M. Doerr

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.

High resolution magnetostriction measurements in pulsed magnetic fields using fiber Bragg gratings

We report on a new high resolution apparatus for measuring magnetostriction suitable for use at cryogenic temperatures in pulsed high magnetic fields which we have developed at the Hochfeld-Magnetlabor Dresden. Optical fiber strain gauges based on fiber Bragg gratings are used to measure the strain in small (approximately 1 mm) samples. We describe the implementation of a fast measurement system capable of resolving strains in the order of 10(-7) with a full bandwidth of 47 kHz, and demonstrate its use on single crystal samples of GdSb and GdSi.

First-order structural transition in the magnetically ordered phase of Fe1.13Te

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and high-resolution synchrotron x-ray powder diffraction investigations of single crystals Fe(1+y) Te (0.06 = 0.13. Most strikingly, all measurements on identical samples Fe(1.13)Te consistently indicate that, upon cooling, the magnetic transition at T(N) precedes the first-order structural transition at a lower temperature T(s). The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic c axis display a small distortion close to T(N) due to a lattice striction as a consequence of magnetic ordering, and a much larger change at T(s). The lattice symmetry changes, however, only below T(s) as indicated by powder x-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.

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.

Frustration in R2PdSi3 (R = Tb,Er) compounds: spin-glass or magnetic short range order? Neutron diffraction studies

Measurements of macroscopic magnetic properties of the isostructural compounds Tb2PdSi3 and Er2PdSi3 show an additional phase transition below the ordering temperature. The behaviours of the two compounds are similar and are interpreted as spin-glass-like phase transitions in the literature. In this contribution, we present detailed neutron diffraction studies of the mechanism of the spin-glass-like phase transition on a microscopic scale. We find a fundamental difference between the two compounds in their magnetic structures and, therefore, the spin-glass-like phase transitions. In the Tb2PdSi3 compound an additional antiferromagnetic short range ordered phase is found within the long range ordered phase. The appearance of the short range order is linked to the spin-glass-like transition. In contrast, the Er2PdSi3 compound shows only long range order. However, the antiferromagnetic order undergoes a modification within the ordered state. The temperature of the transition matches the spin-glass-like transition.

Variation of the intersublattice exchange coupling due to hydrogen absorption in Er2Fe14B: a high-field magnetization study

Single crystals of a series of hydrides Er2Fe14BHx (x ≤ 2.5) have been produced and studied in pulsed magnetic fields up to 60 T. The magnetization curve of Er2Fe14B in the easy direction [100] features a stepwise anomaly at about 45 T, corresponding to the first-order phase transition. A similar magnetization jump is also present in the curve along [110], but at a higher field, ∼52 T. The [100] data of the parent and hydrogen-charged Er2Fe14BHx with x = 0.25, 1.5, 2.5 were used to deduce the Er-Fe molecular field Hmol as a function of hydrogen content x. After moderate initial decrease, Hmol(x) drops abruptly above x = 1.5. Hydrogenation results in a 12% reduction of the Er-Fe molecular field in Er2Fe14BH2.5 as compared to Er2Fe14B. For reference, influence of hydrogen on Hmol in an Er2Fe17-H system is also presented.

Magnetic-field-induced band-structure change in CeBiPt.

We report on a field-induced change of the electronic band structure of CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic fields. Above approximately 25 T, the charge-carrier concentration increases nearly 30% with a concomitant disappearance of the Shubnikov-de Haas signal. These features are intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt the Fermi surface remains unaffected. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Magnetoelastic effects in ErNi2B2C single crystal: probing the H-T phase diagram

Using a capacitive dilatometer we investigated the magnetoelastic behaviour of single-crystalline ErNi2B2C in the temperature range 1. 8K T 16 K and for external magnetic field in the tetragonal ab-plane µ0H 6T . For T TN = 6.4 K the longitudinal magnetostriction coefficient is negative, whereas the transverse one is positive. The thermal evolution of the lattice distortion e γ = (λ⊥ − λ� )/ √ 2 is in agreement with that of the previously reported spontaneous tetragonal-to-orthorhombic distortion determined by neutron diffraction. The behaviour is influenced by the distribution of the magnetic domains in the crystal. The H –T phase diagram, constructed from the magnetostriction curves, shows the well-known cascade of metamagnetic transitions between several phases (antiferromagnetic AF1, ferrimagnetic F1, F2) in the field range 0 <µ 0 H< 2.1 T below TN . On the basis of a Hamiltonian consisting of an exchange, a crystal-field, a Zeeman, and a magnetoelastic term, we were able to reproduce reasonably well the H –T phase diagram as well as various forced magnetostriction curves. (Some figures in this article are in colour only in the electronic version)

Magnetostructural irreversibilities in R5Ge3 (R = Gd, Nd) intermetallics

Magnetoelastic phenomena of irreversible character were investigated on the rare-earth germanides Nd5Ge3 and Gd5Ge3 which are prominent members of the hexagonal R5Ge3 series. Both compounds order antiferromagnetically at 52 K and 76 K, respectively. A strong magnetostructural irreversibility (i.e. a relative length change of about 10-3 which can be induced by a magnetic field and stays stable after ramping down the field) was detected for both samples by measurements of magnetostriction and thermal expansion using capacitive dilatometry. This transition can be reversed by heating the sample near the ordering temperature. Additional experiments by X-ray and neutron scattering at Gd5Ge3 in order to analyze the effect itself and the structural reversal on an atomistic scale indicate the polymorphic (or metastable) magnetic character (e.g. several propagation vectors (0 0 0.4) and (0.3 0.3 0) were found) which allow to induce strong lattice distortions by an external magnetic field via the magnetoelastic coupling.