Traditionally crystal-field wave functions in rare earth compounds are determined with inelastic neutron scattering. However, in the presence of strong correlation the magnetic excitation are strongly broadened and the magnetic intensities are hard to determine accurately. Consequently the resulting wave functions are not very reliable. We therefore started some years ago to use soft XAS at the cerium M-edge to target specifically the 4f ground state wave function [see Hansmann et al., doi: 10.1103/PhysRevLett.100.066405]. Here we take advantage of the dipole selection rules for linear polarized light that give sensitivity to the initial state ground state symmetry.

2. Absence of orbital rotation in superconducting CeCu₂Ge₂ [J.-P- Rueff et al., PRB 91, 201108 (R), doi: 10.1103/PhysRevB.91.201108]

Non-resonant inelastic x-ray scattering (NIXS) with hard x-rays at the rare earth N-edge is a new, state-of-the-art approach for determining ground state wave functions in rare earth compounds. Here we exploit the direction dependence of the scattering function S(q,w) in analogy to the polarization dependence in an XAS experiment.

The use of a scattering experiment with hard x-rays has several major advantages: 1) it is truly bulk sensitive, 2) large momentum transfers (≈10Å^-1) can be reached so that scattering from terms higher than dipole occur in the scattering function S(q,w), and 3) it can be modelled quantitatively due to the absence of an intermediate state. The method is particularly useful for measuring charge densities or anisotropies with a higher than two fold symmetry (cubic, tetragonal basal plane, trigonal, etc) where a dipole technique is insensitive. The beyond-dipol-limit gives rise to extra excitations from the higher order scattering terms that are not observable at low momentum transfers (dipole limit) and these extra excitations are direction sensitive even in higher rotational symmetry. We had shown the feasibility of such an experiment at the prototypical heavy fermion compound CeCu₂Si₂ [see Willers et al.,  doi: 10.1103/PhysRevLett.109.046401].

In the present study the recently proposed orbital transition scenario [Pourovski et al., doi: 10.1103/PhysRevLett.112.106407] in heavy fermion compounds exhibiting two superconducting domes was addressed. The charge densities of CeCu₂Ge₂, a model system of this class, were probed with NIXS and modelled quantitatively with the full mulitplet code Quanty by M.W. Haverkort. Our NIXS results are well described by considering the same sequence of orbitals at all temperatures, thus putting into question whether orbital transitions are related to the formation of the second superconducting dome.

3. A complete high-to-low spin state transition of Co³⁺ ion in SrCo_0.5Ru_0.5O_3-d. [J.-M. Chen et al., J. Am. Chem. Soc. 136, 1514 (2014), doi: 10.1021/ja4114006]

The complex metal oxide SrCo_0.5Ru_0.5O_3‑δ possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy (XAS). We have discovered that Co³⁺ ion is purely high-spin at room temperature, which is unique for a Co³⁺ in an octahedral oxygen surrounding. We attribute this to the crystal-field interaction being weaker than the Hund’s rule exchange due to relatively large mean Co−O distances of 1.98(2) Å. In the FIgure the evolution of the Co K_β x-ray emission line is shown as a function of pressure from ambient up to 39.6 GPa at 300 K. The intensity of the K_β satellite peak is widely applied to estimate the local 3d spin moment of transition metal ions. The inset shows the so-called integrated absolute difference (IAD) as a function of pressure, which is proportional to the spin moment of the Co ion. A gradual high-to-low spin state transition is completed by applying hydrostatic pressure as high as 40 GPa. Across this spin state transition, the Co K_β emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. The highly debated intermediate spin state of Co³⁺ is absent in this material.

4. Spin-state order/disorder and metal–insulator transition in GdBaCo₂O_5.5: experimental determination of the underlying electronic structure [see Z. Hu et al., NJP 14, 123025 (2012), doi:10.1088/1367-2630/14/12/123025]

The layered cobaltates RBaCo₂O_5.5 (R = rare earth) composed of an equal number of Co³⁺O6 octahedra and Co³⁺O5 pyramids have attracted considerable attention of the scientific community in recent years because of their peculiar magnetic and transport properties, showing giant magneto-resistance as well as metal–insulator (MIT) and antiferro–ferro-paramagnetic transition phenomena. This MIT is commonly attributed to a sudden spin-state switch of Co³⁺ at octahedral site, but its modality is controversially discussed with scenarios including full or partial LS-IS or LS®HS state transitions without direct experimental evidence for any of the claimed spin-state transitions.

We have utilized O-K and Co-L_2,3 XAS spectroscopy to reveal that half of the Co³⁺ ions at the octahedral sites are in the low spin (LS) and the other half in the high spin (HS) state, while the Co³⁺ ions at the pyramidal sites are in the HS configuration. Upon increasing the temperature across the MIT, part of the LS octahedral Co³⁺ undergoes a spin-state transition into the HS configuration. We infer that this destroys the spin-state ordering and thus explains the decrease in resistivity. We observed that the band gap is reduced but not closed in the high-temperature phase.

5. Coupled valence and spin state transition in (Pr_0.7Sm_0.3)_0.7Ca_0.3CoO₃ [see F. Guillou et al., Phys. Rev. B 87, 115114 (2013), doi: 10.1103/PhysRevB.87.115114]

The coupled valence and spin state transition (VSST) taking place at T*∼89.3 K in (Pr_0.7Sm_0.3)_0.7Ca_0.3CoO₃ was investigated by XAS experiments carried out at the Pr-M_4,5, Co-L_2,3, and O-1s edges. At T <T* , we found that the praseodymium displays a mixed valence Pr3+ /Pr4+ with about 0.13 Pr⁴⁺/f.u., while all the Co³⁺ ions are in the LS state. At T ∼ T*, the sharp valence transition converts all the Pr⁴⁺ to Pr³⁺ with a corresponding Co³⁺ to Co⁴⁺ compensation. This is accompanied by an equally sharp spin state transition of the Co³⁺ from the low to an incoherent mixture of low and HS states. An involvement of IS state can be discarded for the Co³⁺. While above T* and at high temperatures the system shares rather similar properties as Sr-doped LaCoO₃, at low temperatures, it behaves much more like EuCoO₃ with its highly stable LS configuration for the Co³⁺. Apparently, the mechanism responsible for the formation of Pr⁴⁺ at low temperatures also helps to stabilize the Co³⁺ in the LS configuration despite the presence of Co⁴⁺ ions. We also found out that the Co⁴⁺ is in an IS state over the entire temperature range investigated in this study (10–290 K). The presence of Co³⁺ HS and Co⁴⁺ IS at elevated temperatures facilitates the conductivity of the material.

6. Electronic and spin states of SrRuO₃ thin films: An x-ray magnetic circular dichroism study [S. Agrestini et al., Phys. Rev. B 91, 075127 (2015), http://dx.doi.org/10.1103/PhysRevB.91.075127]

SrRuO₃ is one of the few known 4d transition-metal oxide ferromagnets with a Curie temperature as high as 160 K. Its rare physical properties have raised the interest of the scientific community during the last fifty years and recently also of the applied science sector due to its capacity for being used as an electrically conducting layer within magnetic heterostructure devices as used in storage technologies.

The systematic growth of thin films on (111) oriented SrTiO₃ substrates is very recent and first SQUID measurements suggest the surprising possibility of the stabilization of the high spin state (HS) S = 2 for Ru⁴⁺ ion. In this study, we have investigated strained SrRuO₃ thin layers on (001)- and (111)- oriented SrTiO₃ substrates and compared them with unstrained SrRuO₃ single crystals.

Using synchrotron light from the BOREAS beamline at the synchrotron ALBA, we wanted to determine if the compressive strain from the SrTiO₃ substrate can indeed induce a spin-state transition of the Ru⁴⁺ cations and if the Ru orbital magnetic moment is quenched (i.e., close to zero). To this end, X-ray magnetic circular dichroism (XMCD) studies of the Ru L_2,3 absorption edges were performed.

Results show that for the strained as well as the unstrained samples the Ru orbital moment is close to zero and that the Ru spin (and thus the associated spin magnetic moment) is close to the low-spin value of S=1, as could be verified using theoretical calculations on the shape of the Ru L_2,3 absorption spectra (see Figure).

From a comparison of the experimental with simulated spectra we could determine the effective crystal field. The hypothesis of a compressive strain-induced spin-state transition, as proposed in literature on the basis of SQUID measurements, can be ruled out as the stabilization of the high spin state with S = 2 would be too costly in energy.

7. Spectroscopic evidence for exceptionally high orbital moment induced by local distortions in α-CoV₂O₆ [N. Hollmann et al., Phys. Rev. B 89, 20110 (R) (2014), doi 10.1103/PhysRevB.89.201101]

We have examined the local magnetism of the Co spin chain compounds CoV₂O₆, which crystallizes in two different allotropic phases, α- and γ-CoV₂O₆ with x-ray magnetic circular dichroism. We have found an exceptionally high and a moderate orbital contribution to the magnetism in α-CoV₂O₆ and γ -CoV₂O₆, respectively. Full-multiplet calculations indicate that the differences in the magnetic behavior of α- and γ-CoV₂O₆ phases originate from different local distortions of the CoO₆ octahedra. In particular, the strong compression of the CoO₆ octahedra together with the unusually small octahedral crystal-field splitting in α-CoV₂O₆ lead to a strong mixture of t_2g and e_g orbitals which, via the local atomic Coulomb and exchange interactions, results in an exceptionally large orbital moment close to 2 µ_B.

8. Orthorhombic BiFeO3 [J.C. Yang, et al., Phys. Rev. Lett. 109, 247606 (2012), doi: 10.1103/PhysRevLett.109.247606]

Multiferroic BiFeO₃ (BFO) with its high ferroelectric Curie temperature of 820 Cº and Neel temperature of 370 Cº has received great research interest from a fundamental point of view but also because of its potential for applications to next-generation logic, memory, and high-frequency devices. Up to now, most studies focus on the ferroelectricity. The antiferromagnetism (AFM) is still far less studied because of the limited tools for investigating the orientation of the AFM axis in    thin films. Here using strain should give more insight into the rich spin physics of the AFM response in BFO thin.

Bulk BiFeO₃ has cycloidal spin structure and epitaxial BiFeO₃ films exhibit four AFM axis. The spin orientation in the BiFeO₃ film films can be determined by XLD at the Fe-L_2,3 edges. Figure (a) and (b) show the experimental Fe L_2,3 edge XAS spectra for linear polarized light with perpendicular ad parallel to the c-axis [E^c (red) and E||c (black)] for two different samples; (a) for BFO thin films on DyScO₃ (DSO c/a = 1.005) with very weak compressive strain and (b) for BFO thin films on orthorhombic NdScO3(110) (NSO c/a = 0.981) with in-plane tensile strain. There is distinct polarization dependence. The A and C intensities are larger for E^c and the intensities at B and D are larger for E||c in the BFO/DSO thin films while it is the opposite in BFO/NSO.

To extract the orientations of theses AFM axes, we have simulated the experimental spectra using configuration interaction cluster calculations. Figure (c)–(g) show the theoretical Fe-L_2,3 E^c (red line and E//c (black line) XAS spectra of BFO thin films as a function of the angle θ between the AFM axis and the c axis. We found that the best agreement between theoretical and experimental spectra occurs at θ = 34º for BFO/NSO and θ = 66º for BFO/DSO. Our results indicate that under tensile strain and compressive strain the AFM axis is close to c axis and ab-plane, respectively.

9. k=0 magnetic structure and absence of ferroelectricity in SmFeO₃ [C.-Y. Kuo et al., Phys. Rev. Lett. 113, 217203 (2014), doi: 10.1103/PhysRevLett.113.217203]

It is hard to rotate the spin orientation in antiferromagnetic materials by applying a magnetic field because there is no net magnetic moment. However, for a high spin Fe³⁺ ion with half-filled 3d orbitals, the orbital moment is strongly quenched resulting in a very weak magnetic anisotropy. Hence the spin direction could be altered by slightly modifying the crystal structure via changing temperatures. The antiferromagnetic SmFeO₃ is a nice example for the spin reorientation by change of temperature.

The Figure shows a considerable size of the XMLD signals between the electric field E//b and E//c in (a), between E//a and E//c in (b), but nearly no difference between E//a and E//b (c). The sign of the XMLD signals is reversed when going from 440 K to 490K in (a-b). The experimental spectra are nicely reproduced by the calculated spectra shown below experimental data with spins parallel to c- and a-axis at 440 K and 490 K, respectively.

10. Analysis of charge and orbital order in Fe₃O₄ by Fe L_2,3 resonant x-ray diffraction [A. Tanaka et al., Phys. Rev. B 88, 195110 (2013), doi: 10.1103/PhysRevB.88.195110]

Resonant X-ray Diffraction (RXD) is a useful tool to investigate systems with multiple sites, because of its site selectivity. Particularly, RXD at the transition-metal L_2,3 edge is expected to provide direct information on the 3d electronic state of transition-metal compounds, since the 2p to 3d dipole excitation occurs to its intermediate state. Here we exploit the azimuth angle, incident photon polarization, and energy dependence of the (001/2)_c and (001)_c reflection intensities to investigate the orbital and charge order below the Verwey transition temperature (T_v).

Magnetite (Fe₃O₄) is a magnetic mineral so abundant even known as a loadstone in the Greek era. Nowadays, in addition to its magnetic properties, considerable attention is drawn to a mysterious transition accompanied by a lattice deformation at temperature T_v ~125 K, where an abrupt increase of resistivity by two orders of magnitude on cooling was discovered by Verwey in 1939. Although extensive studies have been made both on experimental and theoretical sides even after more than 70 years, the mechanism of this Verwey transition still remains a highly controversial problem.

To elucidate charge and orbital order below the Verwey transition temperature, a thin layer of magnetite partially detwined by growth on the stepped MgO (001) substrates has been studied by means of soft RXD at the Fe L_2,3 resonance. The azimuth angle, incident photon polarization, and energy dependence of the (001/2)_c and (001)_c reflection intensities have been measured, and analysed using a configuration-interaction FeO₆ cluster model. The azimuth dependence of the (001/2)_c reflection intensities directly represents the space-group symmetry of the orbital order in the initial state rather than indirectly through the intermediate-state level shifts caused by the order-induced lattice distortions. From the analysis of the (001/2)_c reflection intensities, the orbital order in the t_2g orbitals of B sites below T_v is proved to have a large monoclinic deformation with the value of Re[F_xy]/Re[F_yz] ~2. This finding extremely limits possible theories on the Verwey transition, since the majority of them so far proposed do not assume charge or orbital orders with such a large monoclinic deformation. Furthermore, the incident photon polarization and energy dependence of the (001/2)_c reflection intensities cannot be explained by real-number orbital and charge orders predicted by the LDA + U and GGA + U band structure theories [Jeng et al., doi: 10.1103/PhysRevLett.93.156403; Leonov et al., doi: 10.1103/PhysRevLett.93.146404], but by the complex-number orbital order (COO) with a large monoclinic deformation [H. Uzu and A. Tanaka, doi: 10.1143/JPSJ.77.074711].