E. Gratz

Isotropic and anisotropic magnetoelastic interactions in heavy and light RCo2 Laves phase compounds

The thermal expansion and anisotropic magnetostriction of the RCo2 Laves phases were studied in the temperature range 4-500 K using the X-ray powder diffraction method. In the heavy RCo2 the magnetic moment of the itinerant d electron subsystem derived from the magnetovolume effect was found to fit well with the magnetization curve of YCo2. A pronounced paraprocess above the metamagnetic transition has been observed when increasing the f-d exchange field. The type and temperature variation of the distortion of the cubic unit cell of the three compounds PrCo2, NdCo2 and SmCo2 have been studied in detail. The corresponding magnetostriction constants lambda 111 or lambda 100 were calculated. At 4 K the following values have been obtained: PrCo2, tetragonal distortion, easy axes (100), lambda 100=-3.4*10-3: SmCo2, rhombohedral distortion, easy axes (111), lambda 111=-4.6*10-3; NdCo2, tetragonal distortion and easy axes (100) for 42 K<T<Tc, orthorhombic distortion and easy axes (110) for T<42 K with mod lambda 111 mod =1.9*10-3 and lambda 100=-4.6*10-3.

Magnetic and transport properties of the intermetallic compound Y4Co3

Abstract The susceptibility, magnetization, electrical resistivity and thermopower of Y 4 Co 3 have been measured and a Curie temperature of (8±1) K has been observed. The results are interpreted in terms of localized d-electrons on some cobalt sites and itinerant d-electrons on the others.

Hydrogen induced phase transitions in YMn2

Abstract X-ray and magnetic investigations of the YMn2Hx hydrides at temperatures ranging from room temperature down to 4.2 K for the concentration range x≤1.2 are presented and analyzed. It is shown, that in the concentration range 0.5 1. The observed phase transitions are interpreted as of martensitic type. In the border ranges for x 0.8 a complex behaviour with several two phase ranges as a function of temperature is observed. For x On the basis of the presented data a phase diagram is proposed. The observed phase transitions are discussed and interpreted in terms of Mn–Mn magnetic interactions in dependence of the lattice expansion caused by hydrogen, taking into account the possible ordering of the hydrogen atoms.

Study of the valence transition in YbInCu4 by inelastic neutron scattering

Abstract In YbInCu 4 a valence transition occurs from an almost stable trivalent phase for T >45K to an intermediate-valence state below 45K. We have investigated the imaginary part of the dynamic susceptibility X ″ of YbInCu 4 above and below the transition and compared it with spectra of the stable valent compound YbInNi 4 . YbInNi 4 and YbInCu 4 in the trivalent phase exhibit crystal-field (CF) excitations, centered at about 4meV. Below the valence transition in YbInCu 4 the CF excitations disappear and instead we observe a broad magnetic signal centered at about 40meV, i.e., energy transfers ten times larger than the position of the CF lines of the stable valent phase.

The transport properties of RCo2 compounds

The transport properties of intermetallic compounds RCo2 (R=Pr, Nd, Sm, Gd-Lu, Sc and Y) in the temperature range from 4.2 up to 1000 K were studied and different contributions were determined. In the non-magnetic compounds the spin fluctuation contribution to the electrical resistivity, rho sf varies as (T/Tsf)2, and thermal resistivity, Wsf varies as (T/Tsf), were found to follow the theoretical predictions. The spin disorder contribution arising from the scattering of conduction electrons on 4f moments was deduced and found to be proportional to the de Gennes factor. It is shown that the spin fluctuations are responsible for the low-temperature features of the temperature-dependent thermopower, whereas the high-temperature behaviour is mainly determined by the density of states function. In the paramagnetic region near the Curie temperature, the critical fluctuations cause a considerable enhancement of the electrical resistivity of the magnetic RCo2 compounds, which is manifested by pronounced upturns in rho versus T dependences.

CeCu4Ga: A high γ heavy fermion compound

Abstract In the search for new Ce intermetallics which display unusual physical properties we have identified CeCu4Ga as a new example of a heavy fermion compound. CeCu4Ga is characterized by an extremely enhanced electronic contribution to the specific heat which exceeds 3000 mJ/mol K2 at 0.9 K. The Ce ion in this hexagonal compound is nearly trivalent. The Kondo temperature estimated from low temperature susceptibility and specific heat data is of the order of a few kelvin.

Structural, magnetic, electronic and transport properties of NdCu2

NdCu2 crystallizes in the orthorhombic CeCu2 structure. Using temperature dependent X-ray diffraction the variations of the lattice parameters with temperature have been measured and compared with those of YCu2. A magnetic ordering temperature at TN=6.5+or-0.1 K and a spin-reorientation temperature (TR=4.1+or-0.1 K) have been observed in the susceptibility, specific heat and resistivity. Further field-induced magnetic transitions around 1, 3 and 15 T (depending on the measuring temperature) were found in the magnetization and magnetoresistance. A preliminary magnetic phase diagram of NdCu2 is presented. The crystal field excitation temperatures determined by inelastic neutron scattering experiments are E1/kB=34 K, E2/kB=58 K, E3/kB=84 K and E4/kB=164 K. For the determination of the nine crystal field parameters Vlm the superposition model has been applied. The deduced set of parameters was used to calculate the crystal field influence on the magnetization, specific heat, thermal expansion and electrical resistivity. From an analysis of the X-ray powder diffraction data the lattice constants a=4.3844 AA, b=7.0350 AA and c=7.4186 AA and the atomic positional parameters (yCu=0.0506, zCu=0.1659, zNd=0.5383) inside the unit cell were obtained.

New magnetic phenomena in TbNi2

Structural investigations at room temperature revealed that TbNi2 does not crystallize in the Laves phase structure, but shows a superstructure of the Laves phase with the space group F-43m. Susceptibility, specific heat, magnetostriction and magnetoresistance measurements on polycrystalline specimens showed an additional magnetic phase transition at TR = 14 K below the Curie temperature of TC = 36±0.2 K. In order to clarify the nature of this magnetic phase transition at 14 K, elastic neutron diffraction below and above TR and TC was performed. The analysis of these data showed that this transition at TR is due to the rotation of the Tb moments on three of the total of eight non-equivalent Tb sites in the rhombohedrally distorted unit cell in the magnetic ordered state. This rotation of these Tb moments is out of the [111] direction into a plane perpendicular to the space diagonal. The cause for this magnetic instability is due to an interplay of the regularly arranged vacancies in the superstructure and the crystal field level position which has been studied by inelastic neutron scattering.

Structural stability of compounds (M = Al, Ni, Cu) studied by ab initio total-energy calculations and high-pressure x-ray diffraction

A combination of ab initio total-energy calculations and high-pressure x-ray diffraction experiments has been used to study the phase stability of Y-based 1:2 compounds. The motivation for these investigations was to clarify the origin of the change of the structure among the compounds (T = 3d transition element) between the neighbouring elements Ni (: defect superstructure of cubic Laves phase C15) and Cu (: orthorhombic -type structure). As a test of the ab initio calculations the cubic Laves phase compound has been included in the investigations. The ab initio calculations confirm the reversal of the relative stabilities of the and C15 phases in and and give also the correct results for . Furthermore a pressure-induced structural transition is predicted for . The high-pressure x-ray experiments show that there is very good agreement between the calculated and the measured pressure dependence of the lattice parameters of up to about 10 GPa. Above 10 GPa the structure starts to become irreversibly amorphous. This instability may be a hint at the structural phase transition predicted by the calculations.

Magnetic excitations in CeCu2

Abstract Inelastic neutron scattering experiments have been performed on polycrystalline CeCu2. We report on magnon-like excitations in the AF ordered state and on the temperature dependence of the quasielastic magnetic scattering. In addition we have evidence for a strong coupling between phonons and a CEF transition, similar to the CeAl2 case. In CeCu2, however, the involved CEF transition starts from an excited state.