H. Ott

Direct observation of t2g orbital ordering in magnetite

Using soft-x-ray diffraction at the site-specific resonances in the Fe L2,3 edge, we find clear evidence for orbital and charge ordering in magnetite below the Verwey transition. The spectra show directly that the (001/2) diffraction peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+ sites and the (001) by a spatial modulation of the t2g occupation.

Magnetic depth profiles from resonant soft x-ray scattering : Application to Dy thin films

We employ the strong variation of the photon penetration depth across an electronic resonance in soft x-ray scattering for a depth resolved study of an antiferromagnetic (AFM) thin film. We directly observed the development of the helical AFM structure in thin Dy(001) films on W(110) across the temperature-induced phase transition from ferromagnetic to helical AFM order.

Resonant magnetic X-ray scattering from ultrathin Ho–metal films down to a few atomic layers

Abstract The magnetic structure of ultrathin Ho–metal films grown on W(110) was studied by resonant magnetic X-ray scattering at the L3 and M5 resonances. For Ho films down to 14 monolayer (ML) thickness, a bulk-like helical antiferromagnetic structure is observed. For a 10-ML thick film, an altered magnetic structure and enhanced layer spacing is found.

Intrinsic and extrinsic x-ray absorption effects in soft x-ray diffraction from the superstructure in magnetite

We studied the (001/2) diffraction peak in the low-temperature phase of magnetite (Fe3O4) using resonant soft x-ray diffraction (RSXD) at the Fe-L2,3 and O-K resonance. We studied both molecular-beam-epitaxy (MBE) grown thin films and in-situ cleaved single crystals. From the comparison we have been able to determine quantitatively the contribution of intrinsic absorption effects, thereby arriving at a consistent result for the (001/2) diffraction peak spectrum. Our data also allow for the identification of extrinsic effects, e.g. for a detailed modeling of the spectra in case a dead surface layer is present that is only absorbing photons but does not contribute to the scattering signal.

Magnetic x-ray scattering at the M{sub 5} absorption edge of Ho

Magnetic x-ray scattering from thin Ho-metal films at M{sub 5} resonance reveals atomic scattering lengths up to 200r{sub 0}--i.e., of the same order of magnitude as predicted theoretically by Hannon et al. [Phys. Rev. Lett. 61, 1245 (1988)]. The photon-energy dependence of first- and second-order magnetic satellites allows a straightforward identification of circular and linear dichroic contributions. A direct comparison to magnetic neutron scattering demonstrates the potential of the method for studies of complex magnetic structures in ultrathin films and highly diluted materials.

Magnetic effects in the band structure of ferromagnetic and antiferromagnetic lanthanide-metal films

Abstract Systematic magnetic effects in the band structure of the heavy lanthanide metals Gd, Tb, Dy, and Ho were studied by angle-resolved photoemission from epitaxial films. The temperature-dependent exchange splitting exhibits a different behavior for delocalized and localized band states. For the delocalized Δ 2 bulk band states, a Stoner-like temperature dependence of the splitting is observed in ferromagnetic and helical antiferromagnetic phases. A more localized oxygen-induced surface state shows a temperature-independent contribution to the splitting. All observed magnetic splittings scale with the 4f spin moment.

Spectroscopy of Stripe Order inLa1.8Sr0.2NiO4Using Resonant Soft X-Ray Diffraction

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.