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.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Low-temperature study of magnetic ordering in gadolinium orthophosphate

The zero-field heat capacity shows an antiferromagnetic ordering of Gd³⁺ in gadolinium orthophosphate at 0.8 K. The application of the external magnetic field leads to the splitting of the Gd³⁺ ground-state multiplet. The antiferromagnetic ordering becomes gradually suppressed with increasing field, and the loss of the long-range magnetic ordering with a threshold field between 0.2 and 0.5 T is indicated by heat-capacity data. Estimated entropy of the anomaly due to magnetic ordering or the Schottky-type anomaly (above 0.5 T) is close to Rln8 as expected for Gd³⁺ ground-state multiplet. Magnetization measurements above 2 K corroborate this magnetic behaviour.     

Heat capacity of paramagnetic nickelocene: Comparison with diamagnetic ferrocene

Nickelocene [bis(η⁵-cyclopentadienyl)nickel: Ni(C₅H₅)₂, electron spin S=1, the ground state configuration ³A_2g] is paramagnetic and belongs to a typical molecule-based magnet. Heat capacities of nickelocene have been measured at temperatures in the 3−320 K range by adiabatic calorimetry. By comparing with those of diamagnetic ferrocene crystal, a small heat capacity peak centered at around 15 K and a sluggish hump centered at around 135 K were successfully separated. The low-temperature peak at 15 K caused by the spin is well reproduced by the Schottky anomaly due to the uniaxial zero-field splitting of the spin S=1 with the uniaxial zero-field splitting parameter D/k=45 K (k: the Boltzmann constant). The magnetic entropy 9.7 J K⁻¹ mol⁻¹ is substantially the same as the contribution from the spin-manifold R ln 3=9.13 J K⁻¹ mol⁻¹ (R: the gas constant). The sluggish hump centered at around 135 K arises from rotational disordering of the cyclopentadienyl rings of nickelocene molecule. The enthalpy and entropy gains due to this anomaly are 890 J mol⁻¹ and 6.9 J K⁻¹ mol⁻¹, respectively. As the hump spreads over a wide temperature region, separation of the hump from the observed heat capacity curve involves a little bit ambiguity. Therefore, these values should be regarded as being reasonable but tentative. The present entropy gain is comparable with 5.5 J K⁻¹ mol⁻¹ for the sharp phase transition at 163.9 K of ferrocene crystal. This fact implies that although the disordering of the rings likewise takes place in both nickelocene and ferrocene, it proceeds gradually in nickelocene and by way of a cooperative phase transition in ferrocene. A reason for this originates in loose molecular packing in nickelocene crystal. Molar heat capacity and the standard molar entropy of nickelocene are larger than those of ferrocene beyond the mass effect over the whole temperature region investigated. This fact provides with definite evidences for the loose molecular packing in nickelocene crystal.     

Structure and low-temperature properties of Ba8Ni6Ge40

Structural, magnetic, heat capacity, electrical and thermal transport properties are reported on polycrystalline Ba₈Ni₆Ge₄₀. Ba₈Ni₆Ge₄₀ crystallizes in a cubic type I clathrate structure with unit cell a=10.5179 (4) Å. It is diamagnetic with susceptibility χ_dia=−1.71×10^-6 emu/g Oe. An Einstein temperature 75 K and a Debye temperature 307 K are estimated from heat capacity data. It exhibits n-type conducting behavior below 300 K. It shows high Seebeck coefficients (−111×10^-6 V/K), low thermal conductivity (2.25 W/K m), and low electrical resistivity (8.8 mΩ cm) at 300 K.     

Investigation of energy transfer and frequency upconversion in Ho/Yb co-doped tellurite glasses

A serials of Ho³⁺/Yb³⁺ co-doped tellurite glasses by pumping 970 nm laser diode (LD) were demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Two intense emission bands were observed in Ho³⁺/Yb³⁺ co-doped tellurite glasses centered at 549 and 664 nm corresponding to Ho³⁺: ⁵S₂(⁵F₄)→⁵I₈ and ⁵F₅→⁵I₈ transitions, respectively. The upconversion intensities of red and green emissions in Ho³⁺/Yb³⁺ co-doped glasses were enhanced largely when increasing Yb₂O₃ content. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model. The energy transfer coefficients were estimated by fitting the simulated curves to the measured ones. The obtained three energy transfer coefficients C_D2, C_D3 and C_D4 were C_D2=5.0×10⁻¹⁸ cm³/s, C_D3=1.5×10⁻¹⁷ cm³/s, C_D4=9.0×10⁻¹⁷ cm³/s.     

Magnetic measurements on single crystals of Ho(CF3SO3)3.9H2O and crystal field investigation

Magnetic measurements on single crystals of holmium trifluoromethanesulfonate (HoTFMS) have been carried out from 300 to 12.5 K. The hexagonal crystal structure of HoTFMS is very similar to that of the hydrated rare earth ethylsulfates with the non-Kramers Ho³⁺ ion occupying a site of C_3h symmetry. A very good theoretical simulation of the principal magnetic susceptibilities, observed by us and the Friedberg group, over the wide range 300 to ∼1.0 K was obtained with the crystal field J-mixed eigenvectors taking into consideration intermediate coupling effects. No ordering effects were noticed by us down to ∼12.5 K indicating the interionic interaction to be of predominantly dipolar type consistent with the discovery of a ferromagnetic transition at T∼0.23 K by the Friedberg group. The g-values are found to be in reasonable agreement with those derived from other sources. The Schottky anomaly in the electronic heat capacity observed at ∼6 K by the Friedberg group is borne out excellently by our theoretical values computed from the crystalline Stark pattern. The thermal behavior of quadrupole splitting and hyperfine heat capacity is calculated and our computed values explain very well the principal anomaly in C_N observed experimentally.     

Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4(TCNQ) showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH₄-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH₄-TCNQ. The enthalpy and entropy of transition are Δ_trsH=(667±7) J mol⁻¹ and Δ_trsS=(2.19±0.02) J K⁻¹ mol⁻¹, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ_trsS_mag=0.61 J K⁻¹ mol⁻¹ and the lattice contribution as Δ_trsS_lat=1.58 J K⁻¹ mol⁻¹. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K⁻¹ mol⁻¹), a majority of the magnetic entropy (∼4.6 J K⁻¹ mol⁻¹) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.     

The effect of Er doping on the spin reorientation transition in Ho1−xErxAl2

The magnetothermal properties of pseudo binary Ho_1−xEr_xAl₂ alloys have been investigated by heat capacity measurements. Two anomalies are observed in the heat capacity of HoAl₂. A sharp peak at 20 K represents the first order spin reorientation transition, and a second order anomaly occurs in the vicinity of the ferromagnetic transition at 32 K. As Ho is partially replaced by Er in Ho_1−xEr_xAl₂ the sharpness of the first order heat capacity peak diminishes with increasing Er concentration, while the temperature of this transition remains practically unaffected. The second order ferromagnetic transition shifts to higher temperature region with increasing Er concentration. The observed behaviors are explained considering the geometry of 4f charge densities of Ho³⁺ and Er³⁺ and the easy magnetization directions of HoAl₂ and ErAl₂.     

Electronic and magnetic properties of layered LnFePO (Ln=La and Ce)

Effects of the replacement of La with Ce on the electronic and magnetic properties of a layered superconductor LaFePO (T_c=∼5 K) were studied. Polycrystalline samples of CeFePO, prepared by a solid-state reaction, showed metallic conduction down to 2 K without exhibiting superconducting transition, although the resistivity decreased largely at temperatures below 30 K. Further, they showed an apparent positive magnetoresistance (MR) below ∼2 K, superposed on a negative MR. Temperature dependence of magnetic susceptibility is decomposed to a temperature-sensitive Curie-Weiss component presumably due to the Ce³⁺ ions with a magnetic moment of 1.98μ_B and a less temperature-sensitive component attributable to itinerant electrons. The magnetic interaction between Ce³⁺ ions and itinerant electrons in CeFePO likely suppresses the superconducting transition observed in LaFePO.     

Magnetic field influence on the low temperature heat capacity of the spin-Peierls compound CuGeO3

We have investigated the effect of magnetic field on the low-temperature heat capacity C_p of the undoped spin-Peierls inorganic compound CuGeO₃ in the dimerized phase. Below 1 K, C_p is dominated by a Schottky anomaly, which is removed above 1 K for field B higher than 3 T. This anomaly is well accounted for by a molar concentration x=0.75×10⁻³ of intrinsic defects, which occur predominantly on the Cu chains. This amount is confirmed by magnetization measurements. A second contribution, varying as T^−ν with ν=1 or 2, rises up with the field for B>1 T in the lower temperature range (from 70 mK to 0.3 K). At high field this contribution becomes very sensitive to the experimental dynamics.     

Temperature dependence of thermal property for lead nitrate crystal

Thermal property was measured in a lead nitrate crystal, Pb(NO₃)₂, at temperatures from 90 to 340 K by use of ac calorimetry technique. The heat capacity derived from the measurements showed temperature dependence with thermal hysteresis, in the temperature region from 240 to 300 K. The anomaly of the heat capacity was found in the vicinity of 275.22 K. The broad temperature variation in the heat capacity was observed in the region from 235 to 260 K.     

High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo₄O₇ has been investigated. It is shown that the oxides CaBaCo₄O_7+δ and CaBaCo_4−xO_7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with T_C~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co³⁺ cations, with respect to the ideal formula CaBaCo₂²⁺Co₂³⁺O_7. This excess Co³⁺ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

Effect of holmium substitution for the improvement of multiferroic properties of BiFeO3

This paper reports on multiferroic properties of Ho substituted BiFeO₃ (Bi_1−xHo_xFeO₃) ceramics. It is observed that for x=0.15, a prominent ferroelectric loop is seen at 300 K even if the system remains in rhombohedral (R3c) phase without appearance of any observable impurity phases. A well shaped M-H loop is observed at 10 K for x=0.15. However it showed ferromagnetism, confirming the contribution of Ho³⁺ towards enhancement of ferromagnetic properties of BiFeO₃ at 300 K. Suppression of impurity phases of pure BiFeO₃ bulk ceramic favors the reduction of mobile oxygen vacancies and reduces leakage current, due to which ferroelectric properties of BiFeO₃ is enhanced. We argue that Ho substitution at Bi site is likely to suppress the spiral spin modulation and at the same time increase the canting angle, which favors enhanced multiferroic properties. XRD, SEM, magnetization, polarization and chemical bonding analysis measurements were carried out to explain the multiferroic behavior.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

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.     

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.     

Specific heat and magnetic susceptibility of single-crystalline ZnCr2−xAlxSe4 (x=0.15, 0.23)

A correlation between the second critical field H_c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr_2−xAl_xSe₄ spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C(T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T_N, a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T_N and is maximal at the spin fluctuation temperature T_sf=34 K. T_sf is related to the maximum of the magnetic susceptibility at T_m=40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2−x)R ln(2S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr³⁺ 3d³ electronic configuration.     

Calorimetric investigation of the magnetic transition in quasi-one-dimensional molecule-based ferrimagnets: [Mn(OC14H29)4TPP][TCNE]·MeOH and [MnF4TPP][TCNE]·0.5MeOH

Heat capacity study was performed, for the first time, for [MnF₄TPP][TCNE]·0.5MeOH and [Mn(OC₁₄H₂₉)₄TPP][TCNE]·MeOH complexes in the 1.8-100 K temperature range under the 0-9 T magnetic field and disclosed new aspects inherent in such strongly coupled charge-transfer Mn-porphyrin-TCNE linear chain systems, where TPP=5,10,15,20-tetraphenylporphyrinato, TCNE=tetracyanoethylene and MeOH=methanol. Any heat capacity anomaly due to the onset of the magnetic long-range-order was not detected, whereas the magnetic phase transition has clearly been observed around 20 K by previous magnetic studies. As these materials are well approximated by quasi-one-dimensional ferrimagnetic Heisenberg chains with very large intrachain spin-spin interactions, the most part of the magnetic entropy is retained above the phase transition temperature as the dominant short-range order. This is the reason why no magnetic phase transition was detected by calorimetry. On the other hand, the big effect observed in the magnetic susceptibility is well accounted for if the formation of magnetic domains is assumed in the crystal.