XMCD Study Research Articles

XMCD study of magnetism and valence state in iron-substituted strontium titanate

Room temperature ferromagnetism was characterized for thin films of SrTi$_{0.6}$Fe$_{0.4}$O$_{3-{\delta}}$ grown by pulsed laser deposition on SrTiO$_{3}$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe$^{2+}$ cations, whereas Fe$^{3+}$ and Ti$^{4+}$ did not contribute. Films with the highest magnetic moment (0.8 {\mu}B per Fe) had the highest measured Fe$^{2+}$:Fe${^3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies ({\delta} = 0.19). Post-growth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted SrTi$_{1-x}$Fe$_x$O$_{3-{\delta}}$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of Fe$^{2+}$ and Fe$^{3+}$, can be controlled via the oxygen content, either during growth or by post-growth annealing.

Anomalous magnetic properties of Fe3O4 nanostructures on GaAs substrate probed using X-ray magnetic circular dichroism

Abstract We have studied the magnetic properties and electronic structure of 2 nm Fe3O4 film on GaAs substrates, which evolve in a nano-tip structure and compared it with 150 nm thick Fe3O4 film. Valence band studies reveal a drop in density of states at Fermi level for 2 nm film as compared to thicker film. XMCD studies divulge the unusual high magnetization value of ∼5.1 μB/FU in 2 nm film compared to 3.9 μB/FU observed for bulk Fe3O4 or thicker Fe3O4 films. The excess moment is shown to be due to orbital moment contribution.

XMCD studies of magnetic polarization at Mo atoms in CoMo alloy and magnetically coupled Co/Mo multilayers.

Magnetic polarization of Mo atoms in Co96Mo4 alloy film and Co/Mo multilayered structures has been studied by X-ray magnetic circular dichroism. Samples with Mo spacers of two different thicknesses (0.9 nm and 1.8 nm) were investigated. Mo atoms receive a magnetic moment of -0.21μB in the alloy. In the multilayer with the thinner Mo spacer (dMo = 0.9 nm) the magnetic moment is much smaller (-0.03μB). In both cases the measured induced moment at the Mo site is oriented antiparallel to the moment at the Co atoms. The presence of the induced moment in the Mo spacer coincides with antiferromagnetic coupling between the Co component slabs. In contrast, neither measurable induced moment at the Mo site nor interlayer coupling between the Co layers has been found for the multilayer with the thicker Mo spacer. Possible mechanisms of the coupling associated with the induced moment are discussed in detail.

XMCD and TEM studies of as-cast and rapidly quenched Fe50Nd50 alloys

We present the XMCD analysis of as-cast and melt spun Fe50Nd50 samples performed at L2,3-Nd and K-Fe absorption edges at 5 and 50 K in comparison with macroscopic data of XRD, TEM and magnetic properties measurements. In addition, we have measured the magnetic field dependence of XMCD signal for both types of the samples in magnetic fields up/down to 17 T. The obtained results pointed to the strong difference between structure and magnetic properties of the as-cast and melt spun Fe50Nd50 alloys for both macroscopic and local measurements. The element selective XMCD loops for melt spun alloy show almost identical value of the coercive force Hci for L2-Nd and K-Fe edges and practically do not depend on temperature. XMCD loop at K-Fe edge is a sum of contributions of the Fe-based phases. The main Fe-rich phase has high Hci ≈ 2,4 T as a highly anisotropic phase. The absence of the K-Fe XMCD loop saturation in the field up to 17 T points to presence of the second Nd-rich Nd-Fe phase which is ferromagnetic at temperature lower than 50 K. In accordance to the TEM results these both phases may coexist as the mixture of nanocrystals which was formed as a result of decomposition of the amorphous-like matrix phase. The XMCD loop at L2-Nd edge with Hci ≈ 1,9 T is the sum of contributions from two Nd-based phases: hard Fe-rich phase (Hci ≈ 2,4 T) and Nd-Fe matrix phase of medium hardness with Hci ≈ 1,3 T. The macroscopic loop showed the higher Hci compared to XMCD loops. Such discrepancy may be caused by the fact that XMCD signal is collected from a 5-10 mcm thick surface layer, which contains many defects that reduce anisotropy and coercivity.

XMCD Study of Magnetic States in Laves Phase RFe2 (R=Y, Gd) Hydrides under High Pressure

XMCD Study of Magnetic States in Laves Phase RFe₂ (R=Y, Gd) Hydrides under High Pressure

XMCD studies of thin Co films on BaTiO3

Different layer thicknesses of cobalt ranging from 2.6 Å (1.5 ML) up to 55 Å (30.5 ML) deposited on ferroelectric BaTiO3 have been studied regarding their magnetic behavior. The layers have been characterized using XMCD spectroscopy at remanent magnetization. After careful data analysis the magnetic moments of the cobalt could be determined using the sum rule formalism. There is a sudden and abrupt onset in magnetism starting at thicknesses of 9 Å (5 ML) of cobalt for measurements at 120 K and of 10 Å (5.5 ML) if measured at room temperature. Initial island growth and subsequent coalescence of Co on BaTiO3 is suggested to explain the sudden onset. In that context, no magnetically dead layers are observed.

Magnetic Properties of Endohedral Metallofullerenes: Single-Molecular Magnetism and Anisotropy

Endohedral metallofullerenes (EMFs) encapsulating lanthanide ions (Ln) are of particular interest because of the specific magnetic properties resulting from the combination of the partially filled 4f shell of metal ions, their exchange interactions in endohedral clusters, and shielding role of the carbon cage, which protects endohedral spin states from the outer environment. Inhomogeneous environment of Ln ions inside clusterfullerenes induces magnetic anisotropy and results in a number of magnetic phenomena. In particular, Dy-based nitride clusterfullerenes are the first EMFs to exhibit single molecular magnet (SMM) behaviour.1 Furthermore, completely different SMM behaviour of DySc2N@C80, Dy2ScN@C80, and Dy3N@C80 molecules shows that exchange interactions between Ln ions in EMFs has strong influence on the magnetic properties.2 Results of the SQUID and XMCD studies of different EMFs at low temperatures will be discussed in this contribution. Whereas SQUID and XMCD are of particular use for the study of the low-temperature effects, magnetic properties of EMFs at room-temperature are conveniently addressed by paramagnetic NMR spectroscopy,3 since paramagnetic chemical shift is proportional to the anisotropy of magnetic susceptibility. New results on the NMR studies for a series of LnSc2N@C80molecules will be reported.1. R. Westerström, J. Dreiser, C. Piamonteze, M. Muntwiler, S. Weyeneth, H. Brune, S. Rusponi, F. Nolting, A. Popov, S. Yang, L. Dunsch and T. Greber, J. Am. Chem. Soc., 2012, 134, 9840-9843.2. R. Westerström, J. Dreiser, C. Piamonteze, M. Muntwiler, S. Weyeneth, K. Krämer, S.-X. Liu, S. Decurtins, A. Popov, S. Yang, L. Dunsch and T. Greber, arXiv:cond-mat/0604365v1, 2013, 1310.5040.3. Y. Zhang, A. A. Popov, S. Schiemenz and L. Dunsch, Chem.-Eur. J., 2012, 18, 9691-9698.

New Ternary Boride EuRh4B4 Synthesized under High Pressure and Temperature

We report the results of a detailed study of EuRh4B4 compound synthesized by an un-common method under high pressure temperature conditions for the first time. Synthesized sample has been initially characterized by X-ray diffraction, macroscopic electrical resistivity and ac-susceptibility methods. The ferromagnetic and superconducting phases were found below 83K and 5.8K respectively. Element-and shell-selective XANES and XMCD techniques have been further applied to check the possible valence transition of Eu ions and their magnetic states. XANES spectra recorded at the Eu L2,3 absorption edges clearly indicate the presence of either magnetic Eu2+ or non-magnetic Eu3+ ions in the sample. The XMCD studies have also shown that local magnetic moments of Eu2+ ions exist in the superconducting EuRh4B4, at least above the transition.

Engineering Biogenic Magnetite for Sustained Cr(VI) Remediation in Flow-through Systems

In this work, we report a route to enhance the reactivity and longevity of biogenic magnetite in Cr(VI) remediation under continuous-flow conditions by combining functionalization of the biomagnetite surface with a precious metal catalyst, nanoscale palladium, and exposure to formate. Column influent conditions were varied to simulate oxic, anoxic, and nitrate cocontaminated environments. The addition of sodium formate as an electron donor for Pd-functionalized magnetite increased capacity and longevity allowing 80% removal of Cr(VI) after 300 h in anoxic conditions, whereas complete breakthrough occurred after 60 h in anoxic nonformate and nonfunctionalized systems. Removal of Cr(VI) was optimized under anoxic conditions, and the presence of oxidizing agents results in a modest loss in reductive capacity. Examination of reacted Pd-functionalized magnetite reveals close association of Fe with Cr, suggesting that Pd-coupled oxidation of formate serves to regenerate the reactive surface. XMCD studies revealed that Cr(III) is partially substituted for Fe in the magnetite structure, which serves to immobilize Cr. No evidence for a mechanistic interference by nitrate cocontamination was observed, suggesting that this novel system could provide robust, effective and sustained reduction of contaminants, even in the presence of common oxidizing cocontaminants, outperforming the reductive capacity of nonfunctionalized biogenic magnetite.

XMCD study of the induced magnetic moment in non-magnetic Ga and Ge atoms in intermetallic compounds

In the past few years, we have witnessed the appearance of exotic magnetism of atoms in nano-scaled systems associated with compounds that are non-magnetic in their bulk form. Different models ranging from orbital magnetism delocalized over the whole molecule to localized one associated to the modification of the electronic structure have been proposed. Hence, one of the main challenges is to determine the mechanism that induces both the magnetic moment and the exchange interactions, leading to the ferromagnetic-like behaviour in these nanostructures. Aimed at obtaining a deeper insight into the induction of magnetic moments on nominally non-magnetic atoms, we have performed an XMCD study on rare-earth–iron Laves phase compounds containing Ga or Ge. XMCD probes the magnetic polarization of both Ga and Ge in these compounds. Our results show that when the rare-earth (R) is non-magnetic this polarization is uniquely due to the hybridization with the Fe neighbours, while it is enhanced when R is magnetic due to the Fe–R hybridization. As a consequence, the magnetic polarization of both Ge and Ga atoms differs as this hybridization does.

Magnetic switching of Fe-porphyrin molecules adsorbed on surfaces: An XAFS and XMCD study

Metalorganic molecules like Fe-porphyrin or haemoglobin have been investigated in great detail in the past. Its importance is obvious and has been measured mostly with molecules in random orientation. Here we report on experiments of a single monolayer of Fe-porphyrin in UHV aligned flat on ferromagnetic Ni and Co films. NEXAFS with linear and circular polarization is the spectroscopy of choice; it is elementspecific and measures the electronic structure as well as the magnetism at once. For the flat oriented monolayer of porphyrin molecules we have measured the angular dependence of XAFS at the C and N K-edge and XMCD at the Fe L-edges. The paramagnetic Fe-spin is aligned with respect to the ferromagnetic substrate. This can be parallel or antiparallel. Also nonmagnetic substrates like Cu (100) plus an external magnetic field will align the magnetic Fe-moment. This altogether opens a huge field for switching the 3d-spin from parallel to perpendicular of the molecular plane, which in turn will modify the electronic transport properties and act as a single molecular switch.

EXAFS and XMCD studies of correlations of atoms and magnetic moments in the disordered nanocrystalline alloys Fe with sp-elements (Al, Si, Sn)

The results of the studies of an extended X-ray absorption fine structure (EXAFS, at the Fe and Sn K edges) and X-ray magnetic circular dichroism (XMCD, at the Fe K edge) of binary nanocrystaline oversaturated solid solutions of Fe with sp-elements Al, Si, and Sn containing an sp-element up to 50 at % are presented. The parameters of the obtained partial pair correlation functions demonstrate an increase in the short-range chemical order as the concentration of the sp-element increases and with a high degree of local static distortions of the crystal lattice. A considerable change in the XMCD signal observed for Fe-Al and Fe-Sn alloys as the sp-element content increases can be qualitatively explained using a model of local magnetic moments corresponding to the case of the Mattis magnet.

Depth-Resolved XMCD Study of Ultrathin Mo/Co/Au Films

The effect of Mo overlayer on the magnetism of ultrathin Co films grown on Au(111) was studied. The structure and magnetism at the Mo/Co interface were studied by means of depth-resolved X-ray magnetic circular dichroism. The magnetic moment of Co near the Mo/Co interface was reduced in comparison to the inner part of the film for the samples which have in-plane anisotropy. This effect is more prominent in the case of Mo overlayer with higher roughness, and can be attributed to Co–Mo intermixing at the interface.

Synchrotron radiation spectroscopy study of FeCr2X4 (X=S and Se)

Electronic structures of FeCr2X4 (X=S and Se) have been investigated by employing soft-x-ray absorption spectroscopy and soft-x-ray magnetic circular dichroism (XMCD). It is found that FeCr2S4 and FeCr2Se4 have similar electronic structures. The valence states of Cr and Fe ions are nearly trivalent (Cr3+) and divalent (Fe2+), respectively. The Fe 3d states are strongly hybridized to the X p states. The Fe and Cr 2p XMCD study provides evidence for the antiparallel alignment between Cr and Fe spins and the strong hybridization between the Fe 3d and X p electrons.

Induced vanadium polarization in intermetallic iron based compounds: XMCD and ab initio study

Induced vanadium polarization in intermetallic iron based compounds: XMCD and ab initio study

XMCD study of the magnetic behavior of R(Al 1−xFe x) 2 compounds

We present an XMCD study performed at the Fe–K-edge through the Ho(Al 1− x Fe x ) 2 Laves phases. In agreement with other series, a rare-earth contribution, XMCD R, is found. We have also observed that in these compounds XMCD R is clearly larger than that obtained in series with a lower R:Fe ratio, indicating a relation between the size of XMCD R and the number of R neighboring atoms. In addition, the rare-earth sublattice also affects the peak at lower energy, contrary to results obtained on other R –T series, where this lower-energy peak was typically considered as purely of Fe origin. The influence of the R sublattice even at ∼0 eV prevents from using Fe–K-edge XMCD as a fingerprint to study the magnetic character of Fe.

Revealing Fe magnetism in lanthanide-iron intermetallic compounds by tuning the rare-earthL2,3-edge x-ray absorption edges

We present a systematic x-ray magnetic circular dichroism XMCD study performed at the rare-earth L2,3 edges in RAl1�xFex2 Laves phase compounds. The progressive substitution of Al by Fe reveals the existence of a non-negligible contribution of Fe to the rare-earth XMCD spectra. This contribution has been isolated and shown to be similar to the dichroic spectrum recorded at the Fe K edge. These results open the possibility of monitoring the Fe magnetism in lanthanides-iron intermetallic compounds by probing the rare-earth L2,3-edge x-ray absorption edges. Intermetallic compounds containing rare earths R and iron are of great technological significance owing to their industrial applications as high-performance permanent magnets. The intrinsic magnetic properties of these materials are determined by the exchange interaction between the R-4f and Fe-3d electrons. This interaction takes place through the hybridization of the Fe 3d states with the 5d ones of the rare earth. 1 Consequently, the study of the R-5d states is of great significance to the search of new advanced magnetic materi

XMCD study of the anisotropy of nanometric Co clusters in insulating and metallic matrices

The orbital to spin magnetic moment ratio of Co clusters homogeneously dispersed in Al2O3 is found to be much larger than in bulk Co and inversely proportional to the cluster diameter. For the same size, covering the clusters with a Cu layer increases mL/mS. An enhancement on mL/mS in the surface atoms is one order of magnitude more effective as a source of anisotropy than in the bulk.

A depth profile XMCD study of Au/CoO/Co

A magnetic depth profile study was performed on the system Au/CoO/Co. In situ sputtering at 300 K was followed by X-ray magnetic circular dichroism measurements from the CoO layer to the pure Co layer. Sum rules were applied to determine magnetic moments of Co. The orbital moment and the m l/ m s ratio tends to decrease close to the interface.

X-MCD study of mixed valence manganites

A study of the X-MCD spectra and X-MCD magnetometry, i.e. the X-MCD effect vs. applied field, of the colossal magnetoresistive perovskites La 0.67− y Tb y Ca 0.33MnO 3 is reported. The spectra were measured on polycrystalline samples with y=0.0, 0.1, 0.22 at 10 and 77 K at the K, L 2, L 3 edges of manganese, the L 2, L 3 edges of lanthanum and the L 2, L 3, M 4, M 5 edges of terbium. From the X-MCD signal at the M 4, M 5 edges of terbium a parallel alignment of terbium sublattice magnetisation to the manganese magnetisation and to the applied magnetic field is deduced. The X-MCD magnetometry curves at the Mn K edge and La L edges show a saturation in the applied field, whereas Tb L edge signals do not saturate. The effect is especially pronounced in La 0.45Tb 0.22Ca 0.33MnO 3. The results are discussed in terms of the influence of terbium doping on the magnetic and electronic phase separation and on the strength of the coupling between magnetic moments of Tb and Mn and their relation to the colossal magnetoresistance of the compounds.