Low temperature thermodynamical properties of ErCu2Si2

ErCu₂Si₂ crystallises in the tetragonal ThCr₂Si₂-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k=(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.     

A magnetic study of the ThCr2Si2-type RPd2Ge2 (R=Pr, Nd) compounds. Magnetic structure of PrPd2Ge2 from powder neutron diffraction

The magnetic properties of the PrPd₂Ge₂ and NdPd₂Ge₂ compounds have been investigated by magnetic measurements, specific heat measurements and neutron diffraction experiments. The PrPd₂Ge₂ compound orders antiferromagnetically below T_N=5.0(2) with an original modulated magnetic structure characterized by a magnetic cell three times larger than the chemical one by tripling of the c parameter. The palladium atom is non magnetic and the Pr moments are parallel to the c-axis with a value of ≈2.0 μ_B at 2 K. The specific heat measurements clearly detect a low temperature transition for the NdPd₂Ge₂ compound, interpreted as a Nd sublattice antiferromagnetic ordering below 1.3(2) K.     

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.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Magnetic properties of NdAu2Ge2

Physical properties of NdAu₂Ge₂, crystallising with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric and electrical transport measurements as well as by neutron diffraction. The compound exhibits antiferromagnetic ordering below T_N=4.5 K with a collinear magnetic structure of the AFI-type. The neodymium magnetic moments are parallel to the c-axis and amount to 1.04(4) μ_B at 1.5 K. The observed magnetic behaviour is strongly influenced by crystalline electric field effect.     

Growth, thermal and spectroscopic characterizations of BaNd2(MoO4)4 crystal

A new crystal, BaNd₂(MoO₄)₄, has been grown from the flux melt based on Li₂Mo₃O₁₀ by a spontaneous nucleation method. The phase structure of the obtained crystals was determined by X-ray powder diffraction. The result shows that the as-grown crystals are well crystallized and indexed in a monoclinic crystal system with space group B2/b. The specific heat of BaNd₂(MoO₄)₄ crystal at 20 °C is 0.485 J/g K. Absorption and fluorescence spectra were also measured at room temperature. There are several strong and broad absorption peaks from 200 to 1200 nm and three emission transition bands located at 890, 1060, and 1334 nm are detected.     

Magnetic properties of PrRh2Si2: A neutron diffraction study

The magnetic properties of polycrystalline PrRh₂Si₂ sample have been investigated by neutron diffraction measurements. Antiferromagnetic transition with an anomalously high ordering temperature (T_N∼68 K) is clearly observed in magnetic susceptibility, specific heat, electrical resistivity and neutron diffraction measurements. Neutron diffraction study shows that Pr³⁺ ions carry an ordered moment of 2.99(7)μ_B/Pr³⁺ and align along the crystallographic±c-directions for the ions located at the (0,0,0) and positions. The magnetoresistance at 2 K and 10 T is rather large (∼35%).     

Magnetic structure of TbNiAl4

Magnetisation and specific heat measurements, in the range 2 K to room temperature, demonstrate that three magnetic phases exist for the intermetallic compound TbNiAl₄. Powder neutron diffraction, also carried out over a wide temperature range, establishes that the intermediate magnetic phase is incommensurate, and confirms that the lowest temperature phase has a linear antiferromagnetic structure with a (0 1 0) propagation vector. The respective transition (Néel) temperatures, in zero applied magnetic field are 34.0 and 28.0 K.     

Magnetic and magnetocaloric properties of Gd1−xScxNi2 solid solutions

Magnetic and heat capacity measurements have been carried out on the polycrystalline Gd_1−xSc_xNi₂ solid solutions (0≤x≤1), which crystallize in the cubic C15 Laves phases superstructure (space group F4?3m). These solid solutions are ferromagnetic with a Curie temperature below 76 K. Their Curie temperature decreases from 75.4 K for GdNi₂ to 13.6 K for Gd_0.2Sc_0.8Ni₂. At high temperatures, all solid solutions, except ScNi₂, are Curie-Weiss paramagnets. The Debye temperature as well as phonon, conduction electron and magnetic contributions to the heat capacity have been determined from heat capacity measurements. The magnetocaloric effect has been estimated both in terms of isothermal magnetic entropy change and adiabatic temperature change for selected solid solutions in magnetic fields up to 3 T.     

Heat capacity and magnetic properties of pyrochlore Hg2Os2O7

Hg₂Os₂O₇, which has the cubic pyrochlore structure, remains metallic down to the liquid helium temperature unlike its isostructural counterpart Cd₂Os₂O₇, which shows metal-insulator transition at 226 K. Magnetization and heat capacity data for Hg₂Os₂O₇ are presented. The magnetic anomaly at T_N=88 K shares many characteristics in common with the metal-insulator transition in Cd₂Os₂O₇, though Hg₂Os₂O₇ remains metallic below T_N. The heat capacity C_p shows no or very little change in the magnetic entropy around T_N, supporting the view that there is no long-range ordering of localized spins. The measured value of electronic heat-capacity coefficient γ=21 mJ K⁻²mol⁻¹ is comparable to the value obtained from band-structure calculation on Cd₂Os₂O₇, suggesting that mass-enhancement is small in Hg₂Os₂O₇. There is a pronounced peak in C_p/T³ at 13.1 K, which corresponds to a peak in the phonon density of states at 40 cm⁻¹.     

Magnetic order and crystal field in Dy2Ru2O7 and Yb2Ru2O7

The magnetic properties of R₂Ru₂O₇ pyrochlore compounds (R=Yb, Dy) were studied using specific heat down to 0.4 K and bulk magnetic measurements. These two rare-earth elements were chosen to demonstrate the effect of Ru-R exchange interaction on R magnetic sublattice, in two cases of anisotropy: axial in Dy and planar in Yb. Dy₂Ru₂O₇ undergoes a second order transition to a fully ordered state at 1.85 K with no signs of the spin-ice state. In Yb₂Ru₂O₇ the Yb sublattice orders gradually around 8 K due to the Ru molecular field and no further transition is observed down to 0.4 K. Including the Ru molecular field at the R site in calculations based on crystal field parameters known from titanates R₂Ti₂O₇, allowed us to interpret experimental data.     

Magnetic ordering and low-temperature thermodynamic properties of ErFe2Ge2

Polycrystalline sample of ErFe₂Ge₂ was investigated by means of magnetic susceptibility, heat capacity and electrical resistivity measurements, as well as by powder neutron diffraction. All these experiments yielded an evidence of magnetic ordering setting at about 3 K. The low-temperature neutron data revealed the formation of a sine-modulated commensurate antiferromagnetic structure characterized by the propagation vector k=(0, 0, ). The erbium magnetic moment is aligned parallel to the crystallographic a-axis. At T=1.55 K it is equal to 7.06(5) μ_B.     

Physical properties of the cubic spinel LiMn2O4

We have performed an ab initio study of structural, electronic, magnetic, vibrational and thermal properties of the cubic spinel LiMn₂O₄ by employing the density functional theory, the linear-response formalism, and the plane-wave pseudopotential method. An analysis of the electronic structure with the help of electronic density of states shows that the density of states at the Fermi level (N (E_F)) is found to be governed by the Mn 3d electrons with some contributions from the 2p states of O atoms. It is important to note that the contribution of Mn 3d states to N(_EF)$N\left(E_F\right)$ is as much as 85%. From our phonon calculations, we have obtained that the main contribution to phonon density of states (below 250 cm⁻¹) comes from the coupled motion of Mn and O atoms while phonon modes between 250 cm⁻¹ and 375 cm⁻¹ are characterized by the vibrations of all the three types of atoms. The contribution from Li increases rapidly at higher frequency (above 375 cm⁻¹) due to the light mass of this atom. Finally, the specific heat and the Debye temperature at 300 K are calculated to be 249.29 J/mol K and 820.80 K respectively.     

Crystal and magnetic structures of Y2CrS4

Chromium(II) sulfide, Y₂CrS₄, prepared by a solid-state reaction of Y₂S₃ and CrS, showed an antiferromagnetic transition at 65 K. The neutron diffraction patterns at 10 and 90 K were both well refined with the space group Pca2₁. At 90 K, cell parameters were a=12.5518(13) Å, b=7.5245(8) Å, and c=12.4918(13) Å. At 10 K, magnetic peaks were observed, which could be indexed on the same unit cell. Magnetic moments of chromium ions were parallel to the b-axis and antiferromagnetically ordered in each set of the 4a sites.     

Magnetic and crystallographic study of Tb0.75Y0.25Co3B2

Tb_0.75Y_0.25Co₃B₂ was studied as a function of temperature by neutron powder diffraction, ac susceptibility and SQUID magnetization measurements. The solid solution, which is of hexagonal symmetry and is paramagnetic at 300 K, undergoes a magnetic Co–Co ordering transition at ∼150 K, and a second magnetic Tb–Tb ordering transition at ∼17 K. The latter induces a spin-reorientation transition, in which the magnetic axis rotates from the c-axis toward the basal plane. The component of the magnetic axis, which is perpendicular to c, leads to a crystal symmetry reduction from hexagonal to monoclinic. The observed magnitude of the magnetic moment of the Tb ion is 1.5 μ_B, unusually small relative to the free ion and parent compound (TbCo₃B₂) values. These magnetic and crystal properties are discussed and compared with what was previously published for the parent compound.     

Inelastic neutron scattering an ab-initio calculation of negative thermal expansion in Ag2O

The compound Ag₂O undergoes large and isotropic negative thermal expansion over 0–500 K. We report temperature dependent inelastic neutron scattering measurements and ab-initio calculations of the phonon spectrum. The temperature dependence of the experimental phonon spectrum shows strong anharmonic nature of phonon modes of energy around 2.4 meV. The ab-initio calculations reveal that the maximum negative Grüneisen parameter, which is a measure of the relevant anharmonicity, occurs for the transverse phonon modes that involve bending motions of the Ag₄O tetrahedra. The thermal expansion is evaluated from the ab-initio calculation of the pressure dependence of the phonon modes, and found in good agreement with available experimental data.     

A re-evaluation of the heat capacity of cerium zirconate (Ce2Zr2O7)

The heat capacity of cerium zirconate pyrochlore, Ce₂Zr₂O₇, was measured from 0.4 to 305 K by hybrid adiabatic relaxation method for various magnetic field strengths. Magnetisation measurements were performed on the sample also. The results revealed a low-temperature anomaly that showed Schottky-type characteristics with increasing magnetic field strength. The estimated entropy due to the magnetic ordering of the two Ce³⁺ moments is 1.37R, close to the theoretical value for a doublet ground state (1.39R). The enthalpy increments relative to 298.15 K were measured by drop calorimetry from 531 to 1556 K. The obtained results significantly differ from those reported in the literature; the origin of the discrepancy is due to the probable oxidation of the pyrochlore structure into fluorite.     

Unusual thermal conductivity of the negative thermal expansion material, ZrW2O8

In order to investigate the relationship between negative thermal expansion and other thermal properties, the thermal conductivity of the α-phase of ZrW₂O₈ has been determined from 1.9 to 390 K. In addition, the heat capacity was measured from 1.9 to 300 K. The thermal conductivity of ZrW₂O₈ is low, glass-like and close to its theoretical minimum value. The phonon-phonon coupling of the highly anharmonic low-frequency modes which are responsible for negative thermal expansion in ZrW₂O₈ appears to be highly efficient, leading to short phonon mean free paths and exceptionally low thermal conductivity.     

Magnetic anomalies in single crystalline ErPd2Si2

Considering certain interesting features in the previously reported ¹⁶⁶Er Mössbauer effect, and neutron diffraction data on the polycrystalline form of ErPd₂Si₂ crystallizing in the ThCr₂Si₂-type tetragonal structure, we have carried out magnetic measurements (1.8–300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in features due to magnetic ordering in the plot of magnetic susceptibility χ versus temperature T at low temperatures. The χ(T) data reveal that there is a pseudo-low-dimensional magnetic order setting in at 4.8 K, with a three-dimensional antiferromagnetic order setting in at a lower temperature (3.8 K). A new finding in the χ(T) data is that, for H∥〈1 1 0〉 but not for H∥〈0 0 1〉, there is a broad shoulder in the range 8–20 K, indicative of the existence of magnetic correlations above 5 K as well, which could be related to the previously reported slow-relaxation-dominated Mössbauer spectra. Interestingly, the temperature coefficient of electrical resistivity is found to be isotropic; no feature due to magnetic ordering could be detected in the electrical resistivity data at low temperatures, which is attributed to magnetic Brillioun-zone boundary gap effects. The results reveal the complex nature of magnetism of this compound.     

Heat capacity of EuMnO3 and Eu0.7A0.3MnO3 (A=Ca,Sr) compounds

We carried out the heat capacity calculation of the magnetoresistance compounds EuMnO₃ and Eu_0.7A_0.3MnO₃ (where A=Ca and Sr) as a function of temperature from 5 to 100 K, using the Rigid Ion Model (RIM). The results on heat capacity for EuMnO₃ and Eu_0.7A_0.3MnO₃ (A=Ca and Sr) obtained by us are in good agreement with the measured values. Although strong electron–phonon interactions are present in these compounds but the lattice part of the specific heat also deserves proper attention. The parent compound EuMnO₃ exhibits two magnetic transitions at 35 and 47 K due to weak ferromagnetic (FM) component and antiferromagnetic (AF) ordering. In addition, we have reported cohesive energy (φ), molecular force constant (f), compressibility (β), Restrahalen frequency (υ₀), Debye temperature (θ_D) and Gruneisen parameter (γ) in the temperature range 5 K?T?100 K.