Low temperature measurements of specific heat and magnetic properties were performed on granular Cu90Co10 ribbons prepared by melt spinning technique. The thermal and magnetic behavior of an as-quenched sample is compared to that of an annealed sample at 500°C for 1h. It was found that the electronic specific heat γ decreases about 50% and the magnetic properties change from spin-glass to superparamagnetic with the annealing process. These results are interpreted and discussed considering the role of isolated Co atoms in the RKKY interaction among small nanoparticles.

We investigated the influence of B variation on the structural, thermal, and electronic properties of the ternary intermetallics Al3Ni20Bx (x=6,7,…,12). On increasing the metalloid content, the cubic unit-cell parameter (space group Fm3¯m) is observed to increase linearly. Down to 2K (for selected cases down to 120mK), no evidence of superconductivity or (de)localized magnetism is observed in any of the studied samples; rather, all exhibit a nonmagnetic and metallic character. Furthermore, the Pauli susceptibilities, the coefficients of the specific heats, and the coefficients of the resistivities are found to correlate strongly (but non-montonically) with the percentage of the Ni content per unit formula (the Ni 3d band is the dominant contributor to the density of states N(EF)). Such a non-monotonic evolution of the electronic properties is attributed to a corresponding non-monotonic character of the N(E) curve within the neighborhood of EF.

A new quaternary intermetallic borocarbide TmCo2B2C has been synthesized via rapid-quench of an arc-melted ingot. Elemental and powder-diffraction analyses established its correct stoichiometry and single-phase character. The crystal structure is isomorphous with that of TmNi2B2C (I4/mmm) and is stable over the studied temperature range. Above 7 K, the paramagnetic state follows modified Curie-Weiss behavior (chi = C/(T - theta) + chi(0)) wherein chi(0) = 0.008(1) emu mol(-1) with the temperature-dependent term reflecting the paramagnetism of the Tm subsystem: mu(eff) = 7.6(2) mu(B) (in agreement with the expected value for a free Tm3+ ion) and theta = -4.5(3) K. Long-range ferromagnetic order of the Tm sublattice is observed to develop around similar to 1 K. No superconductivity is detected in TmCo2B2C down to 20 mK, a feature which is consistent with the general trend in the RCo2B2C series. Finally, the influence of the rapid-quench process on the magnetism (and superconductivity) of TmNi2B2C will be discussed and compared to that of TmCo2B2C.

Based on magnetization, specific heat, magnetostriction, and neutron diffraction studies on single-crystal TbCo2B2C, it is found out that the paramagnetic properties, down to liquid nitrogen temperatures, are well described by a Curie-Weiss behavior of the Tb+3 moments. Furthermore, below Tc= 6.3 K, the Tb-sublattice undergoes a ferromagnetic (FM) phase transition with the easy axis being along the (100) direction and, concomitantly, the unit cell undergoes a tetragonal-to-orthorhhombic distortion. For fields up to 90 kOe, no field-induced splitting of the Co 3d orbitals was observed; as such the internal field must be well below the critical value needed to polarize the Co 3d subsystem. The manifestation of a FM state in TbCo2B2C is unique among all other isomorphous borocarbides, in particular TbNi2B2C (Tn=15 K, incommensurate modulated magnetic state) even though the Tb-ions in both isomorphs have almost the same crystalline electric field properties. The difference in the magnetic modes of these Tb-based isomorphs is attributed to a difference in their exchange couplings caused by a variation in their lattice parameters and in the position of their Fermi levels.

The low-temperature properties of single-crystal CeCoGe were investigated by specific heat C(T, H), magnetoresistivity rho(T, H), and differential susceptibility measurements chi(T, H). The zero-field low-temperature specific heat evolves as C = gamma T + beta T(3) = 42T +23.5T(3) mJ mol(-1) K(-1). On comparing its gamma = 42 mJ mol(-1) K(-1) with that of LaCoGe (12 mJ mol(-1) K(-2)) it is inferred that both 3d (Co) and 4f (Ce) orbitals contribute to the density of states at the Fermi level. Assuming that its phonic contribution to the specific heat is similar to LaCoGe (beta = 0.5 mJ mol(-1) K(-4)), then the extra cubic term in the specific heat (23T(3) mJ mol(-1) K(-1)) must be due to magnon excitation within the antiferromagnetically ordered state, T < T(N). On the other hand, the thermal evolution of the resistivity is found to be dominated by the following scattering processes: magnon scattering operating within the ordered state at T < T(N) leading to a T(4) resistive contribution and a spin fluctuation process associated with the Co subsystem giving rise to both a quadratic resistive term below 15 K and a saturated resistive term at higher temperatures. The isothermal magnetoresistivity below T(N), rho(T < T(N), H), manifests a peak which is centered at the same critical field that appears in the magnetization isotherms. This peak, together with the peak observed at a temperature 0.7 K below T(N), is attributed to a spin rearrangement of the AFM structure of the Ce sublattice.

We present low temperature specific heat and ac-susceptibility measurements of Fe-8 powdered sample. Below 1.3 K, super-paramagnetic blocking effects and an excess specific heat contribution are observed. The latter is attributed to a splitting of the ground state doublet in an inhomogeneous local field of hyper fine and dipolar origin. The local field contributions are evident at the resonances observed in the field dependent ac-susceptibility and specific heat below 0.5 K. The low temperature Schottky contribution is in agreement with known crystal field parameters of effective spin Hamiltonians proposed to simulate EPR and inelastic neutron scattering experiments. (C) 2008 Published by Elsevier B.V.

Magnetic and EPR data have been collected for complex [Cu(L-Arg)2](NO3)2·3H2O (Arg=arginine). Magnetic susceptibility χ in the temperature range 2–160K, and a magnetization isotherm at T=2.29(1)K with magnetic fields between 0 and 9T were measured. The observed variation of χT with T indicates predominant antiferromagnetic interactions between Cu(II) ions coupled in 1D chains along the b axis. Fitting a molecular field model to the susceptibility data allows to evaluate g=2.10(1) for the average g-factor and J=−0.42(6)cm−1 for the nearest neighbor exchange coupling (defined as Hex=-∑JijSi·Sj). This coupling is assigned to syn–anti equatorial–apical carboxylate bridges connecting Cu(II) ion neighbors at 5.682Å, with a total bond length of 6.989Å and is consistent with the magnetization isotherm results. It is discussed and compared with couplings observed in other compounds with similar exchange bridges. EPR spectra at 9.77 were obtained in powder samples and at 9.77 and at 34.1GHz in the three orthogonal planes of single crystals. At both microwave frequencies, and for all magnetic field orientations a single signal arising from the collapse due to exchange interaction of resonances corresponding to two rotated Cu(II) sites is observed. From the EPR results the molecular g-tensors corresponding to the two copper sites in the unit cell were evaluated, allowing an estimated lower limit |J |>0.1cm−1 for the exchange interaction between Cu(II) neighbors, consistent with the magnetic measurements. The observed angular variation of the line width is attributed to dipolar coupling between Cu(II) ions in the lattice.

Superconductivity has been recently found in two Li containing compounds, Li2Pd3B and Li2Pt3B. They show superconducting transition at the temperatures, 7.5 K and 2.17 K respectively. The structure analysis has been reported by Eibenstein et. al. in 1997 and they take the same cubic structure with the symmetry of P4(3)32. Heat capacity of these compounds was measured for confirming their bulk superconductivity and investigating superconducting properties. Delta C/T-c are evaluated to be around 18 mJ/mol K-2 for Li2Pd3B and 9.7 mJ/mol K-2 for Li2Pt3B. Electronic heat capacity (gamma) and Debye temperature (theta(D)) are derived from the normal state data and lead to 9.0 mJ/mol K-2 and 221 K for Li2Pd3B, 7.0 mJ/mol K-2 and 228 K for Li2Pt3B. Those physical parameters of the two compounds are discussed.

The magnetic properties of the Cu(II)-peptide compounds (L-tyrosyl-L-leucinato)Cu(II) and (L-tryptophyl-glycinato)Cu(II), to be identified as Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively, have been investigated by specific heat (0.08 < T < 28 K), dc magnetization (2 < T < 80 K, with B(0) = mu(o)H < or = 9 T), and ac magnetic susceptibility (with B(0) = 0 for 0.03 < T < 3 K and B(0) up to 9 T for 2 < T < 80 K) measurements. Above approximately 1 K, the specific heat and magnetization of both compounds display a ferromagnetic (FM) spin chain behavior sustained by syn-anti carboxylate bridges connecting equatorially Cu(II) ions at about 5 A. To model this behavior, we calculated the eigenvalues of Heisenberg chains with up to 20 spins 1/2 and used the method of Bonner and Fisher. A global fit of the model to the specific heat and magnetization data gives 2J(0)/k(B) = 3.60(5) K and 2.59(5) K for the intrachain exchange interactions in Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively (H(ex)(i,j) = -2J(0) S(i).S(j)). These values of 2J(0) are discussed in terms of structural properties of the carboxylate bridges in the two compounds. Using the parameters obtained from the global fit, we calculated isothermal susceptibilities in agreement with the ac susceptibilities measured with small applied dc magnetic fields. However, the ac susceptibility measured with applied dc fields larger than 1 T lie between the values calculated for the isothermal and adiabatic susceptibilities. At 0.16 K for Cu(II)Tyr-Leu and 0.53 K for Cu(II)Trp-Gly, the observed specific heat and magnetic susceptibility display peaks associated to three-dimensional magnetic phase transitions. The interchain exchange couplings 2J(1) producing the 3D magnetic order are ferromagnetic and have magnitudes 2J(1)/k(B) approximately 0.015 and 0.073 K for Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively.

Superconducting materials of Li2Pd3B and Li2Pt3B were identified. These materials show a superconducting transition at temperatures of 7.5 K and 2.17 K, respectively. They have a cubic structure composed of distorted Pd6B octahedrons, and their superconductivity is induced by Li addition. Li contains a single valence electron and reacts strongly to most materials. Furthermore, it easily occupies interstitial sites while maintaining its structure. The samples were prepared by a two-stage arc-melting method followed by an annealing process. The specific heat measurements were carried out by heat-pulse relaxation and semi-adiabatic methods. The Sommerfeld (γT) and Debye (βT3) terms of Li2Pd3B from a conventional formula C = γT + βT3 were given as γ = 9.0 mJ mol−1 K−2 and β = 1.08 mJ mol−1 K−1, respectively. The Debye temperature was θD = 221 K. The value of ΔC/γT at Tc was calculated to be 2.0. The same parameters were described for Li2Pt3B as γ = 7.0 mJ mol−1 K−2, β = 0.98 mJ mol−1 K−1, θD = 228 K, and ΔC/γTc = 1.39. The pressure effect was measured with a SQUID magnetometer using a low-temperature hydrostatic micro-pressure cell. The result of dTc/dP showed a negative effect on the superconducting transition temperature of Li2Pd3B.

We investigated the magnetic, thermal and resistive properties of PrT2B2C(T=Co,Ni) and their solid solutions Pr1−xDyxT2B2C. These compounds behave as normal metallic paramagnets at higher temperatures. At lower temperatures, the parent and the low Dy-substitutions show anomalous features; as an example, their TN are unexpectedly high and, furthermore, the magnetic phase diagram (TN versus x) of Pr1−xDyxT2B2C manifests a strong and non-monotonic dependence on x. At and below liquid helium temperatures and for pressures up to 13kbar, the parent Pr-based compounds manifest a seemingly quadratic-in-temperature resistive contribution which is attributed to a magnon–electron scattering process. We observed that all isobar resistivity curves can be collapsed onto a single curve when plotted against T/To where To (13–15K) is a scaling factor. The combined influence of strong crystalline electric field effects and a weak exchange enhancement on the observed properties of these compounds will be discussed. In particular, the manifestation of a weak exchange enhancement in these compounds and in the isomorph nonmagnetic YCo2B2C will be evaluated.

We describe a systematic series of experiments on thermalization of electrons in lithographic metallic thin films at millikelvin temperatures using Coulomb blockade thermometry (CBT). Joule dissipation due to biasing of the CBT sensor tends to drive the electron system into non-equilibrium. Under all experimental conditions tested, the electron-electron relaxation is fast enough to ensure thermal electron distribution, which is also in agreement with the theoretical arguments we present. On the other hand, poor electron-phonon relaxation plays a dominant role in lifting the electron temperature above that of the bath. From a comparison of the results with the theoretical current-voltage characteristics of the thermometers we precisely determine the electron-phonon coupling constant for the common metals used. Our experiments show that it is a formidable task to attain thermal equilibrium with the bath using single-electron devices under non-zero bias conditions at 20–50 mK temperatures that are typically encountered in experiments. The conclusion concerning Coulomb blockade thermometry is more optimistic and two-fold: (1) One can now correct the errors due to bias heating in a satisfactory manner based on known material properties and the size of the metal films in the sensor. (2) Reliable thermometry down to 20 mK requires islands whose volumes are >10 −15 m 3 , which is still acceptable both from the parameter (capacitance) and fabrication points of view.

We report the structure and the magnetic properties of a cobalt(II) compound with the amino acid l-threonine, Co(C(4)H(8)NO(3))(2)(H(2)O)(2). It crystallizes in the orthorhombic chiral space group C222(1), with a = 5.843(5) A, b = 10.120(10) A, c = 22.36(3) A, and Z = 4. The Co(II) ion is in a deformed octahedral environment on a 2-fold symmetry axis parallel to the crystallographic axis b. It is bonded to two threonine molecules in a bidentate fashion, via one oxygen from the carboxylate end and the alpha-amino nitrogen. A water molecule occupies the third independent site. The Co(II) ions are arranged in layers with intralayer and interlayer distances of 5.84 and 11.18 A, respectively. Magnetic measurements data reflect the molecular character of a compound with weak exchange interactions. EPR measurements in polycrystalline and single-crystal samples indicate a distorted axial symmetry around the Co(II) ion, as expected from the structural results. Eigenvalues and eigenvectors of the g tensor are determined. The measured principal g values (5.81, 4.56, and 2.23) reflect a high-spin Co(II) ion, as suggested by the type of ligands and the molecular symmetry. From the incomplete collapse of the hyperfine structure we estimate 0.25 < |J| < 1.2 cm(-1) between neighboring Co(II) ions within a layer, transmitted through H-bonds. A higher limit |J'| < 0.07 cm(-1) is estimated for the exchange interactions between Co(II) ions in neighboring layers. From a global fit of a spin Hamiltonian with spin (3)/(2) to magnetization and EPR data we obtain a zero field splitting delta approximately 231 cm(-1) between the two lowest doublet states. The results are discussed in terms of the molecular and electronic structure of the compound.