TCNQ-based Supramolecular Architectures at Metal Surfaces

In this thesis we combine surface chemistry and surface physics to architecture molecular layers in a bottom-up approach. The formation of self-assembled molecular structures at surfaces on the basis of dipole-dipole interactions, H-bonding, metal coordination and covalent bonding is studied. The molecules of different structure and with specific functional groups are investigated on selected substrates, namely Au(111), Ag(111), Cu(111), Cu(100) or Bi reconstructed Cu(100). A number of model cases for controlling on-surface architectures and their properties has been found and is reported about in this thesis: 1) Architecture control of a coordination polymer, comprised from chiral and flexible molecular building blocks, by tuning of the intermolecular bonding motif; 2) Chirality transfer in a 1D coordination polymer formed from chiral molecules; 3) Dimensionality (0D, 1D, 2D) control via selection of the transition metal adatom, which modifies a ligand and participates in an on-surface coordination complex; 4) Demonstration of a 2D molecular ‘sponge’, created on the basis of a borylene derived covalent link with angular flexibility; 5) Investigation of confined 2D electron states in quantum boxes of different size and shape; 6) Self -sorting of bi-molecular system in a 2D array by the coulomb interaction of the surface dipole which depends on band-alignment, charge transfer and the screening in the substrate. Specifically, we show that chiral and flexible [7]helicene molecules with cyano-groups, covalently attached in symmetric positions, give rise to a 1D arrangement. The intrinsically chiral species imprints its chirality onto the weakly H-bonded assembly, which occurs if molecules are deposited on samples held at low (~90 K) temperatures. This imprint vanishes under the influence of stronger metal-coordination bonds formed after providing metal coordination centers to the H-bonded assembly. The flexibility of the helicene as well as the competition between intermolecular and molecule-surface interactions allow the coordinated chains to assemble in structures with the mirror symmetry apparently being reduced. The next important issue, addressed in this thesis, is the on-surface modification of the ligand as an approach to control the dimensionality of the resulting on-surface polymer. We present a novel metal-specific reaction of amino- /imino- functionalized perylene derivatives. This precursor is modified upon addition of Co, Fe or Ni at room-temperature into an endo-ligand. In contrast, the presence of Cu adatoms in conjunction with thermal activation leads to the formation of an exo-ligand. Thus the type of metal ligand defines whether a 1D or 2D coordinated polymer can be formed. We show that borylene-functionalized molecules react upon thermal activation with trimesic acid in a novel on-surface reaction. Moreover, the covalent connection, formed in this reaction, exhibits a high degree of flexibility and allows for the formation of the differently sized pores. The resulting molecular ‘sponge’, created this way, serves as a template confining the surface state electrons. We investigate the effect of size and shape of the pores on this quantum phenomenon. Furthermore, we present a new way of creating highly-ordered bimolecular self-sorted chessboard arrays. The bi-component mixture of Mn-phthalocyanine (MnPc) and Cu-phthalocyanine (CuPc) on Bi/Cu(100) self-assembles without participation of any chemical bonding or molecular functionalization but only on the basis of the lateral 2D Coulomb interactions. We resolve charge-transfer channels of two types, directing the supramolecular self-assembly: one oriented perpendicular to the substrate surface, the other oriented in-plane. These investigations are performed in ultra-high vacuum conditions (UHV) with the use of variable temperature Scanning Tunneling Microscopy / Spectroscopy (STM/STS), X-Ray Photoelectron Spectroscopy (XPS) and synchrotron-based Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The experimental results are supported by Density Functional Theory (DFT) calculations, performed by research partners.

This thesis deals with self-organized supramolecular structures on metal surfaces and their magnetic and electronic properties. To study the systems, various experimental methods such as Scanning Tunneling Microscopy (STM), X-Ray Absorption Spectroscopy (XAS), and X-Ray Magnetic Circular Dichroism (XMCD) combined with Density Functional Theory (DFT) are applied. We focus on several aspects of supramolecular structures on metal surfaces. First, the role of intermolecular and molecule-substrate interactions are discussed regarding their effect on the self-assembling, charge state, and charge transfer. Second, we are interested in the magnetic state of metal atoms embedded in metal-organic networks and the possibility of producing magnetically coupled supramolecular structures. In the first part, three different systems of charge-transfer materials on a Au(111) surface were examined. In the first case, Na atoms and TCNQ molecules organize into ordered structures accompanied with a charge transfer from the Na atoms to the TCNQ molecules. Thus, the TCNQ molecules are negatively charged with exactly one electron. The unpaired spin can be detected by its interaction with Au(111) surface conduction electrons by a Kondo resonance. Moreover, it could be shown that the singly occupied TCNQ orbital is deformed by the presence of the positively charged Na cations affecting the spatial distribution of the Kondo resonance. When we exchange TCNQ with TNAP molecules we observe that the underlying metal surface plays a significant role on the electronic properties of the TNAP molecules. The TNAP molecules within the Na-TNAP layer are charged with one electron. But only specific TNAP molecules show a spin-Kondo resonance depending on their adsorption site on the Au(111) herringbone reconstruction. This is attributed to adsorption-site dependent molecule- substrate interactions. Replacing the Na atoms by organic TTF molecules also leads to the formation of ordered structures. By the interplay of molecule- molecule and moleculesubstrate interactions the TNAP molecules in the TTF-TNAP system are charged with more than one electron enhancing the charge compared to the bulk compound. The second part of the thesis is focused on supramolecular structures consisting of molecules and transition metal atoms. We show that the network architecture is determined by the choice of the molecules and metal atoms and can be selectively altered. The different network architectures also imply different electronic and magnetic properties. The first described network consists of Cu atoms and T4PT molecules and exhibits a hexagonal structure on both the Ag(111) and Cu(111) surfaces. The identical structural and electronic properties on both surfaces indicate that the formation of the system is mainly ruled by the metal-ligand bonds. Furthermore, the Cu atoms within the network have no magnetic moment. By replacing the Cu with Fe atoms an ordered Fe-T4PT network with a bilayer structure is formed on a Au(111) surface. The first layer exhibits a three- fold bonding node consisting of three pyridine end groups and a central Fe atom. An additional T4PT molecule is located on top of each Fe site. Thus, the Fe atoms are placed in a three-dimensional coordination cavity. The Fe atoms are in a high spin state and exhibit a magnetic anisotropy with an easy-axis out-of-plane orientation. Moreover, for the first time, a ferromagnetic coupling between the Fe atoms in such a system can be observed. The exchange coupling mechanism is presumably based on super-exchange through the T4PT molecules. In the final part the dependence of the supramolecular architecture with respect to the molecular functional end groups is investigated. In this case we could change the network structure from bilayer to single-layer by the adequate by the adequate choice of ending groups. This has also an impact on the network electronic properties.

This thesis reports on the magnetism of low coordinated Fe and Co atoms in different chemical and electronic environments. The objective of this research was to contribute to the fundamental understanding of the magnetic properties, such as magneto-crystalline anisotropy energy (MAE), orbital and spin magnetic moment, of surface supported transition metal (TM) atoms and one atomic layer thick epitaxial TM films. A detailed knowledge on the interplay of electronic hybridization and magnetic properties is mandatory to elaborate new materials with high perpendicular magnetic anisotropy and acceptable writing field for future use in magnetic recording devices. The magnetic properties of the different experimental systems were investigated by x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the effect of electronic hybridization between isolated 3d TM atoms and different kinds of substrates. First, we considered 4d and 5d TM substrates known for their high magnetic polarizability due to the close onset of ferromagnetism. Deposition of single Fe and Co atoms on Rh(111), Pd(111), and Pt(111) produces an increasing MAE going from Rh to Pd and finally to Pt that we directly correlate to the increasing spin-orbit constant of the substrate interlinking the induced magnetic moment and the MAE. Further we investigated single Fe and Co atoms on two different insulating layers, namely Al2O3/Ni3Al(111) and Cu2N/Cu(001), providing an efficient decoupling of the TM 3d states from the conduction electrons of the substrates. In case of the alumina film the absorption spectra of Fe and Co show a pronounced multiplet structure. On Cu2N the multiplet features of the adatoms are less defined due to the lower thickness of the screening layer. On both insulating layers we find a common out-of-plane easy axis, highly unquenched orbital moments, and giant MAE values of several meV/atom which are due to the formation of strong anisotropic bonds with the O and N atoms, respectively. The second part is dedicated to single atom thick layers of Fe and Co deposited on Pt(111) and Rh(111). We find the MAE to be strongly enhanced compared with bulk and we establish a direct proportionality between the induced magnetic moment of the topmost substrate layer and the MAE. The study of bimetallic Fe1-xCox alloys on platinum reveal a material with high MAE (0.5 meV at the equiatomic composition), large saturation magnetization (3μB to 2μB going from Fe to Co), and perpendicular magnetization easy axis. These characteristics hint at new possibilities for the further miniaturization of the grain size in recording devices. In the last part the theoretical description of the MOKE for ultra-thin films is extended to the transverse geometry and improved analytic formulae for the polar and longitudinal geometry are presented. This finding was used to optimize the Kerr signal for a new MOKE setup which is fully integrated in an ultrahigh vacuum chamber combining in situ MOKE and STM. The setup performance was demonstrated for a continuous film of 0.9 monolayers (ML) Co/Rh(111) with in-plane easy axis and for a superlattice of nanometric double layer Co islands on Au(11,12,12) with out-of-plane easy axis. The detection limit is 0.5 ML for transverse and 0.1 ML for polar Kerr geometry corresponding for island superlattices with the density of Co/Au(11,12,12) to islands composed of about 50 atoms. Moreover, the setup holds two independent ways to measure the MAE which is either by thermally induced magnetization reversal or by applying a torque onto the magnetization by turning the magnetic field. Both ways yield similar results for 1.1 ML Co/Au(11,12,12).

Organic-molecular magnets based on a metal-organic framework with chemically tuned electronic and magnetic properties have been attracting tremendous attention due to their promising applications in molecular magnetic sensors, magnetic particle medicines, molecular spintronics, etc. Here, we investigated the magnetic behavior of a heterojunction comprising a ferromagnetic nickel (Ni) film and an organic semiconductor (OSC) 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) layer. Through the magneto-optical Kerr effect (MOKE), a photoemission electron microscopy (PEEM), X-ray magnetic circular dichroism (XMCD), and X-ray photoelectron spectroscopy (XPS), we found that the adsorption of F4-TCNQ on Cu(100)/Ni not only reverses the in-plane magnetization direction originally exhibited by the Ni layer but also results in enhanced magnetic ordering. Furthermore, the cyano group (CN) in adsorbed F4-TCNQ was found spin-polarized along with conspicuous charge transfer with Ni. The density functional theory (DFT) calculations suggest that the experimentally found spin polarization originates from hybridization between the CN group's π orbitals and Ni's d band. These findings signify that the hybrid states at the organic-ferromagnet interface play a key role in tailoring the magnetic behavior of interfaces. For the case of the F4-TCNQ and Ni heterojunction reported here, interface coupling is an antiferromagnetic one.

The theoretical calculation of the interaction of electromagnetic radiation with matter remains a challenging problem for contemporary electronic structure methods, in particular, for x-ray spectroscopies. This is not only due to the strong interaction between the core hole and the photoexcited electron, but also due to the elusive multiplet effects that arise from the Coulomb interaction among the valence electrons. In this work we report a method based on density functional theory in conjunction with multiplet ligand-field theory to investigate various core-level spectroscopies, in particular, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The developed computational scheme is applied to the L2,3 XAS and XMCD edges of magnetite (Fe3O4) as well as cobalt ferrite (CoFe2O4) and nickel ferrite (NiFe2O4). The results are in overall good agreement with experimental observations, both regarding the XAS L2/L3 branching ratio, the peak positions, as well as the relative intensities. The agreement between theory and experiment is equally good for XAS and the XMCD spectra, for all studied systems. The results are analyzed in terms of eg and t2g orbital contribution, and the robustness of the spectra with regard to the uncertainties of the Slater parameters is investigated. The analysis also highlights the strong effect of the 2p−3d interaction in x-ray spectroscopy. Published by the American Physical Society 2024

Within this thesis, two classes of transition metal complexes are studied on surfaces. Firstly, monomolecular layers of Fe(II) spin-crossover (SCO) complexes, prepared by in-situ sublimation onto Au(111) substrates and by self-assembly on Au(111)/mica, are investigated by means of X-ray absorption spectroscopy (XAS). For a multilayer of Fe(phen)2(NCS)2 and phenanthroline molecules on Au(111), a partly reversible SCO transition is demonstrated. It is found that the transition is suppressed if the Fe(phen)2(NCS)2 molecules are in direct contact with the Au(111) surface, possibly due to a chemical reaction of their ligands with the gold substrate. The intact deposition of Fe(bp)2 is demonstrated, being relatively unperturbed even when in direct contact with the Au(111) surface. The self-assembly of Fe(bppmc)2, which results in a monomolecular layer with both linker groups bound to the gold surface, is demonstrated by S 2p X-ray photoelectron spectroscopy. In both cases a high-spin state of the Fe centers is observed, implying that the strength of the ligand field would need to be increased to realize an SCO transition of the molecules on the surface. The second class of transition metal complexes are quasi-planar Fe and Co octaethylporphyrin (OEP) molecules. Their magnetic properties on non-magnetic and ferromagnetic (FM) surfaces are analyzed in the submonolayer regime by X-ray magnetic circular dichroism (XMCD). The angle-dependent electronic structure at the metal center of Fe and Co OEP molecules on non-magnetic Cu(100) and oxygen-covered O/Cu(100) substrates, is determined by means of XAS. Measurements of the magnetic properties are carried out in a magnetic field of B = 5 T at T = 8 K. For Fe OEP on O/Cu, a very strong magnetic anisotropy is found, owing to the interaction with the oxygen and resulting in a factor of five between the in and out-of-plane XMCD signal. The magnetism of Co OEP on O/Cu(100) is dominated by the contributions of the dz2 orbital. This results in a characteristic angular dependence of the XMCD signal, due to the anisotropy of the spin-density. By means of XMCD measurements it is shown that the magnetic moment of the Fe centers of Fe OEP molecules can be aligned at room temperature if they are deposited onto FM Ni and Co substrates. A simple theoretical model is utilized to determine the magnetic coupling energies from temperature-dependent measurements of the Fe and substrate magnetizations. A much stronger coupling is found for Fe OEP on Co than on Ni substrates. Tailoring of the magnetic coupling is achieved by placing atomic oxygen between the molecules and the FM Ni and Co substrates. For the first time, an antiferromagnetic coupling of Fe porphyrin molecules to FM substrates is realized here, as evidenced by the opposite sign of the Fe and substrate XMCD signals.

The magnetic and electronic properties of CeCo2 bulk and nanoparticles have been investigated using element‐specific techniques, X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD). The results of Ce L3‐edge XAS spectra shows a higher spectral weight of 4f1 configuration in nanoparticles with respect to the bulk, which indicates that the valence of Ce is decreased in nanoparticles. In addition, spectroscopic results from the XMCD at Co L2, 3‐edges and Ce M4, 5‐edges indicate that CeCo2 undergoes a nonmagnetic to magnetic transition with size reduction. The variations in electronic and magnetic properties of nanoparticles were attributed to the charge transfer related to the sample dimension. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In our information society, the importance of energy conservation is increasing year by year. To tackle this issue, the transition from volatile memory to non-volatile memory can be a solution. One of the candidates for non-volatile memories is a racetrack memory[1], driven by current-induced domain wall motion (CIDWM). For practical use, faster CIDWM and lower threshold current density are key factors. In order to reach this goal, we have been focusing on and investigating Mn4N as a promising material. Mn4N film is an antiperovskite ferrimagnet without rare-earth elements, which is advantageous in fast magnetization reversal due to its perpendicular magnetic anisotropy (Ku ~ 1.1×105 J/m3) and a small saturation magnetization (MS ~ 80 kA/m)[2]. Previous study on 1-2μm-wide Mn4N strips showed the fastest spin-transfer-torque-driven domain wall motion (vDW ~ 900 m/s at 1.3×1012 A/m2) at room temperature (RT)[2], comparable to those reported in the system including rare-earth 4f magnets or heavy metals. To achieve faster vDW, Mn4N based mixed crystals have been studied in pursuit of the use of magnetic and/or angular momentum compensation for more efficient CIDWM thanks to diverged damping constants. Recently, Mn4-xNixN films have been suggested to have a magnetic compensation (MC) point between x = 0.1 and 0.25 at RT[3]. We found that Mn4-xCoxN films have a compensation point between x = 0 and 0.8 from x-ray magnetic circular dichroism (XMCD) measurements[4]. However, there is a lack of information about the magnetic behavior of Mn4-xCoxN at values x much smaller or larger than 0.8. Considering that the compensation in Mn4-xNixN takes place in a small range of composition x, further compensation points can be found when x is far from 0.8. In this work, we performed XMCD measurements on Mn4-xCoxN epitaxial films at x = 0.2 and 1.3 and investigated the change in magnetic structures by composition ratio to verify MC at RT.20-30 nm-thick Mn4-xCoxN films with x = 0-1.3 were epitaxially grown on SrTiO3(001) substrates by molecular beam epitaxy. SiO2 or Ta capping layers were sputtered in-situ on the surface to prevent oxidation. X-ray absorption spectroscopy (XAS) and XMCD measurements were performed at BL-16A of KEK-PF for x = 0.2 and 1.3. In these measurements, we applied an external magnetic field of 5 T perpendicular to sample surfaces. The incidence angle of the circularly polarized x-ray was 54.7°(magic angle) to the plane in order to simplify the sum rule calculation. For x = 0.8, the XMCD measurements were carried out at BL23SU of SPring-8[4].Figures 1(a)-1(c) show the XAS and XMCD spectra of Mn in Mn4-xCoxN at x = 0.2, 0.8[4], and 1.3, respectively. In these figures, the sharp peak near 640 eV comes from Mn atoms at corner sites (I sites), and the broad peak near 643 eV originates from those at face-centered sites (II sites)[4]. We observed the sign reversals of XMCD signals between x = 0.2 and 0.8, and also between x = 0.8 and 1.3. Similar sign reversals were also observed in the XMCD signals of Co atoms. These results indicate that MC occurs twice in the range of x = 0-1.3 in Mn4-xCoxN. Besides, the XAS spectra of Co indicate that Co preferentially occupied the corner sites. We also calculated the mean magnetic moment of Co by using the sum rule analysis. Figure 2 shows the expected magnetic structures derived from the XMCD measurements. Around x = 0.2, Co atoms preferentially replaced Mn(I) and thus the total magnetic moment of I site and that of face-centered site (II site) became closer. With further increasing x, the total magnetic moment became zero. In this manner, the first MC occurred between x = 0.2 and 0.8. After the first MC, the total magnetic moment of II sites became larger and this led to the reversal of magnetic moments of all atoms in order to minimize the Zeeman energy. Above x = 0.8, Co atoms gradually got to occupy II sites and thus the second MC occurs. As far as we investigated, Mn4-xCoxN is the only material which has two MC points at RT. This property can be useful for spintronic devices, for example, the use of compensation in a wide range of composition.The XMCD experiment for x = 0.2 and 1.3 was performed with the approval of the Photon Factory Program Advisory Committee (Proposal No. 2019G574). **

Ferromagnetism above room temperature has been observed in Mn/Si3N4 multilayered films for the first time. Characterization of the structural and electronic properties was performed to study the ferromagnetic behavior of this system. X-ray absorption spectroscopy at the Mn K and L edges, as well as x-ray magnetic circular dichroism, evidences the presence of divalent Mn in the films. X-ray absorption near edge structure measurements, which are compared to calculations, confirms the presence of a slightly distorted Mn3N2 phase that is proposed to be the origin of the ferromagnetism in this system.

연 x선 광흡수 분광법(soft x-ray absorption spectroscopy: XAS)과 연 x선 자기 원편광 이색성(soft x-ray magnetic circular dichroism: XMCD)을 이용하여 스피넬 준강자성 산화물인 <TEX>$FeV_2O_4$</TEX>의 전자 구조를 연구하였다. Fe 2p XAS와 V 2p XAS 측정으로 부터 <TEX>$FeV_2O_4$</TEX>에서 Fe 이온과 V이온의 고유한 원자가는 각각 약 <TEX>$Fe^{2.3+}$</TEX>의 혼합원자가 상태와 약 <TEX>$V^{3+}$</TEX>의 상태임을 알 수 있었다. 실험적으로 측정된 Fe 2p XMCD의 신호는 거의 <TEX>$Fe^{2+}$</TEX> 상태에서 기인하였으며, <TEX>$Fe^{3+}$</TEX> 상태는 Fe 2p XMCD의 신호에 거의 기여하지 않는다는 사실이 발견되었다. 그러므로 <TEX>$FeV_2O_4$</TEX>의 자성 특성을 결정함에 있어서 자기적으로 정렬된 <TEX>$Fe^{2+}$</TEX> 상태 이온들이 중요한 역할을 한다고 생각된다. The electronic structure of ferrimagnetic spinel oxide of <TEX>$FeV_2O_4$</TEX> has been investigated by employing soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD). The Fe 2p and V 2p XAS spectra show that the valence states of Fe and V ions are <TEX>${\sim}Fe^{2.3+}$</TEX> mixed-valent states and <TEX>${\sim}V^{3+}$</TEX> states, respectively. In Fe 2p XMCD spectra, finite XMCD signals are observed for divalent <TEX>$Fe^{2+}$</TEX> states only, but not for <TEX>$Fe^{3+}$</TEX> states. This finding indicates that the magnetic moments of <TEX>$Fe^{2+}$</TEX> ions are ordered ferromagnetically but that those of <TEX>$Fe^{3+}$</TEX> ions are cancelled, implying that <TEX>$Fe^{2+}$</TEX> ions play an important role in determining magnetic properties of <TEX>$FeV_2O_4$</TEX>.

We report on a comparative study of electronic and magnetic properties of Mn6 single-molecule magnets (SMMs) grafted on gold surface. Two derivatives with spin-ground states S=4 and S=12 have been functionalized with 3-tp-CO2- (3-thiophene carboxylate, tpc) ligands and characterized as thick films (TFs) as well as sub-monolayers (sMLs) by synchrotron based techniques. X-ray absorption spectroscopy at the Mn L2,3 edges shows the modification of the spectral lineshape in the sMLs with respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules are deposited on gold surface. In spite of this, the expected MnIII oxidation state is preserved. X-ray magnetic circular dichroism (XMCD) spectra show that the total magnetic moment is only given by spin part because of the quenched orbital moment. Moreover, variable temperature and variable field XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives.

We combine x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) data with first principles density functional theory (DFT) calculations and a multiorbital many-body Hamiltonian approach to understand the electronic and magnetic properties of Co atoms adsorbed on h-BN/Ru(0001) and h-BN/Ir(111). The XAS line shape reveals, for both substrates, an electronic configuration close to 3d8, corresponding to a spin S = 1. Magnetic field dependent XMCD data show large (14 meV) out-of-plane anisotropy on h-BN/Ru(0001), while it is almost isotropic (tens of μeV) on h-BN/Ir(111). XMLD data together with both DFT calculations and the results of the multiorbital Hubbard model suggest that the dissimilar magnetic anisotropy originates from different Co adsorption sites, namely atop N on h-BN/Ru(0001) and 6-fold hollow on h-BN/Ir(111).

An ultrahigh vacuum chamber that enables the in situ growth of thin films and multilayers by magnetron sputtering techniques is described. Following film preparation, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements are performed by utilizing an in vacuum electromagnet. XMCD measurements on sputtered thin films of Fe and Co yield spin and orbital moments that are consistent with those obtained previously on films measured in transmission geometry and grown in situ by evaporation methods. Thin films of FeN prepared by reactive sputtering are also examined and reveal an apparent enhancement in the orbital moment for low N content samples. The advantages of producing samples for in situ XAS and XMCD studies by magnetron sputtering are discussed.

We have investigated the electronic and magnetic properties of a MnAs:GaAs granular film with MnAs clusters embedded in the GaAs matrix fabricated by high-temperature annealing of Ga1−xMnxAs using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The XAS line shapes in the Mn 2p core level changed from a localized structure to an itinerant NiAs-type one. Magnetic-field dependence of the XMCD revealed no hysteresis curves at the fixed photon energy where the large XMCD signals were observed, suggesting the superparamagnetic behavior at 100 K.

We investigated Zn0.94Co0.06O thin films on sapphire (0001) substrates with respect to their structural and magnetic properties. X-ray diffraction shows a axis oriented growth and no secondary phases within its resolution. A clear improvement of the crystalline quality was obtained by post annealing under vacuum conditions. Further information about the local electronic structure is obtained by X-ray absorption spectroscopy at the Co L2,3 and the O edge. Magnetic properties were investigated with a superconducting quantum interference device (SQUID) and by X-ray magnetic circular dichroism at the Co L edg2,3e. Both techniques yield mainly paramagnetic behavior of the samples. For low temperatures, an additional small ferromagnetic contribution was observed in SQUID measurements. Several possible origins of this ferromagnetic contribution are discussed.