The magnetic fieldtemperature (BT) phase diagram of the Mn0.9Co0.1P single crystal is studied in the vicinity of the Lifshitz point by means of isothermal magnetocaloric coefficient (MT) and alternating current (AC) susceptibility measurements. Results confirm previously reported shape of the BT phase diagram and locations of characteristic temperatures and fields. At the Curie temperature (TC) the critical exponent ?, which describes a singularity of MT as a function of magnetic field ($M_{T} \,{\propto} \,B{{-} \omega } $), is estimated for B parallel to the easy axis to be equal to ???0.35. Below TC an evidence of a new enigmatic phase, reported previously for pure manganese phosphide (MnP), is found in susceptibility data also for Mn0.9Co0.1P. However, the range of existence of this phase is significantly larger here, than in MnP. At the LP we observe a sharp peak in the imaginary part of the magnetic susceptibility. A phenomenological theory is introduced to describe the field dependence of the critical lines from the disordered phase (paramagnetic) to ordered phases (ferromagnetic and modulated). The temperature and field dependences of the magnetocaloric coefficient and susceptibility are also calculated within the same framework.

High-quality single crystals of UPd2Si2 have been studied by means of heat capacity and magnetization measurements. The obtained data has yielded a H-T phase diagram that significantly differs from those reported before in the literature. The main finding is identification of a multicritical point that seemingly exhibits Lifshitz characteristics.

RPdBi (R = Er, Ho, Gd, Dy, Y, Nd) compounds were studied by means of x-ray diffraction, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermoelectric power and Hall effect measurements, performed in the temperature range 1.5-300 K and in magnetic fields up to 12 T. These ternaries, except diamagnetic YPdBi, exhibit localized magnetism of $R^{3+}$ ions, and order antiferromagnetically at low temperatures ($T_{N}$ = 2-13 K). The transport measurements revealed behavior characteristic of semimetals or narrow-band semiconductors. Both, electrons and holes contribute to the conductivity with dominant role of p-type carriers. The Hall effect of ErPdBi is strongly temperature and magnetic field dependent, reflecting complex character of the underlying electronic structures with multiple electron and hole bands. RPdBi, and especially DyPdBi, exhibit very good thermoelectric properties with a power factor coefficient $PF$ ranging from 6 to 20 $\mu$Wcm$^{-1}$K$^{-2}$.

The article discusses selected properties of the non- and superconducting polycrystalline samples of RuSr2GdCu2O8 and comments the consequences of introducing insignificant sub-stoichiometry of Ru into the nominal formula. The magneto-resistive and the magnetic characteristics are interpreted in favour of the formation of the intrinsically inhomogeneous superconducting phase, which seems to be stabilized along with the structural modifications likely enhanced with the modification of starting stoichiometry. The specific heat data reveals the shift of temperature of the magnetic ordering T_{m}, suggesting the dilution in magnetic sublattice of the Ru moments. The measurements of the magnetic field dependences of the isothermal magnetocaloric coefficient M_{T} show that there is no gain in magnetic entropy in a broad range of the accessed fields and temperatures. Whereas the multi-component character of the probed magnetic system precludes from concluding on the ground state for the Ru ordering, the maximum in M_{T}(H) which occurs at weak magnetic fields for temperature vicinity of T_{m} may reflect dominance of the ferromagnetic type interactions with a constrained correlation range. The literature explored models for the Ru magnetic ordering and possible phase separation in the RuSr2GdCu2O8 are brought into the discussion.

The temperature dependencies of the resistivity for the superconducting ruthenocuprates of nominal compositions RuSr 2 GdCu 2 O 8 , Ru 0.98 Sr 2 GdCu 2 O 8 and Ru 0.5 Sr 2 GdCu 2.5 O 8− δ were examined for the magnetic field dependent characteristics of the superconducting transitions. The effect of the insignificant diminishing of the Ru/Cu ratio in parent RuSr 2 GdCu 2 O 8 was confirmed as relevant for the stabilisation of the superconducting phase. Noted differences in the compared characteristics are interpreted for possible inhomogeneous nucleation of the superconducting phase in the parent ruthenocuprate. The phase anisotropy in RuSr 2 GdCu 2 O 8 and Ru 0.98 Sr 2 GdCu 2 O 8 , in presence of the compounds Ru magnetism, appears to be a cause of a significant softening of the H c 2 ( T ) phase line. An anomalous lowering of the magnetoresistivity was observed in the approx. 10 K range above the onset of the superconducting transition, which may suggest the presence of enhanced superconducting fluctuations in the samples. The positive magnetic field shift of the temperatures, which limit the magnetoresistivity and the specific heat signatures of the magnetic ordered state of the Ru sublattice, suggests probing the influence of the ferromagnetic Ru interactions in an effective metallic-like conduction channel present in the samples. Superconducting characteristics of the Ru 0.5 Sr 2 GdCu 2.5 O 8− δ reveal a significant contribution of the Gd paramagnetic signal at low temperatures, interpreted for the presence of a significant anisotropy of the superconducting phase. It is concluded that the Ru–Cu substituted phases of ruthenocuprates may present an opportunity to investigate the effectively anisotropic superconducting phase despite its comparatively high T c in the compounds related to the 123-type cuprate superconductor.

The evolution of the magnetic field-temperature phase diagram of UAs1-xSex with x in the range of 0-0.1 is studied by means of magnetocaloric and specific-heat measurements. Our interest is focused on the high-temperature phase transitions and especially on the point, where the paramagnetic (P) and two ordered phases meet. For undoped UAs these two ordered states are the ferrimagnetic (Fi) and the type-I antiferromagnetic phases. According to Sinha et al. [Phys. Rev. Lett. 45, 1028 (1980)] the antiferromagnetic phase transition is in the vicinity of a Lifshitz point. Furthermore, Kuznietz et al. [J. Magn. Magn. Mater. 61, 246 (1986)] showed that an incommensurate phase (IC) emerges between the type-I (or type-IA) antiferromagnetic and paramagnetic phases in the case of UAs1-xSex with 0 < x < 0.15 in zero magnetic field. The results reported in this paper show the existence of a T-Fi/IC(B) line that separates the ferrimagnetic region from a phase, which cannot be identified on the basis of our thermodynamic measurements. However, one may assume it is the IC phase, consistent with the above mentioned zero field results. The T-Fi/IC(B) line merges with the order-disorder line at point (B-p, T-p), where the critical line (IC/P) meets with two first order transitions lines: Fi/P and Fi/IC. This point can be considered as an analog of a Lifshitz point. A simple phenomenological description of the phase transitions near this special point is provided.

High-quality single crystal of Ce2RhSi3 was studied by means of thermoelectric power measurements carried out down to 2 K in external magnetic fields up to 13 T. The results obtained above 50 K were interpreted in terms of a modified two-band model that takes into account temperature variation of the width of 4f-derived narrow band located near the Fermi level. At lower temperatures the thermopower exhibits more complex temperature dependences that likely involve interplays of magnetic exchange, Kondo and crystal-field interactions.

Data on the magnetothermopower and specific heat of three compounds belonging to ``1111'' oxypnictides family are reported. One specimen $({\text{SmAsFeO}}_{0.8}{\text{F}}_{0.2})$ is a superconductor with ${T}_{c}=53\text{ }\text{K}$, while two others (SmAsFeO and NdAsFeO) are nonsuperconducting parent compounds. Our results confirm that spin-density-wave (SDW) order is present in SmAsFeO and NdAsFeO. In these two samples a strict connection between the thermoelectric power and electronic specific heat is found in the vicinity of SDW transition, which indicates that the chemical potential of charge carriers strongly depends on temperature in this region. Low-temperature data suggest presence of significant contribution magnon drag to the thermoelectric power.

The magnetic properties of superconducting Ru1-xSr2GdCu2O8 (x = 0, 0.02) and non-superconducting RuSr2Gd1-xCexCu2O8 (x = 0.07, 0.1) were investigated by means of magnetocaloric experiments with complementary magnetoresistivity and ac susceptibility measurements. The isothermal magnetocaloric coefficient M-T(B) assumes positive values in a broad range of temperatures (20 K <= T <= 231 K) and magnetic fields (0 T <= B <= 13 T), i.e. also in the magnetically ordered state (T-m = 132 K for RuSr2GdCu2O8 and Tm = 150 K for RuSr2Gd0.93Ce0.07Cu2O8) which indicates no gain in the system's magnetic entropy with increasing magnetic field. The maximum in the M-T(B) dependence was observed for RuSr2GdCu2O8 in the temperature vicinity of T-m, which indicates a ferromagnetic character of the accessed magnetic correlations. No spontaneous ferromagnetic order was revealed as the M-T assumes limiting zero values at zero magnetic field for the whole range of investigated temperatures. Temperature dependences of the specific heat reveal the magnetic-field-induced positive temperature shift of the anomaly associated with the magnetic transition in the Ru spin system. The M-T(B) dependences and the magnetoresistivity data suggest that the magnetic system may be inhomogeneous.

A simple method of in-magnetic-field calorimetric measurements is presented. It is based on commercial Peltier elements used as sensitive heat-flow meters of high thermal conductivity. In the presented experimental setup the Peltier element thermally connects the sample to a heat sink of constant temperature. Application of the magnetic field (B) ramp results in the sample temperature change due to magnetocaloric effect, but, due to high thermal conductivity of the Peltier element, any temperature difference is quickly thermalized. The ensuing heat flux, detected as a voltage on the Peltier element terminals, is proportional to a quantity called the isothermal magnetocaloric coefficient, MT. It could be measured quasi-continuously up to the maximum field attainable by the superconducting magnet used in the experiment (Bmax = 13 T in our case). The available temperature range spans between 20 and 300 K. The obtained set of MT(B) curves holds a valuable information on magnetic and thermodynamic properties of the matter. It is particularly useful if compared with the set of specific heat curves CB(T) taken at constant field, which could be obtained using the very same experimental setup. Several examples of the behavior of the isothermal magnetocaloric coefficient for various magnetic (Mn0.9Co0.1P and UAs0.97Se0.03 single crystals) and superconducting (YBa2Cu3O7- single crystal and Bi2223 tape) materials are given.

The half-Heusler compounds ErPdSb and YPdSb were studied by means of x-ray diffraction, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermoelectric power, Hall effect, thermal conductivity, and specific heat measurements, performed in the temperature range 1.5-300 K and in magnetic fields up to 12 T. The Er-based compound is a paramagnet down to 1.7 K and shows localized magnetism of Er3+ ions, while YPdSb exhibits a weak diamagnetic behavior. The crystal field effect in ErPdSb brings about a distinct Schottky anomaly in the specific heat. The total splitting of the erbium I-4(15/2) multiplet in a cubic crystal field potential is of the order of 200 K, with a doublet being the ground state. Electrical properties of ErPdSb and YPdSb are governed by the formation of narrow gaps in the electronic band structures very close to the Fermi level. The Hall effect measured for ErPdSb indicates a complex electronic structure with multiple electron and hole bands with different temperature and magnetic field variations of carrier concentrations and mobilities. The thermal conductivity of ErPdSb is relatively low and dominated by the phonon contribution. At room temperature the Seebeck coefficient is of order 150 mu V/K for Er- and 200 mu V/K for Y-based compound, respectively, making these materials promising candidates for thermoelectric applications. This conjecture is supported by the value of the figure of merit of ErPdSb, which is about 0.15 at room temperature.

The authors report lattice specific heat of bulk hexagonal GaN measured by the heat flow method in the temperature range of 20-300 K and by the adiabatic method in the range of 5-70 K. The best fit with the accuracy of 3% was obtained for the temperature-independent Debye temperature Theta(D)=365 K and Einstein temperature Theta(E)=880 K. The authors relate these temperatures to the function of density of states. Using their results for heat conduction coefficient, they established in the temperature range of 10-100 K the explicit dependence of the phonon mean free path on temperature ...(ph)proportional to T-2. Above 100 K, there is an evidence of contribution of the Umklapp processes, which limits phonon free path at high temperatures. (c) 2006 American Institute of Physics.

Single crystals of UNi0.5Sb2 were investigated by means of Seebeck coefficient and Hall effect measurements in the temperature range 5–300K. The results corroborated the occurrence of two magnetic phase transitions: from para- to antiferromagnetic state at TN=161.5K and a spin-reorientation near Tt=64K. The first-order character of the latter feature was proved by studying in detail the electrical resistivity and the magnetic susceptibility of single-crystalline UNi0.5Sb2 in the vicinity of Tt.

A simple model for the irreversible contribution to the isothermal magnetocaloric coefficient ${M}_{T}$ in type-II superconductors is presented. The account is based on the Bean model. Similarities and differences between ${M}_{T}$ and irreversible magnetization are described. The model predictions are confirmed by measurements of the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ superconductor. The usefulness of the isothermal magnetocaloric effect as another tool for the critical current density determination was indicated.

Single crystals of UNi0.5Sb2 have been studied by means of specific heat (C-p) and isothermal magnetocaloric coefficient (M-T) measurements using a heat-flow technique. The C-p(T) variation exhibits a pronounced lambda-shaped anomaly at T-N=161.1 K, associated with an antiferromagnetic phase transition. Another feature, less distinct and strongly hysteretic, was found at T-tr=63.5 K (cooling) and 71 K (heating), which signals a change in the magnetic structure. In an applied magnetic field T-N decreases, while T-tr remains constant. The analysis of the zero-field anomaly at T-N indicated the effective total angular momentum J=1/2 for U ions and placed UNi0.5Sb2 between the two- and three-dimensional Ising systems. The M-T(B) curves measured below the onset of antiferromagnetic (AF) order revealed a similar to ln vertical bar B-B-N vertical bar anomaly at the transition field B-N, which increases with lowering temperature. All the calorimetric data form unanimously an AF phase-transition line of the square-root type. Its shape was described by a mean-field model for the tetragonal lattice with ferromagnetic coupling (J) in the ab plane and AF coupling along c axis (J(1)). This analysis revealed strong anisotropy in the magnetic vertical bar J(1) vertical bar/J=0.25.

MER Michel DECROUX, Alfred MANUEL Postdocs Louis ANTOGNAZZA, Morten ESKILDSEN, Isabelle JOUMARD, Edmond KOLLER, Olivier KUFFER, Martin KUGLER, Ivan MAGGIO-APRILE, Serge REYMOND, Shukichi TANAKA PhD students Laurent BESSON, Cédric DUBOIS, Bart HOOGENBOOM, Pascal REINERT, Emmanuel TREBOUX Diploma students Estelle DE CHAMBRIER, Daniel GUTIERREZ RIOS Technicians Paul-Emile BISSON, Jean-Gabriel BOSCH, Arthur STETTLER

A recently developed technique for measuring the isothermal magnetocaloric coefficient (MT) is applied to the study of a superconducting NdBa2Cu3O7 single crystal. Results are compared with magnetization (M) and specific heat (C). In the reversible region both C and MT follow the scaling law of the 3D-xy universality class. The anomalies connected with flux-line lattice melting are visible on MT(B) curves as peaks and steps, similar to C(T) curves yet with much smaller background. At lower temperature, in the irreversible region the MT(B) behaviour resembles more that of M(B), exhibiting the 'fishtail' effect. Our results confirm that the peculiarities of the phase diagram known from the high-temperature superconductor YBa2Cu3O7, e.g. vortex melting, dominance of critical fluctuations and absence of a Bc2 critical field line, are a common property of RE-123 systems.

Heusler alloys ErPd2Sb, ErPd2Bi, ErPdSb and ErPdBi were studied by means of X-ray diffraction, magnetic susceptibility, electrical resistivity, thermoelectric power and Hall effect measurements. All these compounds are paramagnetic down to 1.7K due to localized magnetism of Er3+ ions. Their electrical behavior is governed by the formation of narrow gaps in the electronic band structures near EF. Both electrons and holes contribute to the conductivity, yet the dominant role is played by p-type carriers. The Hall response is strongly dependent on temperature and applied magnetic field, thus reflecting complex character of the underlying electronic structures. The Seebeck coefficient in ErPdSb is of the order of 150μV/K at room temperature making this material promising candidate for thermoelectric applications. Common to all four compounds studied is a rapid drop in the resistivity below 6–8K that seems manifest a phase transition of magnetic or superconducting origin, despite the lack of any corresponding anomalies in the low-temperature magnetic susceptibility and specific heat.