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Register a new userStructural, electronic, magnetic, and thermal properties of single-crystalline UNi0.5Sb2
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Milton Torikachvili
Publication date
2011
fields
Physics
and research's language is
English
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We studied the properties of the antiferromagnetic (AFM) UNi0.5Sb2 (TN approx 161 K) compound in Sb-flux grown single crystals by means of measurements of neutron diffraction, magnetic susceptibility ({chi}), specific heat (Cp), thermopower (S), thermal conductivity ({kappa}), linear thermal expansion ({Delta}L/L), and electrical resistivity ({rho}) under hydrostatic pressures (P) up to 22 kbar. The neutron diffraction measurements revealed that the compound crystallizes in the tetragonal P42/nmc structure, and the value of the U-moments yielded by the histograms at 25 K is approx 1.85 pm 0.12 {mu}B/U-ion. In addition to the features in the bulk properties observed at TN, two other hysteretic features centered near 40 and 85 K were observed in the measurements of {chi}, S, {rho}, and {Delta}L/L. Hydrostatic pressure was found to raise TN at the rate of approx 0.76 K/kbar, while suppressing the two low temperature features. These features are discussed in the context of Fermi surface and hybridization effects.
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We studied single-crystals of the antiferromagnetic compound UNi0.5Sb2 (TN ~ 161 K) by means of measurements of magnetic susceptibility (chi), specific heat (Cp), and electrical resistivity (rho) at ambient pressure, and resistivity under hydrostatic pressures up to 20 kbar, in the temperature range from 1.9 to 300 K. The thermal coefficient of the electrical resistivity (drho/dT) changes drastically from positive below TN to negative above, reflecting the loss of spin-disorder scattering in the ordered phase. Two small features in the rho vs T data centered near 40 and 85 K correlate well in temperature with features in the magnetic susceptibility and are consistent with other data in the literature. These features are quite hysteretic in temperature, i.e., the difference between the warming and cooling cycles are about 10 and 6 K, respectively. The effect of pressure is to raise TN at the approximate rate of 0.76 K/kbar, while progressively suppressing the amplitude of the small features in rho vs T at lower temperatures and increasing the thermal hysteresis.
We report the electrical resistivity, Hall coefficient, thermoelectric power, specific heat, and thermal conductivity on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and R_H over a wide temperature range indicate that electrons dominate electrical transport properties. Both rho(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 uV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10^19 cm^(-3). Hall mobility u_H shows a maximum of 62 cm^2/Vs around 70 K. The analysis of temperature dependence of u_H suggests a crossover of dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.
The trigonal compound EuMg2Bi2 has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic transport properties of EuMg2Bi2 single crystals are presented. The Eu{+2} spins-7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a temperature TN = 6.7 K, as previously reported. By analyzing the anisotropic magnetic susceptibility chi data below TN in terms of molecular-field theory (MFT), the AFM structure is inferred to be a c-axis helix, where the ordered moments in the hexagonal ab-plane layers are aligned ferromagnetically in the ab plane with a turn angle between the moments in adjacent moment planes along the c axis of about 120 deg. The magnetic heat capacity exhibits a lambda anomaly at TN with evidence of dynamic short-range magnetic fluctuations both above and below TN. The high-T limit of the magnetic entropy is close to the theoretical value for spins-7/2. The in-plane electrical resistivity rho(T) data indicate metallic character with a mild and disorder-sensitive upturn below Tmin = 23 K. An anomalous rapid drop in rho(T) on cooling below TN as found in zero field is replaced by a two-step decrease in magnetic fields. The rho(T) measurements also reveal an additional transition below TN in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of TN on the c-axis magnetic field Hperp was derived from the field-dependent chi(T), Cp(T), and rho(T) measurements. This TN(Hperp) was found to be consistent with the prediction of MFT for a c-axis helix with S = 7/2 and was used to generate a phase diagram in the Hperp-T plane.
We report temperature and thermal-cycling dependence of surface and bulk structures of double-layered perovskite Sr3Ru2O7 single crystals. The surface and bulk structures were investigated using low-energy electron diffraction (LEED) and single-crystal X-ray diffraction (XRD) techniques, respectively. Single-crystal XRD data is in good agreement with previous reports for the bulk structure with RuO6 octahedral rotation, which increases with decreasing temperature (~ 6.7(6)degrees at 300 K and ~ 8.1(2) degrees at 90 K). LEED results reveal that the octahedra at the surface are much more distorted with a higher rotation angle (~ 12 degrees between 300 and 80 K) and a slight tilt ((4.5pm2.5) degrees at 300 K and (2.5pm1.7) degrees at 80 K). While XRD data confirms temperature dependence of the unit cell height/width ratio (i.e. lattice parameter c divided by the average of parameters a and b) found in a prior neutron powder diffraction investigation, both bulk and surface structures display little change with thermal cycles between 300 and 80 K.
The magnetic behavior of single-crystalline CeCuGa3 has been investigated. The compound forms in a tetragonal BaAl4-type structure consisting of rare-earth planes separated by Cu-Ga layers. If the Cu-Ga site disorder is reduced, CeCuGa3 adopts the related, likewise tetragonal BaNiSn3-type structure, in which the Ce ion are surrounded by different Cu and Ga layers and the inversion symmetry is lost. In the literature conflicting reports about the magnetic order of CeCuGa3 have been published. Single crystals with the centrosymmetric structure variant exhibit ferromagnetic order below approx. 4 K with a strong planar anisotropy. The magnetic behavior above the transition temperature can be well understood by the crystal-field splitting of the 4f Hunds rule ground-state multiplet of the Ce ions.
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