DyBaCo2O5.5 has shown a complex phase diagram, which is based on the interplay of different energy scales, related to magnetism, orbital ordering and for example Co spin-state transitions. For a detailed understanding of these fascinating materials it is therefore necessary to identify the order of the different energy scales. Small changes in the corresponding energy relations strongly influence the electronic structure and ground state properties, like low and high spin configurations, which have been controversially discussed in order to interpret the metal-to-insulator (MIT) transition in REBaCo2O5.5 (RE = rare earths). To clarify unambiguously the microscopic nature of the spin-state evolution associated with this MIT, we performed detailed temperature and angular dependent x-ray magnetic circular dichroism measurements in DyBaCo2O5.5 single crystals above and below the MIT and at the onset of the ferromagnetic phase. Anisotropic contributions of spin and orbital moments have been observed with an extremely high signal-to-noise ratio. We can identify a higher-spin- to lower-spin-state change across the MIT, which is in contrast to previous macroscopic experimental findings. Only the Co ions in octahedral environment are found to be in a reduced spin configuration in the high-temperature metallic state.

Fe3O4 has been investigated since decades, because of its magnetic and unusual electronic transport properties, which exhibit significant changes at the so-called Verwey transition, despite the promising high-spin polarization at the Fermi energy. We will show detailed and angular-dependent magnetic (XMCD) and nonmagnetic (XAS) absorption spectroscopy results of a high-quality single crystal, comparing cleaved and polished surfaces from the same single crystal. While the cleaved sample exhibits the full spin moment of magnetite, the same sample with a polished surface exhibits a magnetic moment reduced by a factor of 2, indicating a reduced surface magnetization. Angular-dependent XMCD measurements do not exhibit any significant variation. These results are consistent to recently published photoemission data.

A Comment on the Letter by D. J. Huang et al., Phys. Rev. Lett. 93, 077204 (2004). The authors of the Letter offer a Reply.Received 21 March 2005DOI:https://doi.org/10.1103/PhysRevLett.96.039701©2006 American Physical Society

We show detailed magnetic absorption spectroscopy results of an in situ cleaved high quality single crystal of magnetite. In addition the experimental setup was carefully optimized to reduce drift, self absorption, and offset phenomena as far as possible. In strong contradiction to recently published data, our observed orbital moments are nearly vanishing and the spin moments are quite close to the integer values proposed by theory. This very important issue supports the half metallic full spin polarized picture of magnetite.

Soft x-ray magnetic circular dichroism spectra and in situ element specific hysteresis loops of epitaxially grown CrO2 thin films have been investigated in the temperature range from 25 to 330 K for two different crystallographic projections. The quantitative temperature dependence of the microscopic magnetic moments (spin, orbital, and magnetic dipole term) and the magnetocrystalline anisotropy energy give strong evidence for the validity of the models by Bruno and by van der Laan, which describe the generation of the orbital moment and its relation to the magnetic anisotropy energy.

The temperature dependence of the O K-edge threshold has been investigated in a temperature range encompassing the Verwey transition. Both above and below T-V the O K-edge threshold changes linear with temperature whereas there is discontinuous energy shift of 13 meV at T-V. A comparison of our results to published low and high energy photoemission experiments suggests the presence of an asymmetric gap with respect to the Fermi level. This explains present fundamental differences between photoemission and infrared derived gap energies, and supports the polaronic image of the electrical conductivity of Fe3O4 above the Verwey transition temperature.

By the use of sum rules and X-ray magnetic circular dichroism (XMCD) integral spectral values, experimentally determined element specific spin- and orbital-magnetic moments could be extracted. Therefore, sum rules neglect all additional spectral shape information. On the basis of so called ground state moments and their spectral representations, XMCD spectra could be fitted. The gain of information due to this method is directly related to the analysis of the spectral shape. Simple and complex spectra, with many different observable spectral features, could be quantitatively analyzed. Different unoccupied parts of the bandstructure have been resolved and intuitively interpreted by the moment analysis procedure. Focusing on this fitting method, we will review recent applications to Iron-Garnets and CrO2 and show new results for Fe-, Mn-, and V-L2,3 XMCD spectra. In addition, spectral overlap between the L2 and L3 edges at the light transition metal site could be handled by this method, and quantitative magnetic moments have been extracted from the spectra. This is the basis for a possible future element specific renormalization technique, used for the whole series of the transition metals, which is very important at the light transition metal site.

The role of delocalization and hybridization in complex magnetic oxides has been investigated by magnetic oxygen K-edge absorption of circular polarized soft X-rays in epitaxial grown CrO2 as a function of the azimuthal angle at grazing incidence. Unusual strong variations in the typically small X-ray magnetic circular dichroism (XMCD) signal have been observed. Those angular dependencies of the typical CrO2 oxygen K-edge XMCD signal could be quantitatively interpreted in terms of an induced anisotropic Cr 3d orbital magnetism for different spectral regions of the unoccupied density of states. The results strongly suggest a delocalized non-ionic magnetic behavior of the conduction electrons.

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.

The total electron yield current is strongly influenced by external magnetic fields. As known before, this side effect can be slightly reduced by applied external bias voltages increasing the total sample drain current nearly up to saturation. Nevertheless those effects are not perfectly reduced in almost all X-ray Magnetic Circular Dichroism (XMCD) applications and are more prominent in very small XMCD signals, like O K edge spectra. We show that asymmetries in the total electron yield field response will result in XMCD offset signals, which are strongly photon energydependent and follow the nonmagnetic absorption signal. A simple but effective method to prevent those offset signals, is the use of asymmetric magnetic fields. A quantitative analysis and a numerical reduction method for those offset signals are shown.

We have investigated the magnetism of Holmium-Iron-Garnet (Ho3Fe5O12) at the Ho M4,5 - the Fe L2,3 - and the O K - edges. As expected switching of the sub-lattice magnetization is observed at the compensation temperature. We will give detailed analysis ofthe Ho and Fe XMCD signals, using sum rules. Fe dichroism is analyzed in terms of 3d ground state moments and compared to Gadolinium-Iron-Garnet (Gd3Fe5O12). Contributions of octahedral and tetrahedral Fe sites could be separated and analyzed quantitatively. At the oxygen K edge two different structures are observable. Those structures show different temperature dependencies. Therefore we address these features separately to Fe and Rare Earth contributions.