M. Gutowska

Giant magnetostriction and magnetostriction jumps in superconducting single crystalline

We have studied magnetostriction in single crystalline using the strain gauge technique in magnetic fields up to 12 T at temperatures 4.2 K < T < 35 K. We have observed giant transverse magnetostriction (of the order of at 4.2 K) for magnetic field and longitudinal magnetostriction (of the order of at 4.2 K) for orientation. Pronounced jumps in transverse magnetostriction have been observed in the temperature range between 4.2 K and 8.5 K. Experimental magnetostriction results are compared with magnetization measurements using a vibrating sample magnetometer and gradientometric Hall sensors measurements. The calculations were performed on the basis of a pinning-induced mechanism of magnetostriction. The simple adiabatic theory seems to be insufficient to describe the observed flux instabilities.

Microstructural magnetic phases in superconducting FeTe0.65Se0.35

In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogeneities in the iron chalcogenide system FeTe1?xSex and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe0.65Se0.35 studied with transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate the presence of nanoscale hexagonal regions coexisting with a tetragonal host lattice, a chemical disorder demonstrating a nonhomogeneous distribution of host atoms in the crystal lattice, as well as iron-deficient bands hundreds of nanometres in length. From the magnetic data and ferromagnetic resonance temperature dependence, we attribute magnetic phases in Fe?Te?Se to Fe3O4 inclusions and to hexagonal symmetry nanoscale regions with a structure of the Fe7Se8 type. Our results suggest that a nonhomogeneous distribution of host atoms might be an intrinsic feature of superconducting Fe?Te?Se chalcogenides and we find a surprising correlation indicating that a faster grown crystal of inferior crystallographic properties is a better superconductor.

Crystal structure and magnetic properties of potassium erbium double tungstate KEr(WO4)2

Results of structural, magnetic and specific heat investigations of the potassium erbium double tungstate, KEr(WO 4 ) 2 , are presented. Potassium erbium double-tungstate KEr(WO 4 ) 2 single crystals have been grown by the top-seeded solution growth method (TSSG) and modified Czochralski techniques. It crystallizes in the monoclinic crystal structure (C2/c space group). The unit cell contains four formula units and is described by parameters a = 10.615(2) A, b = 10.316(2) A, c = 7.534(2) A, β = 130.73(3)°. From the x-ray diffraction measurements the fractional atomic coordinates, displacement parameters and interatomic distances have been determined. The specific heat C(T) of the KEr(WO 4 ) 2 crystal has been measured over a temperature range of 0.6-300 K. The susceptibility has been studied at T = 0.25-4.0 K. The magnetic phase transition was observed at a temperature of 0.48 K. The magnetization has been measured in the temperature region from 4.2 to 60 K and in magnetic field up to 1.6 T. A strong anisotropy of magnetic properties was found. The temperature and field dependences of susceptibility and magnetization data were used for both elucidation of character of the magnetic ordering and calculation of the exchange and dipole-dipole interaction energies as well as for determination of the possible magnetic structure of KEr(WO 4 ) 2 .

Specific heat in CeNi4Cu and YbNi4Cu

The temperature dependence of specific heat for the compounds CeNi4Cu and YbNi4Cu is analysed. These studies are supported by magnetic susceptibility and x-ray photoemission spectroscopy measurements. The scheme of the energy levels created by the splitting due to the crystal electric field is determined from the Schottky contribution to the specific heat. Anomalies observed at low temperatures are discussed in the framework of heavy-fermion/Kondo physics. It is found that an external magnetic field has a strong influence on the low temperature part of the specific heat.

Direct and specific heat study of magnetocaloric effect in La0.845Sr0.155MnO3

Magnetocaloric effect and specific heat of a La0.845Sr0.155MnO3 sample were measured over a temperature range of 4.2–300 K, to verify whether large entropy changes induced by magnetic field make possible the use of manganites for magnetic refrigeration and whether a magnetostrictive contribution to magnetocaloric effect is significant. Isothermal entropy change and adiabatic temperature change induced by ΔB=7 T were determined as functions of temperature. Near to the Curie temperature, TC≈234 K, they reach, respectively, ∼−6.6 J/(kg K) and ∼3.4 K, i.e., the values ∼2 and ∼4 times smaller than for Gd. Despite the presence of large magnetostriction, its contribution to magnetocaloric effect was found negligible.

Multiple magnetic phase transitions in Tb3Cu4Si4

A polycrystalline sample of Tb(3)Cu(4)Si(4) was investigated by means of magnetometric, electrical resistivity, thermopower and heat capacity measurements. This paper also includes reanalyses of former neutron diffraction data. The compound crystallizes in a Gd(3)Cu(4)Ge(4)-type orthorhombic structure (Immm space group), in which the Tb ions occupy two inequivalent sites (2d and 4e). The bulk results indicate that these two magnetic sublattices order antiferromagnetically at different temperatures, in agreement with the neutron diffraction data.

Electronic origin of the step-like character of the discharge curve for NaxCoO2-y cathode

This paper shows electronic approach to an explanation of the step-like character of the discharge/charge curve of Na/Na+/NaxCoO2-y battery. On a basis of comprehensive experimental studies of physicochemical properties of NaxCoO2-y cathode material (XRD, electrical conductivity, thermoelectric power, electronic specific heat) supported by calculations performed using the Korringa–Kohn–Rostoker method with the coherent potential approximation to account for chemical disorder, it has been shown that the observed step-like character of the discharge curve reflects the variation of the chemical potential of electrons (Fermi level) in the density of states of NaxCoO2-y, which is anomalously perturbed by a presence of the oxygen vacancy defects and sodium ordering.

Specific heat and magnetic order of La0.2Ca0.8MnO3

Measurements of specific heat of the electron-doped La0.2Ca0.8MnO3 manganite are reported for 2<T<400 K. It was found that La0.2Ca0.8MnO3 exhibits an orbital ordering at TOO=204 K and a long-range antiferromagnetic order below TN=178 K. The entropy changes (ΔS) around both TOO[ΔS(TOO)≈0.38 J/mol K] and TN[ΔS(TN)≈3.58 J/mol K] were determined. The entropy change associated with the orbital ordering was found to be much smaller than that for charge ordered manganites. The low temperature specific heat indicates that La0.2Ca0.8MnO3 has rather high Debye temperature ΘD=685 K in comparison with existing results for electron-doped La1−xCaxMnO3 manganites. Moreover, the presence of a large 1/T2 hyperfine contribution and of a considerable linear term, unexpected for La0.2Ca0.8MnO3 with antiferromagnetic insulating ground state, was revealed. The possible relationship between the behavior of the heat capacity and the phase separation is discussed.

Specific heat and the cooperative Jahn-Teller effect in

The specific heat of has been measured over the wide temperature range from 3.5 K to 200 K. Only one anomaly, indicating the phase transition driven by the cooperative Jahn-Teller effect, has been found, at . The phonon specific heat has been estimated and subtracted from the heat capacity measured. The Schottky anomaly originating from the population of the lowest two doublets of the dysprosium ions, and the -anomaly connected with the Jahn-Teller phase transition have been described theoretically using the molecular-field approximation. Satisfactory agreement between experimental points and theoretical dependences calculated using the material parameters known from the literature was obtained.

Magnetic phase transition in KGd(WO4)2 double tungstate

Investigations of the specific heat of the potassium gadolinium double tungstate KGd(WO4)2 have been performed over the temperature range from 0.05 K up to 4 K in zero magnetic field. The λ -type anomaly observed at T = 0.42 K was interpreted as an indication of a second order magnetic phase transition within the Gadolinium sublattice. The critical exponent α was determined from experimental data.