G. Schaudy

Magnetism and specific heat of UIrGa5 and UPdGa5

Abstract Magnetization and specific heat measurements on UPdGa5 indicate antiferromagnetism at temperatures below 50 K. Neutron diffraction study shows, however, that the ordered magnetic moment of U should be very low (probably less than 0.5μB). UIrGa5 exhibits weak paramagnetism with susceptibility values around 1×10-8 m3/mol. The feeble magnetic properties of UTGa5 compounds are probably due to a strong hybridization of U 5f states with 4p states of Ga and d states of the transition metal.

Rare-earth magnetism and superconductivity in RBaSrCu3O7-x (R=Gd, Pr)

Abstract We present specific-heat, susceptibility, X-ray absorption (EXAFS and XANES) and 155 Gd Mossbauer experiments of RBaSrCu 3 O 7−δ ( R = Gd , Pr ) in order to examine the effect of isoelectronic substitution of Ba by Sr upon superconductivity and magnetism with respect to the parent reference compounds RBaSrCu 3 O 7−δ . The RBaSrCu 3 O 7−δ compounds offer the possibility to vary the oxygen content without changing the long-range tetragonal structure. From 155 Gd Mossbauer spectroscopy a small local orthorhombic distortion is observed for tetragonal GdBaSrCu 3 O 7 with a superconducting transition at 69 K. The XANES spectra of PrBaSrCu 3 O 7−δ indicate that Pr is mainly trivalent with a considerable extent of hybridization, in agreement with a recent description of the specific-heat and susceptibility data in terms of Pr 3+ crystal-field effects. From the comparison of the isoelectronic Ba/Sr substitution with hole doping (oxygen removal or huydrogen absorption) we deduce that the hybridization of the Pr 4f states with the Cu-O valence bands appears to be important for the suppression of superconductivity and the high antiferromagnetic ordering temperature in these Pr compounds.

Heat capacity of Sm2CuO4 and Sm1.85Ce0.15CuO4

Abstract We present specific heat measurements of Sm 2 CuO 4 and Sm 1.85 Ce 0.15 CuO 4 in external fields up to 11 T. Entropy considerations indicate that Ce is tetravalent in these systems.

U(Ni1−xGax)5 novel compounds with CaCu5-type: crystal structure and magnetism

Abstract A novel ternary solid solution U 1-y ▭ y (Ni 1-x Ga x ) 5 has been observed, whose homogeneous region at 800°C has been determined to range from 0.20 ≤ x ≤ 0.60 and 0 ≤ y ≤ 0.17. Employing quantitative room-temperature, Rietveld-type X-ray powder analyses, the crystal structures of the novel solution range have been evaluated to be isotypic with the CaCu5-type for practically the entire homogeneity region. An enclave of alloys with an orthorhombically distorted unit cell has been observed in samples quenched from 800°C for x=0.45-0.55 and y≈0.09. There are monotonic increases in the lattice parameters and in the volumes for nickel-rich alloys when nickel atoms at a constant U level are randomly replaced by gallium atoms in the 3g sites, whereas on further Ni/Ga replacement the nickel atoms preferentially occupy the prism centres at 2c, causing a maximum for the c-parameter at UNi3Ga2. The magnetic measurements for the gallium-rich compounds (x>0.5) show the typical broad maximum in the susceptibility at T≈60 K that is commonly observed in heavy-fermion systems. Furthermore, two samples exhibit an additional kink at T ≈ 26 K (TN≈23 K) possibly due to an antiferromagnetic coupling of the uranium spins. Anomalies in the specific heat versus temperature observed at 30.5 K for U16.5Ni40Ga43.5 confirm the indications of the onset of AF magnetic order. The enhanced γ-value of ≈ 100 mJ/mol K2 suggests that this is a strongly correlated electron system.

Heat capacity of (R1−xPrx)Ba2Cu3O7 (R=Y, Gd)

Abstract We present specific heat measurements of (R, Pr)Ba 2 Cu 3 O 7 (R=Y, Gd) in external fields up to 11 T. The results are analyzed in terms of crystalline electric field (CEF) effects and indicate that in these systems Pr appears to be trivalent rather than tetravalent.

Specific heat and susceptibility of PrNi2

Abstract The specific heat of the cubic PrNi 2 , YNi 2 and LuNi 2 intermetallic compounds has been measured in magnetic fields up to 11 T in the temperature range 1.6–80 K and in zero field up to room temperature. The comparison of the measured susceptibility data with those of the static crystal field calculation yields an overall agreement but allows no decision with respect to a singlet or doublet ground state. Our specific heat data show that the singlet Γ 1 is the ground state in PrNi 2 . The excess specific heat observed below 20 K can be attributed to a broadening of the first excited level.

Magnetoelastic interactions in RE2CuO4 system

Abstract We present thermal expansion data for R 2 CuO 4 (R = Pr, Nd, Sm, Gd, Eu) compounds and specific heat measurements of Sm and Eu compounds in the temperature range from liquid helium to room temperature. In the case of Pr, Nd, Gd, and Eu compounds the coefficient of thermal expansion (α) increases monotonously with rising temperature whereas α of the Sm compound shows a pronounced minimum at about 60 K. These findings are interpreted in terms of the crystal field induced electric quadrupole moment of the 4 f electrons.

The effect of chemical pressure upon the magnetism of (Ba1−xSrx)2PrNbCu2O8

Abstract We present specific-heat and susceptibility measurements and a structural analysis of (Ba1−xSrx)2PrNbCu2O8 in order to explore the effect of the chemical pressure on the magnetic properties of the Pr sublattice. These systems are similar to PrBa2Cu3O7−δ for which the Cu-O chain layers have been fully replaced by layers of Nb-O octahedra. The overall volume compression of - 5.5% across the series is found to be rather inhomogeneous yielding an anomalous expansion of the average Pr-O bond lenght of 3.8% which is accompanied by a reduction of TN from 11.6 K to 2.3 K. The variation of the magnetic properties across the solid solution is discussed in terms of hybridization of the Pr4f states with the Cu-O valence bands.

Magnetic order in Sm2−xGdxCuO4

Abstract We performed specific heat and susceptibility measurements of the pseudoternary system Sm2−xGdxCuO4 in order to establish the magnetic phase diagram of the Rare Earth sublattice.

Magnetism and superconductivity in Sr2RyCe0.5MCu2O10−δ (R = Pr, Nd, Sm, Eu, Gd; M = Nb, Ta)

Abstract We present magnetic and specific heat measurements of the series Sr 2 R y Ce 0.5 M Cu 2 O 10−δ ( R = Pr, Nd, Sm, Eu, Gd; M = Nb, Ta). Superconductivity occurs for y > 0.8 ( R = Nd, Sm, Eu), however, for Gd only in combination with M = Nb, while the corresponding Pr compounds are found to be nonsuperconducting.