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.     

Thermal expansion and specific heat of intermediate valent YbCuAl

The specific heat, thermal expansion and resistivity was measured for the intermediate valence compound YbCuAl and its reference compound LuCuAl between 1.5 and 400 K. All quantities show strong anomalies. The entropy of the specific heat anomaly is found to be nearR·ln9, the value expected for the entropy of a high temperature mixture of Yb-ions in the ratio dictated by the degeneracies of the Hund's rule groundstates of 4f ¹⁴ and 4f ¹³. TheT ³ coefficient of the low temperature specific heat of YbCuAl contains a large electronic contribution. The integrated thermal expansion anomaly indicates a shift of the valence by at least 3.5% towards 3⁺ between 1.5 and 310 K.     

A quantum spherical model of spin glass

A quantum spherical model of spin glass is solved. The solution exhibits the usual spin glass phase transition, but with-out any unphysical features at low temperatures. In particular, the entropy is always positive and vanishes like T³ as T → 0.     

The entropy expression of a Fermi liquid in shielded potential approximation. Application to3He

Using the functional representation of the thermodynamical potential, a simple expression for the entropy of a normal Fermi liquid in shielded potential approximation is derived. The result is valid for arbitrary temperatures above a possible phase transition and contains both Fermi (single-particle) and Bose (collective) contributions. It thus represents an improvement over the quasi-particle approximation. — Applying this result to the fluctuation model of liquid³He, we find a Bose contribution of the orderT ³ logT/Θ from spin fluctuations as well as the usual quasi-particle term.     

Lu-induced spin entropy enhancement in Ca3Co4O9+δ system

The magnetothermopower have been studied in Lu-doped Ca_3?xLu_xCo₄O_9+δ. A strong magnetic field suppression of thermopower indicates large spin entropy contribution. The magnetothermopower for doped samples are overall enhanced compared with that for undoped Ca₃Co₄O_9+δ, providing an experimental evidence for the enhancement of spin entropy. Magnetic results confirm that Co⁴⁺ concentration is reduced by Lu doping. We suggest that the reduction in Co⁴⁺ concentration results in the enhanced spin entropy.     

On the specific heat of fermi liquids

The spin fluctuation contribution to theT ³ logT term in the specific heat of nearly ferromagnetic Fermi liquids is evaluted again. A result different from^1,2 is found, which in the case of liquid Helium agrees better with experiment.     

Thermal expansion and specific heat of intermediate valent YbCuAl

The specific heat, thermal expansion and resistivity was measured for the intermediate valence compound YbCuAl and its reference compound LuCuAl between 1.5 and 400 K. All quantities show strong anomalies. The entropy of the specific heat anomaly is found to be nearR·ln9, the value expected for the entropy of a high temperature mixture of Yb-ions in the ratio dictated by the degeneracies of the Hund's rule groundstates of 4f ¹⁴ and 4f ¹³. TheT ³ coefficient of the low temperature specific heat of YbCuAl contains a large electronic contribution. The integrated thermal expansion anomaly indicates a shift of the valence by at least 3.5% towards 3⁺ between 1.5 and 310 K.     

The melting pressure and entropy of spin ordered solid 3He

We report measurements of the melting pressure of solid ³He between 0.36 mK and 1.2 mK. At 1.030 ± .005 mK we observe a first order phase transition in the solid with a loss in entropy of 0.443R ln². Below ～0.6 mK the melting pressure varies as T⁴, in agreement with antiferromagnetic spin wave theory.     

Pressure effects on the magnetic transition temperatures of NiS2

Both antiferromagnetic (T_N) and weak ferromagnetic (T_c) transition temperatures of NiS₂ have been measured under pressure up to 18.4 kbar. Values of dT_N/dP=(0.9 ± 0.1) deg/kbar and dT_c/dP=(0.4 ± 0.1) deg/kbar are obtained. The present results allow estimates of the magnetic Grüneisen constant γ_m = - d ln T_N/d ln V of (26±5) and of the entropy change of (1.4±0.5) J/mol·deg at the first order transition temperature T_c.     

Scaling of magnetoresistance of carbon nanomaterials in Mott-type hopping conductivity region

A method for analyzing data on Mott hopping conduction in a magnetic field, ρ ～ exp[(T ₀/T)^α], based on scaling relation ln[ρ(H)/ρ(0)] = (T ₀/T)^α F(H/T) for the spin-polarized contribution to the magnetore-sistance is proposed. This general approach is tested for a carbon nanomaterial synthesized from single-wall carbon nanotubes under high pressure (up to 7 GPa). The experiments confirmed the theoretical predictions over the temperature range 1.8–12.0 K in a magnetic field of up to 70 kOe and made it possible to correctly determine all parameters of the localized states involved in the model. The experimental data obtained for carbon nanomaterials synthesized from single-wall carbon nanotubes and a mixture of C_2N fullerenes indicate the possible renormalization of the magnetic moment of electrons involved in hopping transport.     

Effect of pressure on the distribution of hyperfine field about A Sn impurity in iron

The distribution of hyperfine field at the iron atoms in an FeSn alloy has been measured as a function of pressure to 15 kbar. The frequency of the main NMR line was found to change such that (? ln H_m/?P)_T = ? 1.7 × 10^?4 kbar^?1, which is close to the value in pure iron. The value for the satellite line, due to iron atoms which are third nearest neighbours of the impurity, was (? ln H₃/?P)_T = ? 2.4 × 10^?4 kbar^?1. The magnitude of the difference of the pressure derivatives of H_m and H₃ is not consistent with models in which the Sn atom is screened by conduction electrons but may be understood in terms of a perturbation of the 3d band of iron. The discrepancy between spin wave theory and the measured temperature dependence of the hyperfine fields at constant pressure is not removed by correction to constant volume.     

On perturbation theory at finite temperature

We show that the largest temperature-dependent contribution to the fermion self-energy of order α² in QED goes as (α²T⁴/p²)ln[(E + p)/(E ? p)], where p is the 3-momentum and E is the energy, and we speculate on the form of higher-order terms in the perturbation expansion.     

Disappearance of the heat capacity peak of Sc3In around the curie temperature in high magnetic fields

The low temperature (1.3–20.0 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to ～ 10 T. The heat capacity peak observed around T_c = 6.0 K in zero field becomes smaller with increasing fields and at 9.98 T its magnetic entropy is ≈ 18% of the zero field value. Above T_c, the spin fluctuation contribution to the heat capacity, which is enhanced by the magnetic field at lower fields (?5 T), is quenched at higher fields (?5 T). This depression of the spin fluctuation contibution to the heat capacity by the high magnetic fields occurs at lower magnetic fields than had been considered possible heretofore. Our results suggest that the itinerant ferromagnetism is Sc₃In is completely quenched at 12 T.     

Far-infrared investigation of the phase transition at 217 K in layer-structured TiCl3

Far-infrared measurements reveal a distortion in layer-structured TiCl₃ below 217 K. This is explained by the formation of pairs of covalently bonded Ti³⁺ ions below this temperature. The electronic contribution to the entropy increase above 217 K is attributed to the degree of freedom of the spin of the d electron localized on the Ti³⁺ ion.     

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.     

Temperature dependence of the contribution to the transport coefficients of nearly ferromagnetic metals from electron-paramagnon scattering

We present calculations of the temperature dependence of the contribution from electron-paramagnon scattering to the electrical and thermal resistivity of a simple model of a nearly ferromagnetic metal. The purpose of the work is to explore the behavior of these quantities when the temperature is the order of or greater than the spin fluctuation temperature T_sf. As the temperature T is raised from zero through T_sf, the electrical resistivity varies more slowly with temperature than the T² law characteristic of the regime T?T_sf. When T?T_sf, the electrical resistivity becomes proportional to T, although this asymptotic behavior is approached very slowly. The Lorenz number rises monotonically with temperature, and appears to approach the ideal Sommerfeld value when T?T_sf, although this limit is also approached slowly.     

Measurement of complex susceptibility on a metallic spin glass with broad relaxation spectrum

AC susceptibility measurements (0.625 H_z?_v?625 H_z) have been performed in a SQUID magnetometer on the amorphous metallic spin glass (Fe_0.06Ni_0.94)₇₅P₁₆B₆Al₃. The in-phase component of the susceptibility (x') shows a sharp peak at the spin glass freezing temperature T_g≈8 K with a frequency dependence of ?T_g/T_g≈0.015 per decade of frequency increase. The out-of-phase component (x″) exhibits a small anomaly at the spin glass transition with the magnitude of the anomaly almost independent of frequency. In particular we find the relation x″=(π;/2)?x'/? lnω to be valid in the vicinity of T_g. This relation and a frequency independence of x″ are fundamental characteristics of a spin glass having a broad spectrum of relaxation times. We argue that similar results from ac susceptibility measurements should be found in many spin glass systems, metallic as well as non-metallic.     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.     

Temperature dependence of the electronic spin relaxation of DCNQI salts

For the elucidation of the charge and spin dynamics of the radical anion salts of DCNQI with metallic counterions we have performed cw- and pulsed ESR experiments (βB _pp,T _1e ^?1 andT _2e ^?1 ) between 300 and 4 K at nine salts differing in counterions and sidegroups, respectively. We can explain the relaxation rates by dipolar electron-electron interaction and spin-orbit contribution. In the high temperature range we have a gradual decrease in the number of charge carriers by interband transitions without a slowing down of the mobility. With complete localization of the electron spins (no mobile electrons anymore) exchange interaction governs the spectral density, becoming strongly temperature dependent due to effective spin exchangeJ _eff(T), explained by an extended REHAC-model. This effective spin exchangeJ _eff(T) includes for the first time a contribution by the metallic counterions. For spin-orbit interaction we developed a model based on F. Adrian [1] not depending on the mixture of Bloch and spin states as given by Elliott [2]. This is achieved by the inclusion of the electronic probability on atoms with higher atomic numbers, modulated by phonons. This model explains the drastic changes in the ESR linewidth of different radical ion salts of DCNQI and allows inductively the prediction of the electronic properties of new radical ion systems of which just the molecular and crystal structure is known.