H. Böhm

Magnetic properties of GdPdAl single crystals

Single crystals of the intermetallic ternary compound GdPdAl, crystallizing in the hexagonal ZrNiAl-type structure, were grown by the Czochralski method. Lattice parameters, electrical resistivity, magnetic susceptibility give evidence of an isostructural phase transition from a high temperature modification HTM I phase to HTM II phase at 180 K. The jump related to this transition causes a decrease of the lattice parameter a of 0.7% and an increase of the lattice parameter c of 1.3% during the cooling process. These changes are responsible for some microcracks in the single crystal. Besides the structural phase transition, magnetic ordering of the Gd sublattice is observed at 48 K. At about 20 K a spin reorientation process may occur. The XPS measurements of GdPdAl reveal a dominating contribution of d electrons at the Fermi level. For comparison the electronic structure of TbPdAl was also examined. Both compounds reveal formation of a narrow Pd 4d band. Further information on changes of the electronic structure at the isostructural phase transition were obtained from the temperature dependence of the ESR linewidth.

Isostructural phase transition in the GdPdAl single crystals

At 180 K, single crystals of the intermetallic ternary compound GdPdAl undergo an isostructural phase transition from the high temperature hexagonal ZrNiAl-type structure to the low temperature hexagonal structure. The lattice parameters for temperatures neighbouring the phase transition exhibit a hysteresis on heating and cooling which is indicative of a first-order phase transition. In order to understand this phase transition the crystal structure was determined on a single crystal for a few degrees below and above 180 K. Also X-ray photoelectron spectra were measured at 192 K and 173 K and compared with spectra simulated with the help of band structure calculations.

Magnetic properties of DyMn2 single crystals

Abstract Single crystals of the DyMn 2 compound were grown by the Czochralski method from a levitated melt. The electrical resistivity exhibits a magnetic transition at 40 K, a second transition at 25 K, a cubic temperature dependence at low temperatures and a saturation effect above 250 K. The magnetization in strong magnetic fields, and the DC and AC magnetic susceptibility measured along the principal crystallographic directions show an ordering of the dysprosium moments to a spin-canted magnetic structure at about 40 K and a spin reorientation below 25 K. The temperature dependence of the lattice parameter shows a lattice contraction below 40 K.

Magnetic properties of ErMn2 single crystals

Abstract Single crystals of the ErMn 2 compound were grown by the Czochralski method from a levitated melt. The thermal dependence of the lattice parameters exhibits a magnetovolume effect along the a axis, a contraction for both the c axis and unit cell volume below 20 K. The thermal dependence of the electrical resistivity exhibits a magnetic transition at 23 K. A second transition occurs at 13 K. Moreover, for ρ ( T ) a negative curvature above the ordering temperature and a saturation effect above 200 K occurs. The magnetization in strong magnetic fields, the d.c. and a.c. magnetic susceptibilities measured along the principal crystallographic directions show ordering of the erbium moments to a spin-canted magnetic structure at about 15 K and a spin reorientation about 10 K.

X-ray investigation and discussion of the magnetostriction of Gd3T (T = Ni, Rh, Irx) single crystals

The variations of the lattice parameters of Gd3T (T = Ni, Rh, Irx) single crystals with temperature were measured in the range 10–300 K. The compounds with Ni and Rh crystallize in the orthorhombic space group Pnma, but the compound with Ir crystallizes as Gd5Ir2 in the monoclinic space group A2/a. The three compounds exhibit an anomalous anisotropic spontaneous magnetostriction below the magnetic ordering temperature.

The surface superstructures in niobium disulfide and diselenide intercalated by Cu, Co and Fe

Abstract A series of intercalated transition-metal dichalcogenides with nominal compositions AxNbCh2, with A=Cu, Co, Fe, Ch=S, Se and x=0.25, 0.33, were studied by scanning tunneling microscopy (STM). Evidence is given that intercalation influences the STM images. Intercalated Cu atoms occupy tetrahedral interstices in the van der Waals gaps, forming pairs with Nb atoms and adopting the 2Hb–MoS2 stacking of the host structure. Effects associated with non-stoichiometry, such as formation of agglomerates and incomplete ordering of the intercalated Cu are frequently observed. Fe and Co atoms occupy octahedral interstices in the gaps, form chains with alternating Nb and stabilize the 2Ha–NbS2 polytype. In case of (Fe,Co)xNbS2 2a0×2a0 and √3a0×√3a0 hexagonal superstructures are formed for x=1/4 and x=1/3, respectively, while in case of CuxNb(S,Se)2 only the x=1/4 fraction orders into a 2a0×2a0 hexagonal superstructure at room temperature. In all cases the superstructures are visible at the surfaces and are attributed to changes in the local densities of states caused by a charge transfer between the intercalated and the surface atoms. Typically these superstructures were observed to change during prolonged scanning, probably as a result of tip-induced fluctuations in the charge density. The experimental STM images are compared to simulated images calculated with the extended Huckel tight binding approximation.

Electronic structure and magnetic properties of a HoPdAl single crystal

Abstract Lattice parameters, electrical resistivity and magnetic susceptibility temperature dependence of HoPdAl, crystallizing in the ZrNiAl-type structure, show no isostructural phase transition in contrast to GdPdAl and TbPdAl. Effective magnetic moment agrees with the free ion value of Ho 3+ . Palladium does not contribute to the density of states at the Fermi level. The holmium sublattice orders at 16 K.

The surface and domain structure of NbTe2

Scanning tunneling microscopy of freshly cleaved NbTe2 van der Waals surfaces reveals that Te vacancies regularly appear in the topmost chains of the Te surface layer. As a result of reduced symmetry with regard to the parent CdI2 structure microdomains of three possible orientational variants are formed, separated by ortho (601)- or meta (1201)boundaries, while the para (1801)-boundaries are not detected. Regions close to the boundaries are often modulated with periodicities of the adjacent domains, confirming that the boundaries in neighboring Te–Nb–Te triple layers do not necessarily coincide. r 2002 Elsevier Science B.V. All rights reserved.

Electronic structure and magnetic properties of GdRhAl single crystals

Abstract The temperature dependence of the lattice parameters, electrical resistivity, DC and AC magnetic susceptibility of GdRhAl, which crystallizes in a TiNiSi type structure, show no isostructural phase transition, contrary to GdPdAl crystallizing in a ZrNiAl type structure; the ordering temperature was 30 K. Below this temperature a weak reorientation of the magnetic moment was found in measurements of the magnetization in strong magnetic fields. The effective magnetic moment, slightly enhanced in comparison with the free ion value of Gd 3+ , was 8.06 μ B . ESR measurements reveal a small contribution of the Rh 4d electrons to the valence band and XPS measurements show a narrowing of the Rh 4d band in the compound in relation to pure Rh. The Rh 4d electrons preserve band character in the compound and the hybridization of the Gd 5d, Rh 4d and Al 3p electron states determines the magnetic properties.

Magnetic properties of the TbMn2 single crystals

Abstract Single crystals of TbMn 2 were grown by the Czochralski method from a levitated melt. Results of electrical resistivity, magnetization in strong magnetic fields along the principal crystallographic directions, AC and DC magnetic susceptibility and the temperature dependence of the lattice parameter are presented.