April 15, 2009

The properties of iron nanostructures prepared by shallow incidence molecular beam epitaxy on faceted ?-Al2O3 (100) (m plane) are described. Depending on the angle of deposition with regard to the sapphire surface, the morphology of the structures reaches from pearl-necklet-like strung nanodrops to laced nanowires. Crystallographic measurements reveal at least four epitaxial close to (211)-oriented twins. Magnetization measurements performed at room temperature in a vibrating sample magnetometer and magneto-optical Kerr rotation measurements reveal an easy axis of magnetization close to the facet ridges. Still, the shape of the hysteresis loops depends strongly on the morphology of the samples. The magnetization reversal process can be described by the formation of a magnetic vortex structure

Spectroscopic fingerprints for charge localization in the organic semiconductor (DOEO)4[HgBr4]·TCE

Changes of the electronic structure accompanied by charge localization and a transition to an antiferromagnetic ground state were observed in the organic semiconductor (DOEO)4 [HgBr4 ]·TCE. Localization starts in the temperature region of about 150 K and the antiferromagnetic state occurs below 60 K. The magnetic moment of the crystal contains contributions of inclusions (droplets), and individual paramagnetic centers formed by localized holes and free charge carriers at 2 K. Two types of inclusions of 100–400 nm and 2–5 nm sizes were revealed by transmission electron microscopy. Studying the temperature-and angular dependence of electron spin resonance (ESR) spectra revealed fingerprints of antiferromagnetic contributions as well as paramagnetic resonance spectra of individual localized charge carriers. The results point on coexistence of antiferromagnetic long and short range order as evident from a second ESR line. Photoelectron spectroscopy in the VUV, soft and hard X-ray range shows temperature-dependent effects upon crossing the critical temperatures around 60 K and 150 K. The substantially different probing depths of soft and hard X-ray photoelectron spectroscopy yield nformation on the surface termination. The combined investigation using complementary methods at the same sample eveals the close relation of changes in the transport properties and in the energy distribution of electronic states

Design of magnetic materials: Co2_2Cr1−x_{1-x}Fex_{x}Al

Doped Heusler compounds Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure of the ordered, doped Heusler compound Co$_2$Cr$_{1-x}$Fe$_{x}$Al ($x=n/4, n=0,1,2,3,4)$ was calculated using different types of band structure calculations. The ordered compounds turned out to be ferromagnetic with small Al magnetic moment being aligned anti-parallel to the 3d transition metal moments. All compounds show a gap around the Fermi-energy in the minority bands. The pure compounds exhibit an indirect minority gap, whereas the ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism (MCD) in X-ray absorption spectra was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. The experimentally observed reduction of the magnetic moment of Cr can be explained by Co-Cr site-disorder. The presence of the gap in the minority bands of Co$_2$CrAl can be attributed to the occurrence of pure Co$_2$ and mixed CrAl (001)-planes in the $L2_1$ structure. It is retained in structures with different order of the CrAl planes but vanishes in the $X$-structure with alternating CoCr and CoAl planes.Comment: corrected author lis