Polycrystalline samples of PrNi_{1-x}Cu_{x}Al series were studied by X-ray diffraction, magnetization and specific heat. The hexagonal ZrNiAl-type structure is preserved in the whole series. Compounds with x up to 0.4 order antiferromagnetically with the Néel temperatures between 3 and 5 K. The rest of the compounds (x = 0.5-0.9) exhibits a transition into a spin glass state below the freezing temperatures around 4 K. The analysis of the specific heat data reveals a quasi-doublet ground state well separated from higher crystal field levels in the whole series.
The energy spectrum and peculiarities of field and temperature dependences of the basic thermodynamic characteristics of the finite spin-1/2 XX chain closed by one zz (Ising) bond and open ends XX-chain with two zz-impurities at the both ends have been investigated.
The critical exponents of the 3D Ising model were calculated in the approximation of the fourth-order cumulant expansion. Thermodynamic functions in the high-temperature range are obtained.
The mixed spin-1/2 and spin-S (S>1/2) Ising model on a rope ladder is examined by combining two exact analytical methods. By the decoration-iteration mapping transformation, this mixed-spin system is firstly transformed to a simple spin-1/2 Ising model on the two-leg ladder, which is then exactly solved by the standard transfer-matrix method. The thermal variations of the zero-field specific heat are discussed in particular.
The heat capacity in the applied magnetic field up to 9 T, susceptibility and magnetization of polycrystalline CeCu_4Al are presented. The determined electronic heat capacity coefficient γ= 210 mJ mol^{-1} K^{-2} confirmed heavy fermion character of this compound. Magnetic ordering was not observed down to 0.5 K. Magnetic properties confirm these observations.
In this work we present the study of the part of Ce(Ni,Cu)Al series from the pure CeNiAl to 30% of copper concentration, which illustrates the transition from mixed-valence state of CeNiAl to the trivalent state in CeCuAl. The work is based on X-ray diffraction, magnetization and specific heat measurement. The results indicate smooth transition between the mixed-valence and trivalent cerium state. The specific-heat data reveal increase of the Sommerfeld γ coefficient with copper concentration.
We consider a simplified version of the s-f model, supplemented by the intersite Cooper pairs interaction between conduction electrons with opposite spins to investigate ferromagnetic and superconducting properties of an alloy Re_{1-x}^{(1)}Re_{x}^{(2)}X (Re^{(1),(2)} - rare earth elements, X = Rh_{4}B_{4}) using the virtual crystal approximation. For a suitable choice of the model parameters we can reproduce the phase boundaries of the experimentally measured phase diagrams for Er_{1-x}Ho_{x}X, Er_{1-x}Gd_{x} X, Er_{1-x}Tm_{x}X, Ho_{x}Lu_{1-x}X, and Sm_{1-x}Er_{x}X alloys.
The temperature dependence of the magnetic specific heat was studied experimentally and theoretically in the semimagnetic semiconductor Pb_{1-x}Eu_xTe for x=0.027 and x=0.073, over the temperature range from 0.5 K to 15 K, in magnetic fields up to 2 T. In zero magnetic field at about 2 K there was a broad maximum in the magnetic specific heat, which was much higher than that predicted by the model of superexchange interaction between nearest neighbors; the maximum values increased with magnetic field. The experimental data were analyzed in the framework of a model which takes into account the spin splitting of the ground state of a single Eu2+ ion in the presence of local lattice distortions in the Pb_{1-x}Eu_xTe mixed crystal. The model describes well the experimental data, especially for lower x-values, where the contribution from singlets dominates.
We present a comparative study of Na_{0.7}CoO_2 samples obtained from three different sources and prepared by different methods. The specific heat and magnetic susceptibility measurements in the temperature range 2-300 K show substantial influence on the observed anomalies, which underlines that the system is extremely sensitive to preparation protocols.
We investigate the critical behaviour of the chemical potential and average occupation numbers using the extended s -f model with intersite Cooper pairing for systems with fluctuating valence. The model is able to describe phase transitions from normal ferromagnet to normal paramagnet at T=T_{C}, from superconducting paramagnet to normal paramagnet at T=T_{S}, as well as reentrant phase transitions with three critical temperatures T_{S_{1}}, T_{C} and T_{S_{2}} (T_{S_{1}} < T_{C} < T_{S_{2}}). Present investigation, as well as recent results obtained for another models suggest one-to-one correspondence between critical temperatures of the system and kinks appearing in the temperature dependence of the chemical potential and average occupation numbers. This, in turn, indicates a possibility to apply the measurement of the chemical potential vs. temperature as an experimental universal tool when looking for phase transitions in solids.
Here we present the results of the zero-field specific heat study of the LuFe_6Al_6 single crystal. The specific heat data were analyzed as a sum of the phonon, electronic, and magnetic contributions, respectively. The analysis of the phonon part involves three acoustic and 36 optical branches, respectively, all of them corrected for the anharmonicity. The magnetic part of the specific heat was obtained by subtracting the electronic and the phonon part from the experimental specific heat and the magnetic entropy was calculated.
Ac susceptibility (χ_{ac}) and dc magnetization (M_{dc}) of the organometallic magnet Na[FeO_{6}(C_{10}H_{8}N)_{3}] have been studied in temperature range of 4.2-200 K, in an external magnetic field up to 16 kOe. Some peculiarities of the χ_{ac} and M _{dc} behavior, which are characteristic of spin-glasses, e.g. cusps in the susceptibility and zero-field-cooled magnetization, an irreversibility between the field-cooled and zero-field-cooled magnetization and dependences of the T_{cusp} temperature on the frequency and the intensity of the magnetic field, were observed at low temperatures.
Quantum transfer matrix technique and numerically exact diagonalization method are applied to the Heisenberg spin systems to model ring-shaped molecules. Two cases are investigated: (i) a dozen of S = 1 spins with additional biquadratic exchange and (ii) a dimetallic molecule Cr_7Cd, where it is assumed that exchange anisotropy is determined in a local coordination system. In the latter case the calculated susceptibility is compared with experimental results.
We present analysis of the specific heat of the RTAl (R = Y, Lu;T = Cu, Ni, Pd) compounds. We focus on the lattice contribution and analyze the dependence of all the characteristic parameters in the Debye and Einstein models on the atomic masses and interatomic distances.
The magnetocaloric effect of frustrated antiferromagnetic HoB_{12} is calculated. The isothermal entropy change Δ S characterising the magnetocaloric effect shows a small positive change upon magnetisation below the transition temperature T_{N}, indicating a small inverse magnetocaloric effect. At T_{N}, Δ S shows clear scaling behaviour with the applied magnetic field. The adiabatic temperature change Δ T shows paramagnetic behaviour above T_{N}, despite strong magnetic correlations that persist in this temperature region. The adiabatic temperature change calculated for HoB_{12} is appreciable.
Zero-temperature limits of the local and global thermodynamic quantities in the nine-membered antiferromagnetic s=3/2 spin ring are investigated by means of numerical exact diagonalization. An anisotropic Heisenberg model with tunable bond defect reflecting continuously varying topology (from closed to open ring) is exploited. The frustrated and non-frustrated phases are identified in the ground-state phase diagram determined by a bond-defect strength and magnetic field. Near the phase boundaries significance of the thermal fluctuations affecting the estimates of the local magnetic quantities found earlier at T=1 K is revealed. For the global quantities the effects of thermal fluctuations are found much weaker. A sequence of the local magnetic moments is analysed and their experimental verification at the edges of the non-frustrated and within the entire frustrated phase is suggested at sufficiently low temperature.
The present work is focused on the investigation of magneto-structural correlations in Cu(en)Cl₂ and Cu(tn)Cl₂. A comparative study of powder susceptibility and magnetization of both compounds revealed that the replacement of tn by en ligand did not affect single-ion properties. On the other hand, the structure modification led to significant reduction of magnetic interactions as well as lowering the crystal symmetry. The impact of the high pressure on magnetic properties of the compounds is discussed.
Structural analysis of [Ni(en)(H_2O)_4][SO_4]·2H_2O was performed and it suggests that the crystal field should play a dominant role in the magnetic properties of the system. This conjecture coincides well with the specific heat and susceptibility behaviour. The analysis confirmed that the compound can be treated as a spin 1 single molecule magnet with nonmagnetic ground state introduced by easy-plane single-ion anisotropy D/k_B≈11 K and neglecting in-plane anisotropy E/D <0.1.
We present the experimental evidence for the presence of spinodal decomposition of the magnetic ions in the Ge_{1-x-y}Cr_{x}Eu_{y}Te samples with chemical composition varying in the range of 0.015 ≤ x ≤ 0.057 and 0.003 ≤ y ≤ 0.042. The ferromagnetic transition at temperatures 50 ≤ T ≤ 57 K was observed, independent of the chemical composition. The long-range carrier mediated itinerant magnetic interactions seem to be responsible for the observed ferromagnetic order. The magnetic irreversibility with coercive field H_C = 5 - 63 mT and the saturation magnetization M_S ≈ 2 - 6 emu/g are found to strongly depend on the chemical composition of the alloy.
We consider an exactly solvable version of the quantum spin-1/2 orthogonal-dimer chain with the Heisenberg intra-dimer and Ising inter-dimer couplings. The investigated quantum spin system exhibits at zero temperature fractional plateaux at 1/4 and 1/2 of the saturation magnetization and it has a highly degenerate ground state at critical fields where the magnetization jumps. We study the field dependence of the specific heat at low temperature. The lattice-gas description is formulated in a vicinity of critical fields to explain the low-temperature behaviour of specific heat.
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