M. Rotter

A miniature capacitance dilatometer for thermal expansion and magnetostriction

A very small capacitive sensor for measuring thermal expansion and magnetostriction of small and irregular shaped samples has been developed. A capacitive method with tilted plates is used. The tilted plate capacitance formula is used for the calculation of the capacitor gap, the calibration is performed by measuring the signal of a standard material. The active length of the sample can be less than 1 mm. The absolute resolution is about 1 A. All mechanical connections of the dilatometer are carried out by tiny Cu–Be springs, enabling the small force on the sample to be adjusted (50–500 mN) and no additional sample fixing is necessary. The cell has been tested in the temperature range 0.3–200 K and in static magnetic fields up to 15 T. The zero signal of the dilatometer has been determined by measuring a silver sample. The correct operation and reproducibility has been verified by measuring the thermal expansion of Cu. The thermal expansion and magnetostriction of a DyCu2 single crystal has been determine...

Ternary clathrates Ba–Zn–Ge: phase equilibria, crystal chemistry and physical properties

The present paper describes the formation, phase relations at subsolidus temperatures and at 800 °C, crystal chemistry and physical properties of a series of ternary clathrates as part of the solid solution , derived from binary with a solubility limit of 8 Cd per formula unit at 800 °C. Structural investigations in all cases confirm cubic primitive symmetry with a lattice parameter a≈1.1 nm, consistent with the space group type . Both the temperature dependent x-ray spectra and the heat capacity define a low-lying, almost localized, phonon branch. Studies of transport properties show electrons to be the majority charge carriers in the systems. As the Cd content increases, the system is driven towards a metal-to-insulator transition, causing , for example, to show metallic behaviour at low temperatures while at high temperatures semiconducting features become obvious. A model based on a gap of the electronic density of states slightly above the Fermi energy perfectly explains such a scenario. Thermal conductivity exhibits a pronounced low temperature maximum, dominated by the lattice contribution, while at higher temperatures the electronic part becomes more important.

Structure and physical properties of type-I clathrate solid-solution Ba 8 Pt x Ge 46 − x − y ◻ y ( ◻ = vacancy )

Formation, crystal chemistry, and physical properties were investigated for the solid-solution Ba{sub 8}Pt{sub x}Ge{sub 46-x-y}{open_square}{sub y} ({open_square} is a vacancy) deriving from binary clathrate Ba{sub 8}Ge{sub 43}{open_square}{sub 3} with a solubility limit of {approx}3.5 Pt atoms/f.u. at T=800 deg. C. Structural investigations throughout the homogeneity region confirm isotypism with the cubic primitive clathrate type-I structure (space group type Pm3n) and lattice parameters ranging from a=1.0657(2) nm for Ba{sub 8}Ge{sub 43}{open_square}{sub 3} to a=1.0752(2) nm for Ba{sub 8}Pt{sub 3.5}Ge{sub 41.5}{open_square}{sub 1.0}. Phase relations for the region concerning the clathrate solution were derived at subsolidus temperatures as well as at 800 deg. C. Transport properties evidence electrons as the majority charge carriers in the system with a slight dependency on the Pt content. The system is located close to a semiconducting regime with a gap in the electronic density of states of a few thousand K. No low temperature maximum is obvious from thermal conductivity which is dominated by the lattice contribution. Thermal conductivity furthermore documents a high efficiency of phonon scattering on vacancies.

Autocorrelation analysis of bone structure

We propose a method called spatial autocorrelation analysis (SACA) to determine the spatial anisotropy of the trabecular bone in order to investigate osteoporosis. For demonstrating the potential of SACA we first evaluate the method on rectangular, simulated test patterns as a simple model for the anisotropic pore structure of the bone. As a next step towards biomedical application, photographic reference images of human vertebral bone were investigated by SACA. Osteoporotic bone structure could be clearly differentiated from non‐osteoporotic sample images. Moreover, for demonstration of the applicability and potential of the method for in vivo characterization of osteoporosis, the microstructure of the human calcaneus was investigated by MR‐microimaging on a young healthy male subject and an osteoporotic female. The measurements were performed using a high‐field (3T) whole‐body MR tomograph equipped with a special, strong head gradient system. The signal was acquired with a surface coil mounted on an in‐house‐built device for convenient immobilization of the subjects foot. Using a 3D gradient echo sequence a resolution of 0.254 × 0.254 × 2.188 mm3 was achieved in vivo. Selected images were inverted, gradient corrected for the inhomogeneous but sensitive detection by the surface coil, and subsequently analyzed by SACA. The anisotropy of bone structure detected by SACA is a possible candidate for noninvasive determination of the osteoporotic status, potentially complementing standard bone mineral density measurements. J. Magn. Reson. Imaging 2001;14:87–93.

Magnetostriction in rare-earth based antiferromagnets

Magnetostriction in systems with rare-earth elements is a very general topic of interest because it characterizes the electronic magnetism of compounds as well as the interaction of the magnetic and the lattice system. The magnetostrictive behaviour of classical ferromagnets is widely investigated. The interpretation of the data is based on domain dynamics, crystal field effects and other magnetoelastic mechanisms, which are extremely difficult to separate. Therefore, in the last few years there has been great activity in the area of magnetostriction of antiferromagnets. In contrast to ferromagnets, in antiferromagnets a large variety of magnetic structures can be stabilized at different temperatures and magnetic fields. This fundamental property of antiferromagnets allows one to separate and distinguish the different mechanisms of magnetostriction. New classes of magnetic compounds with antiferromagnetic order, for example the borocarbides and some special manganites, have also focused research efforts on the magnetostrictive behaviour in order to gain a more complex picture of these materials. In particular, changes of symmetry caused by lattice distortions attracted great interest. Moreover, some antiferromagnetic compounds with giant magnetostrictive effects of more than 1% have been found and the number of publications in this area is growing. The aim of the present review is to summarize experimental data as well as theoretical models concerning spontaneous and forced magnetostriction in antiferromagnetic systems, where the magnetic properties are dominated by the rare-earth magnetism. Above this, the available data, which were acquired with different motivations and therefore are of heterogeneous character, are systematized. After a summary of experimental methods the standard model of rare-earth based magnetostriction is reviewed. Then experimentally determined expansion and magnetostriction data of the pure rare-earth metals are discussed followed by selected rare-earth compounds and giant magnetostriction materials. The potential of a theoretical model which takes into account the crystal field effects as well as the (anisotropic) exchange striction is demonstrated in the interpretation of some of the experimental data.

Dynamical matrix diagonalization for the calculation of dispersive excitations

The solid state exhibits a fascinating variety of phases, which can be stabilized by the variation of external parameters such as temperature, magnetic field and pressure. Until recently, numerical analysis of magnetic and/or orbital phases with collective excitations on a periodic lattice tended to be done on a case-by-case basis. Nowadays dynamical matrix diagonalization (DMD) has become an important and powerful standard method for the calculation of dispersive modes. The application of DMD to the interpretation of inelastic neutron scattering (INS) data on dispersive magnetic excitations is reviewed. A methodical survey of calculations employing spin-orbit and intermediate coupling schemes is illustrated by examples. These are taken from recent work on rare earth, actinide and transition metal compounds and demonstrate the application of the formalism developed.

Magnetic excitations in multiferroic LuMnO3 studied by inelastic neutron scattering

from inelastic neutron scattering, magnetization and thermal expansion mea-surements. We measured the magnon dispersion along the main symmetry directions and used thisdata to determine the principal exchange parameters from a spin-wave model. An analysis of themagnetic anisotropy in terms of the crystal field acting on the Mn is presented. We compare theresults for LuMnO

Structural Stability of LaCu2 and YCu2 Studied by High-Pressure X-Ray Diffraction and ab initio Total Energy Calculations.

LaCu2 is the only compound among the RCu2 series (R from La to Lu, and Y) which crystallizes in the hexagonal AlB2 -type structure, whereas the other compounds show the orthorhombic CeCu2 -type structure. In agreement with ab initio calculations our high-pressure x-ray diffraction experiments show that LaCu2 transforms at relatively low pressures to the CeCu2 -type structure, which can be regarded as a low-symmetry variant of the AlB2 -type structure. To investigate the stability of the CeCu2 -type structure we performed high-pressure x-ray diffraction experiments on YCu2 using argon as the pressure transmitting medium. These experiments showed in contrast to an earlier study that the CeCu2 structure is stable at least up to the highest attained pressure of 35 GPa. The tendency for amorphization at pressures above 10 GPa, as observed in the earlier study, was probably due to the non-hydrostatic conditions of the pressure transmitting medium, showing that the CeCu2 -type compounds are very sensitive to non-hydrostatic compression. A transition to the MgCu2 -type structure (cubic Laves phase) at about 28 GPa, as predicted by the ab initio calculations, has not been observed.

Magnetic structure and field-dependent properties of CeCu5

A study of the magnetic properties of the hexagonal compound CeCu5 reveals antiferromagnetic order below TN=3.9 K. The magnetic structure comprises moments along the c-axis with a propagation vector k=(0,0, 1/2 ). The presence of the Kondo interaction reduces the magnitude of the moments from about 0.42 mu B, expected for a mod +or- 1/2 ) doublet, to about 0.36 mu B, observed from elastic neutron scattering experiments.

The influence of the crystal field on the anisotropic thermal expansion in ErCu2 and NdCu2

The anisotropic thermal expansion parameters a(T), b(T) and c(T) of some RECu2, intermetallics (RE=Y, Er, Nd) has been measured in the temperature range from 400 K down to 4.2 K using X-ray powder diffraction. The influence of the crystal field on the thermal expansion in ErCu2 and NdCu2 has been determined by comparing the thermal expansion of the nonmagnetic isostructural YCu2 with the Er and Nd compounds. The data thus obtained are described theoretically using a set of crystal-field parameters deduced from the analysis of inelastic neutron diffraction data.