula Fe_1?xM_xSn, where MMn, Co and Ni. In these compounds, Sn atoms occupy two crystallographically inequivalent sites. For FeSn the observed spectrum consists of a quadrupole doublet and a magnetic pattern corresponding to 2(d) and 2(a) sites respectively. The data have been analysed to resolve the controversy regarding hyperfine parameters. On replacing Fe by Mn atoms, additional lines appear in the higher velocity region of the Mössbauer spectrum and the intensity of the nuclear Zeeman pattern increases at the expense of quadrupole doublet. The resulting Mössbauer spectra have been analysed by taking only the nearest neighbour interactions into account. This analysis shows that on replacing each Fe atom by a Mn atom, the hyperfine field at 1(a) Sn site increases by about 40 kOe and a field of about 35 kOe is produced at the 2(d) Sn sites. Further, from the nuclear Zeeman pattern for 2(d) sites, the sign of quadropole splitting for these sites could also be determined and was found to be positive. However, the substitution of Co and Ni in place of Fe atoms results in a broad unresolved pattern suggesting that the hyperfine field at the 1(a) sites decreases and a finite field develops at the 2(d) site. The origin of transferred hyperfine fields at the two inequivalent Sn sites is discussed, the magnetic transition temperatures of these compounds have been estimated and the magnetic moments of M-atoms have been inferred.     

Synchrotron radiation studies of mass-selected Fe nanoclusters deposited in situ

A portable UHV-compatible gas aggregation cluster source, capable of depositing clean mass-selected nanoclusters in situ, has been used at synchrotron radiation facilities to study the magnetic behaviour of exposed and Co-coated Fe clusters in the size range 250 to 540 atoms. X-ray magnetic circular dichroism (XMCD) studies of isolated and exposed 250-atom clusters show a 10% enhancement in the spin magnetic moment and a 75% enhancement in the orbital magnetic moment relative to bulk Fe. The spin moment monotonically approaches the bulk value with increasing cluster size but the orbital moment does not measurably decay till the cluster size is above ∼ 400 atoms. The total magnetic moments for the supported particles though higher than the bulk value are less than those measured in free clusters. Coating the deposited particles with Co in situ increases the spin moment by a further 10% producing a total moment per atom close to the free cluster value. At low coverages the deposited clusters are super-paramagnetic at temperatures above 10 K but a magnetic remanence at higher temperature emerges as the cluster density increases and for cluster films with a thickness greater than 50 ?(i.e. 2-3 layers of clusters) the remanence becomes greater than that of an Fe film of the same thickness produced by a conventional deposition source. Thick cluster-assembled film show a strong in-plane anisotropy. Received 14 December 2000     

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.     

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.     

Imaging of magnetic domains with the X-ray microscope at BESSY using X-ray magnetic circular dichroism

X-ray Magnetic Circular Dichroism (X-MCD), i.e. the change of the absorption of circular polarized Xrays for reversed sample magnetization amounts at the L_{2, 3}-edges of 3d transition metals up to 50% percent. This can be used to obtain in energy-dispersive X-ray imaging techniques a considerable, element-specific magnetic contrast. On the other hand, with the transmission X-ray microscope (TXM) based on the zone-plate technique spatial resolutions of 30nm can be achieved. In this communication it is shown for the first time that the combination of the TXM with XMCD provides a huge contrast and is therefore a powerful new method to visualize in a quantitative and elementspecific manner magnetic domains. Using soft X-rays with a wavelength of 1:7nm corresponding to the energy of the Fe L₃-edge the variation of the shape and magnetization of domains in a magneto-optical GdFe layer system was studied with a lateral resolution of 60nm.