Magnetic versus crystal field linear dichroism in NiO thin films

We have detected strong dichroism in the Ni $L_{2,3}$ x-ray absorption spectra of monolayer NiO films. The dichroic signal appears to be very similar to the magnetic linear dichroism observed for thicker antiferromagnetic NiO films. A detailed experimental and theoretical analysis reveals, however, that the dichroism is caused by crystal field effects in the monolayer films, which is a non trivial effect because the high spin Ni $3d^{8}$ ground state is not split by low symmetry crystal fields. We present a practical experimental method for identifying the independent magnetic and crystal field contributions to the linear dichroic signal in spectra of NiO films with arbitrary thicknesses and lattice strains. Our findings are also directly relevant for high spin $3d^{5}$ and $3d^{3}$ systems such as LaFeO$_{3}$, Fe$_{2}$O$_{3}$, VO, LaCrO$_{3}$, Cr$_{2}$O$_{3}$, and Mn$^{4+}$ manganate thin films

Prediction of huge X-ray Faraday rotation at the Gd N_4,5 threshold

X-ray absorption spectra in a wide energy range around the 4d-4f excitation threshold of Gd were recorded by total electron yield from in-plane magnetized Gd metal films. Matching the experimental spectra to tabulated absorption data reveals unprecedented short light absorption lengths down to 3 nm. The associated real parts of the refractive index for circularly polarized light propagating parallel or antiparallel to the Gd magnetization, determined through the Kramers-Kronig transformation, correspond to a magneto-optical Faraday rotation of 0.7 degrees per atomic layer. This finding shall allow the study of magnetic structure and magnetization dynamics of lanthanide elements in nanosize systems and dilute alloys.Comment: 4 pages, 2 figures, final version resubmitted to Phys. Rev. B, Brief Reports. Minor change

Electronic structure of some 3D transition-metal pyrites

Bremsstrahlung Isochromat spectra of FeS2, NiS2, NiS1.2Se0.8 and NiSe2 are reported. These are the first direct experimental evidence for a sharp antibonding p-like state above the Fermi level. A comparison is made with experimental results in the literature. For FeS2, band-structure calculations are also presented; these are in good agreement with experiment. We show that it is necessary in this calcualtion to include 'empty spheres' when using a muffin-tin approximation. This is because the pyrite crystal structure is far from close packed. Furthermore, we briefly discuss the electrical and magnetic properties of the compounds.

Temperaturabhaengigkeit der Energieluecke in Hochtemperatursupraleitern Schlussbericht

The electronic structure of high-Tc superconductors was studied by means of a novel arrangement of equipment allowing high-resolution photoemission measurement at low temperatures. An energy gap (of about 5 meV) was found in textured, laser-ablated YBaCuO films and in sintered specimens of BiSrCaCuO, the gap being smaller than a B.C.S. gap. In in-situ cleaved monocrystals of the system BiSrCaCaO we observed a relatively large energy gap (20-25 meV), which is in line with results published by other authors. What is new is the temperature dependence: the observed two types of temperature dependence obviously are determined by the emission angle or the k-vector of the initial state, respectively, the first type being a continuous decrease of the energy gap with temperature approaching the transition temperature, and the second type is a gap independent of temperature. The latter case is characterized by a quasiparticle peak in the energy distribution curve, the peak growing in width linearly with temperature. The results were built into a theoretical model and contributed a new approach to exploring and understanding the process of energy gap formation. (orig.)Mit Hilfe einer neu aufgebauten Apparatur zur hochaufgeloesten Photoemission bei tiefen Temperaturen wurde die Elektronenstruktur von Hochtemperatursupraleitern untersucht. Es wurde in texturierten laserablatierten YBaCuO Schichten und in BiSrCaCuO Sinterproben bei tiefer Temperatur eine Energieluecke (ca. 5 meV) gefunden, die kleiner als eine BCS-Luecke ist. Bei in-situ gespaltenen Einkristallen aus BiSrCaCuO beobachten wir in Uebereinstimmung mit anderen Autoren die relativ grosse Energieluecke (20-25 meV). Neu sind die beobachteten Temperaturabhaengigkeiten. Offenbar in Abhaengigkeit vom Emissionswinkel bzw. vom k-Vektor des Anfangszustands beobachten wir zwei Formen der Temperaturabhaengigkeit, eine kontinuierliche Abnahme der Energieluecke bei Annaeherung an die Uebergangstemperatur bzw. eine temperaturunabhaengige Luecke. Im letzteren Falle ist die Energieverteilungskurve durch einen Quasiteilchenpeak gekennzeichnet, dessen Breite linear mit der Temperatur anwaechst. Die Ergebnisse haben durch Zusammenarbeit mit Theoretikern zu einem neuen Ansatz zum Verstaendnis der Energieluecke gefuehrt. (orig.)SIGLEAvailable from TIB Hannover: D.Dt.F.QN1(7,18) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman