N.H. Kim-Ngan

Crystalline electric field and high field magnetization in ErNi5 single crystal

Abstract High field magnetization measurements up to 380 kOe along the three symmetry axes of a single-crystalline sample of hexagonal ErNi 5 , especially the curves along [100] and [120] show us for the first time the strong field induced anisotropy between the two axes in the basal plane. A quantitative analysis of this experiment together with the other data such as paramagnetic susceptibility, specific heat and inelastic neutron scattering allowed us to determine unambiguously the crystalline electric field (CEF) parameters and the paramagnetic exchange coefficient. This leads to a better understanding of the CEF anisotropy in the basal plane. The Er 3+ CEF interactions are compared with those found in other RNi 5 compounds.

The specific heat of NdNi/sub 5/

The specific heat of NdNi/sub 5/ measured on a single-crystalline specimen from 1.5 K to 250 K has been analyzed in terms of two electronic subsystems, i.e. conduction-electron and f-electron subsystems. The conduction-electron subsystem is found in LaNi/sub 5/ to be characterized by a value of the Sommerfeld coefficient /spl gamma/ of 36 mJ/K/sup 2/mol f.u. The contribution of the f-electron subsystem to the specific heat shows a /spl lambda/-type of peak at T/sub c/ of 7.4 K and a non-pronounced Schottky-type of peak above T/sub c/. The f contribution is well described within a single-ion Hamiltonian that includes the crystalline-electric-field (CEF) and exchange interactions of the Nd/sup 3+/ ions. The exchange parameter and the full set of the CEF parameters associated with the hexagonal symmetry have been evaluated. The ground state /spl Gammasub 8/ for the Nd/sup 3+/ ion in NdNi/sub 5/ is highly anisotropic with the magnetic moment lying within the hexagonal plane. This ground-state results from higher-order charge multipolar interactions. >

Effect of pressure on the Néel temperature of YMn2: resistivity of Y(Mn1−xCux)2

Abstract Resistivity measurements have been carried out for Y(Mn1−xCux)2 compounds with values of x of 0, 0.04, 0.07, 0.10, 0.20 and 0.30 in the temperature range from 4.2 up to 300 K at zero pressure and for YMn2 under a pressure of 2 kbar. TN has been found to decrease rapidly by applying external pressure as well as by applying a chemical pressure caused by a partial substitution of Mn atoms by Cu atoms.

Magnetic, thermal and transport properties of NdMn2

Susceptibility, magnetisation, specific-heat, thermal expansion and electrical resistivity measurements have been performed on NdMn2 between 1.5 K and room temperature. Discontinuities are observed in the susceptibility, magnetisation and resistivity and large anomalies in the specific heat and thermal expansion at TN(=104 K). Additional anomalies are found at 7 K and around 20 K in the specific heat and thermal expansion. The results are shown to be compatible with the picture for this compound proposed by Ballou et al.

Specific heat of Nd1−xLuxMn2

Abstract Specific heat measurements have been performed on Nd1−xLuxMn2 compounds with x values of 0.0, 0.02, 0.05, 0.10 and 1.0 for temperatures between 1.5 and 300 K in magnetic fields up to 8 T. The electronic and lattice contributions to the specific heat of the isostructural paramagnetic compound LuMn2 have been taken as a reference for the Nd containing compounds in order to deduce the magnetic and crystal field contributions. By comparing the measured specific heat data with the calculated specific heat contribution from the Nd-ions, the Mn contribution is deduced. The variation of this Mn contribution with x is discussed.

High-field magnetisation of Nd1−xLuxMn2 compounds

Abstract Nd 1− x Lu x Mn 2 compounds with x =0, 0.02, 0.03, 0.05, 0.10, 0.15, 0.20, 0.30 and 1.0 have been investigated in high-field magnetisation measurements up to 38 T at 4.2 K and in electrical resistivity, thermal expansion and specific heat experiments between 1.5 K and room temperature. The high-field magnetisation increases linearly with increasing field for LuMn 2 with a susceptibility of 11 × 10 -9 m 3 / mol , indicating a paramagnetic behaviour. No anomalies are observed in other experiments in the temperature range from 1.5 to 300 K, confirming this observation. For the compounds with x ⩽0.20, saturation is not reached in the highest field whereas around 10 T a transition in the magnetisation curve is observed. For the compound with x =0.30, the magnetisation shows a trend to saturation at higher field values. The value of the saturated moment in this case is close to that expected for Nd 3+ which suggests the absence of a Mn moment in this compound. This result is confirmed by other experimental data.

Thermal properties of UPdSn and UCuSn

Abstract We report on the specific heat and the thermopower of UPdSn and UCuSn, both of which order antiferromagnetically at low temperatures. The compounds show similar behavior in the specific heat, and the large magnetic-entropy changes around T N are evidence for a large degree of 5f-electron localization. For both compounds, we find that thermopower results are consistent with the findings for the electrical resistance. While for UCuSn very abrupt changes at 25 and 60 K are observed for both quantities, more continuous changes at the magnetic transitions (25 and 40 K) are found for UPdSn.

Specific heat of PrNi5

Abstract Specific-heat measurements have been performed on single-crystalline PrNi 5 from 1.3 to 250 K, in order to evaluate the contribution of the Pr subsystem. PrNi 5 does not order magnetically down to the lowest temperatures as crystalline-electric-field (CEF) interactions, producing the non-magnetic singlet ground state Γ 4 , dominate the exchange interactions. Analysis of the specific heat unambiguously establishes the position of the two lowest excited levels at 33.7 K (Γ 1 ) and 45.7 K (Γ 6 ) which levels cannot be observed in inelastic-neutron-scattering experiments. A set of CEF parameters of the Pr 3+ ion in the f 2 configuration has been evaluated that gives the best account for all known experimental results of this compound.

Spin fluctuations in the (Nd,Lu)Mn/sub 2/ system

Using Gruneisen relations for the thermal expansion and the specific heat of (Nd,Lu)Mn/sub 2/ compounds, we succeeded in defining separate contributions, ascribed to electron excitations, lattice contributions and crystal-field effects in the Nd sublattice, with Gruneisen parameters /spl Gammasub e/=3.5, /spl Gammasub ph/=0.7 and /spl Gammasub Nd/=-3.0, respectively. After subtracting these contributions from the measured specific heat and thermal expansion, we applied the same analysis to the remaining part, ascribed to the Mn sublattice. In this way, we identified a spin-fluctuation contribution and a contribution ascribed to the antiferromagnetic ordering of local Mn moments. The latter contribution disappears at substitution of 30% of Nd by Lu. The spin-fluctuation contribution is found in the complete concentration range of the Nd/sub 1-x/Lu/sub x/Mn/sub 2/ compounds. >

Pressure dependence of the Néel temperature in NdMn2 and related compounds

Abstract Resistivity measurements on Nd(Mn 1 − x Cu x ) 2 compounds in the temperature range from 4.2 to 300 K show that magnetic order disappears for x exceeding 0.07. In order to investigate the role of the lattice parameter in the suppression of magnetic order, the effect of pressure on the Neel temperature has been studied by performing resistivity measurements under pressures up to 5 kbar. A linear decrease with pressure has been found with d T N /d p values of -3.45 K/kbar, -2.76 K/kbar and -1.59 K/kbar for the compounds with x = 0, 0.03 and 0.05, respectively. From a comparison with the effect of chemical pressure observed in (Nd, Lu)Mn 2 compounds, it is concluded that the lattice parameter is not the (only) decisive factor, but that the disturbance of the Mn-Mn interaction (caused by substitution of Mn sites) plays an important role.