Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

Specific heat and magnetic ordering of NdNi2B2C

In this work, the specific heat of NdNi₂B₂C was computed with the three sets of crystal-field parameters proposed by previous authors. All curves of the heat capacity plotted with the calculated results exhibit sharp peaks around the magnetic transition temperature TN as experimentally observed. To understand the mechanism of its magnetic ordering, we also calculated the magnetization of the material in low temperature region with the ground crystal-field (CF) level, the two lowest CF levels, and the full CF levels of J=9/2 multiplet respectively for comparison. Using the two eigenstates of the ground CF level, we derived a formula for 〈Jx〉 with mean-field approach for theoretical analysis. Both our numeric and theoretical results suggest that the two lowest CF levels play dominant roles in the magnetic process of the material below TN. It is also very interesting to notice that the ground CF level itself results in a larger TN, but the inclusion of the first excited CF doublet in calculations instead hinders the magnetic ordering, leading to a weakly reduced transition temperature.     

Oscillations in magnetocaloric effect and magnetic properties of RE2Al3Si2 (for RE=Dy,Ho and Er) and REAlSi (for RE=Ce and Pr)

We present the magnetic and thermal properties of a series of compounds RE₂Al₃Si₂ for RE=Dy, Ho, Er, and REAlSi for RE=Pr, Ce. The 2–3–2 family crystallizes with themonoclinic Y₂Al₃Si₂-type structure while the 1–1–1 family crystallizes in the body-centered tetragonal α-ThSi₂-type structure. The measurements were done on single crystals, grown using high-temperature flux technique and molten Al as a solvent . Susceptibility and heat capacity data were taken from 1.8 to 200 K, from the heat capacity data, the isothermal magnetic entropy change was obtained. Our results indicate signal oscillations in magnetocaloric properties for those compounds, in particular, Dy₂Al₃Si₂ that shows an anomaly that can be associated with a spin reorientation. Similar results are known for some Dy discilicides and dialluminades.     

Analysis of low temperature specific heat in Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd,Sm, Dy and Ho) compounds

We report on the specific heat C(T) of doped manganites Nd_0.5Sr_0.5MnO₃, Nd_0.5Ca_0.5MnO₃, Sm_0.5Ca_0.5MnO₃, Dy_0.5Ca_0.5MnO₃ and Ho_0.5Ca_0.5MnO₃ in the temperature range 2?T?300 K using modified rigid ion model (MRIM). The present specific heat results are in general satisfactory agreement with experimental data except at very low temperatures (i.e. T?12 K). Also a sharp peak observed in the experimental results for these compounds around 5 K could not be revealed by our computed results as they arise due to Schottky-like anomaly. Besides, we have reported the cohesive and the thermal properties of these compounds. The results obtained by us are discussed in detail.     

Crystal field in valence-fluctuating CeNi-based compounds

The effects of the crystal field (CF) on the paramagnetic Pr ion in a number of compounds of the type R_1−x Pr_xNi (R = Ce, La, Y), in which a transition of the cerium ions from an intermediate-valence into a Kondo state occurs as La is substituted for Ce, are investigated. The level schemes of the Pr ion in the CF are reconstructed from inelastic neutron scattering spectra and the temperature dependence of the heat capacity in different magnetic fields (B=0–8 T). The parameters of the low-symmetry CF in the compounds RNi are determined from the experimental data. It is established that in the Kondo regime the hybridization of the f electrons with conduction electrons only gives a proportional increase in all the parameters of the CF potential. At the same time, partial delocalization of the f electrons in the intermediate-valence state results in charge redistribution, which is manifested in different scales for the changes in the different CF parameters. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 947–952 (25 June 1996)     

Anisotropic magnetic behavior of PrAg2Ge2—a crystal field study

A crystal field (CF) analysis of the experimental, single crystal magnetic susceptibility data (300-1.8 K) along and perpendicular to the [0 0 1] axis, of the Ag based rare earth intermetallic compound PrAg₂Ge₂, has been carried out for the first time, thus yielding first reliable set of CF parameters for the system. The susceptibility feature at 12 K is possibly due to CF effects rather than the magnetic order as proposed earlier. This removes the issue of the transition temperature being too large to scale properly with the de Gennes factor. We have used the set of CF parameters to find the Stark energies of the ground state and the excited states together with their corresponding eigenvectors, and the thermal variation of the magnetic specific heat. Possible explanation of the absence of magnetic ordering of the Pr sublattice and the nature of variation of the CF parameters with the substitution of transition-metal ion in PrAg₂Ge₂ is studied and discussed in relation to PrAu₂Ge₂. All computations have been carried out using the intermediate coupling scheme including the J-mixing.     

Crystal field investigation on the magnetic properties of Yb in Yb(CF3SO3)3·9H2O

Ytterbium tri-fluoromethanesulfonate (YbTFMS) single crystals are prepared from the slow evaporation of the aqueous solution of YbTFMS and the principal magnetic susceptibility perpendicular to the c-axis of the hexagonal crystal (χ_⊥) is measured from 300 K down to 13 K. Principal magnetic anisotropy Δχ(=χ_∥−χ_⊥) is measured from 300 K down to 80 K which provides principal magnetic susceptibility parallel to the c-axis (χ_∥) down to 80 K. Very good theoretical simulation of the observed magnetic properties of YbTFMS has been obtained using one electron crystal field (CF) analysis having C_3h site symmetry. No signature of ordering effect in the observed magnetic data is noticed down to the lowest temperature (13 K) attained, indicating the inter-ionic interaction to be of predominantly dipolar type. The calculated g-values are found to be g_∥=2.67 and g_⊥=2.51, respectively. CF analysis provides the electronic specific heat which gives two Schottky anomalies in its thermal variation down to ∼13 K. The temperature dependences of quadrupole splitting and hyperfine heat capacity are studied from the necessary information obtained from the CF analysis.     

Properties of hexagonal Al2Ge2RE (RE=Y,La,Ce,Nd,Eu,Gd,Tb,Y b and Lu) : A first-principles study

In this work, we present a study of the structural, elastic and electronic properties of the Al₂Ge₂RE (RE=Y, La, Ce, Nd, Eu, Gd, Tb, Yb and Lu) through the approaches of generalized gradient approximation (GGA) and local spin density approximation with the Hubbard energy (LSDA+U) based on density-functional theory. For most of the compounds (RE=Y, La, Ce, Nd, Eu, Yb and Lu), the results of the structural constants calculated from the approach of LSDA+U are in good agreement with the reported experimental data. Both the approaches of GGA and LSDA+U have been used to calculate DOS. Compared with the results of the GGA approach, the LSDA+U is more credible because it can show the influence of RE-f states and the RE-f states play an important role in the compound. The magnetic property has been investigated according to the result of DOS and it shows that the Al₂Ge₂Y, Al₂Ge₂La, Al₂Ge₂Y b and Al₂Ge₂Lu do not exhibit obvious magnetic property, while the other five ternary compounds are magnetic. The Voigt-Reuss-Hill (VRH) approach is used to calculate the elastic properties including bulk and shear moduli. The results of the calculated Poisson’s ratio ν and the B/G ratio demonstrate that all the Al₂Ge₂RE ternary compounds are brittle materials. The compounds, i.e. Al₂Ge₂Nd, Al₂Ge₂Eu, Al₂Ge₂Gd and Al₂Ge₂Tb which are mechanically unstable, display differences with the other five in the elastic properties.     

Paramagnetic susceptibilities, crystal field Stark energies and hyperfine properties of Eu in europium trifluoromethanesulfonate nonahydrate

Hexagonal single crystals of europium trifluoromethanesulfonate nonahydrate (EuTFMS) have been prepared. The paramagnetic susceptibility perpendicular to the symmetry axis (c-axis, χ_⊥) and the principal magnetic anisotropy (Δχ=χ_||−χ_⊥) were measured in the temperature range of 300-13 and 300-80 K, respectively. The paramagnetic susceptibility parallel to the c-axis (χ_||) was calculated using the values of χ_⊥ and Δχ. χ_|| and χ_⊥ increase slowly with decrease of temperature and below ∼93 K, χ_⊥ is more or less constant. But χ_|| levels off below ∼137 K. The calculated value of the effective magnetic moment (P_eff) of Eu³⁺ in the crystalline environment of EuTFMS decreases with lowering of temperature. The Raman and Fourier transform infrared (FTIR) spectra of EuTFMS were recorded in the wavenumber range of 10-1800 and 370-6000 cm⁻¹, respectively. A good theoretical simulation of the observed magnetic properties including the CF Stark energies of the ground term (⁷F) extracted from the Raman and FTIR spectra of Eu³⁺ in EuTFMS have been achieved using one electron crystal field (CF) analysis. The electronic specific heat and other relevant CF dependent hyperfine properties, viz, nuclear quadrupole splitting and hyperfine specific heat have been computed.     

On the behavior of Ho3+ in LaCl3 and the correlation with its absorption spectra

The crystal field (CF) energies of the electronic ground state of Ho³⁺ ions in a LaCl₃ host have been calculated with the set of CF parameters of Crosswhite et al. The magnetic anisotropy and the average susceptibility have been studied from room temperature down to liquid helium temperature. The g-values and the hyperfine structure parameters have been computed and compared with the experimental values. The Schottky and hyperfine heat capacities have also been determined and some interesting anomalies are predicted. All available observed properties are explained fairly well on the basis of the interaction of the ion with the CF proposed by Crosswhite et al.     

Magnetocrystalline anisotropy in RAu2Ge2 (R = La, Ce and Pr) single crystals

Anisotropic magnetic properties of single crystalline RAu₂Ge₂ (R=La, Ce and Pr) compounds are reported. LaAu₂Ge₂ exhibits a Pauli-paramagnetic behaviour whereas CeAu₂Ge₂ and PrAu₂Ge₂ show an antiferromagnetic ordering with Nèel temperatures T_N = 13.5 and 9 K, respectively. The anisotropic magnetic response of Ce and Pr compounds establishes [0 0 1] as the easy axis of magnetization and a sharp spin-flip type metamagnetic transition is observed in the magnetic isotherms with H // [0 0 1]. The transport and magnetotransport behaviour of these compounds, in particular LaAu₂Ge₂, indicate an anisotropic Fermi surface. The magnetoresistivity of CeAu₂Ge₂ apparently reveals the presence of a residual Kondo interaction. A crystal electric field analysis of the anisotropic susceptibility in conjunction with the experimentally inferred Schottky heat capacity enables us to propose a crystal electric field level scheme for Ce and Pr compounds. For CeAu₂Ge₂ our values are in excellent agreement with the previous reports on neutron diffraction. The heat capacity data in LaAu₂Ge₂ show clearly the existence of Einstein contribution to the heat capacity.     

Low temperature heat capacity of some heavy REA12 compounds (RE = Tb,Dy, Er and Tm) in a magnetic field

The molar heat capacity of single crystalline TbAl₂, DyAL₂ and ErAl₂ and of polycrystalline TmAl₂ has been measured for the temperature range from 1.5 to about 20 K in external magnetic fields up to 7.5 T. The magnetic contribution to the heat capacity has been interpreted by means of a simple theoretical approach containing a cubic crystalline electric field interaction and an exchange interaction in mean field approximation. The three model parameters have been determined from the interpretation of the experimental results for the four compounds. A comparison of the reduced crystal field parameters for the heavy rare earth-Al₂ compounds shows systematic behaviour and supports the physical relevance of the applied model.     

Electronic and magnetic properties of rare-earth ions in REBa2Cu3O7-x (RE=Dy,Ho, Er)

Heat capacity, resistivity, and magnetic susceptability data have been obtained for the compounds REBa₂Cu₃O_7-x, where RE = Dy, Ho or Er. Neutron diffraction data on the Ho compound show a structure identical to that of YBa₂Cu₃O_7-x. Magnetic transitions are observed at T_m=0.95, 0.17 and 0.59 K for Dy, Ho and Er compounds, respectively. It is argued that these are due predominantly to dipolar interactions. Resistivity data show that the magnetic state is coexistent with superconductivity in all cases. From the heat capacity data, the degeneracies of the crystal field ground states are determined, and estimates are given for the magnetic moment in the ground state and the energy separation of the first excited crystal field state.     

Magnetic and thermodynamic properties of DyFe2Si2 further investigated with crystal-field theory and two-ion model

We report our theoretical studies on the magnetic and thermodynamic properties of DyFe₂Si₂ using the crystal-field parameters (CFPs) proposed by Chatterji et al. recently [1]. Our calculated magnetization and magnetic specific heat curves obtained with the above CFPs based on the two-ion model we developed recently show reasonable agreement with the experimental data.     

First-principles study of the structural properties and magnetism of R2CoIn8 (R=Y,Pr, Nd,and Dy) intermetallic compounds

The electronic structure of R₂CoIn₈ (R=Y, Pr, Nd, and Dy) intermetallic compounds is calculated from first principles based on the density functional theory (DFT). The Kohn–Sham single-particle equations of the DFT are solved using two independent computational methods, namely APW+lo and FPLO. First the structural properties of Y₂CoIn₈ are studied. Good agreement of calculated equilibrium volume and c/a ratio with the experimental data is found. Also we minimize the forces at equilibrium volume and calculate the symmetry-free structural parameters of the space group P4/mmm for Y₂CoIn₈. In Y₂CoIn₈, the Co 3d states are almost fully occupied and situated below the Fermi level. We applied the fixed-spin-moment method and a stable paramagnetic ground state for Y₂CoIn₈ was found. Finally, the crystal field (CF) parameters were calculated for R=Pr, Nd and Dy from first principles. The microscopic tetragonal CF Hamiltonian was diagonalized and the obtained eigenvalues and eigenfunctions were used to predict the anisotropy of the magnetic properties of R₂CoIn₈ single crystals.     

Crystal field effects on the magnetic and hyperfine properties of Yb3+ in ytterbium ethylsulfate

The magnetic and hyperfine properties of Yb³⁺ in Yb(C₂H₅SO₄)₃·.9H₂O have been studied using the crystal field (CF) obtained from an analysis of the observed absorption spectra of the crystal. The principal magnetic susceptibilities are in reasonable agreement with those observed both at liquid oxygen and liquid helium temperatures. The observed reversal of magnetic anisotropy at 17.7K is also corroborated. The Schottky heat capacity shows a peak at around 40K. The magnetic hyperfine field due to the 4f electrons at the nucleus is found to be 1.79MG. The hyperfine heat capacity C_N has a Schottky anomaly at about 25 mK. Above 1K, in the liquid helium range, C_N follows a simple T^?2 law as observed by Cooke et al. It is concluded that in the rare earth ethylsulfates a single suitable CF gives a good account of the various properties from room temperature down to liquid He temperatures.     

Magnetization and heat capacity studies of double perovskite compounds Ba2SmRuO6 and Ba2DyRuO6

Here we report the magnetic and heat capacity studies of the double perovskite compounds Ba₂SmRuO₆ and Ba₂DyRuO₆. Antiferromagnetic transitions are inferred at 54 and 47 K in Ba₂SmRuO₆ and Ba₂DyRuO₆, respectively, in the magnetization measurements. Heat capacity measurements show large jumps at the corresponding temperatures and confirm the bulk magnetic ordering. Both the measurements provide clear indication of the ordering of the rare earth moments also along with the Ruthenium moments. However, the heat capacity results suggest that the ordering of rare earth magnetic moments is spread over a large temperature range. An anomaly observed in the magnetization measurements at 42 K (below the magnetic ordering) in Ba₂SmRuO₆ is discerned as a reorientation of Sm³⁺ moments.     

Calculation of thermophysical properties for CO2 gas using an ab initio potential model

The density, isochoric heat capacity, shear viscosity and thermal conductivity of CO₂ gas in the pressure range of 1–50 atm and 300 K are calculated based on a five-centre potential model obtained from ab initio calculations of the intermolecular potential of a CO₂ dimer. The quantum effects of the intramolecular motion are included in a model by the Monte Carlo (MC) Method. Without using any experimental data, the present model achieves excellent agreements between the calculated thermophysical properties and experimental data for all simulated CO₂ densities except the highest one at 135 kg/m³ (3 mol/L). The contributions of potential to the thermophysical properties of the moderate dense CO₂ gas and their dependence on density are investigated in detail.     

Magnetic properties of R2Re2Si2C (R=Ho and Er)

We have investigated magnetic properties of R₂Re₂Si₂C (R=Ho and Er) using magnetic susceptibility, magnetization and heat capacity measurements. Both the materials order antiferromagnetically. The ordering temperatures (T_N) for Ho₂Re₂Si₂C and Er₂Re₂Si₂C are, ∼8.8 and ∼7.6 K, respectively. Our measurements indicate crystal field effects in the bulk properties of both these compounds. The experimental results have been analyzed by taking into account the effect of crystalline electric field and magnetic exchange interaction.     

A first-principles study on the structural, elastic, vibrational, and thermodynamical properties of BaX (X=S, Se, and Te)

Ab-initio calculations based on norm-conserving pseudopotentials and density functional theory (DFT) have been performed to investigate the structural, elastic, thermodynamic, and lattice dynamical (phonon dispersion curves) properties of BaX in rock-salt (B1) and CsCl (B2) structures. The results support the experimental and theoretical data in the existing literature. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as entropy, heat capacity, internal energy, and free energy for the same compounds in the B1 phase.