Kaijun Lin

X-ray and resistance studies of the melting and freezing phase transitions for gallium in an opal

The melting and freezing processes and the structure of gallium in an opal were studied using x-ray powder diffraction within the temperature range 10 to 320 K. Four different modifications of solid confined gallium were found. Two of these modifications did not coincide with any known gallium structures; another two coincided with and disordered . The broadening of the total melting and freezing processes and reduction of the phase transition temperatures compared to bulk phase were obtained. The size of confined gallium crystallites corresponding to the different modifications was estimated for cooling and warming. The reproducible freezing was treated as a result of the steep temperature dependence of the nucleation rate in supercooled melts. Additional measurements of resistance for the opal filled with gallium revealed that it was sensitive mainly to the melting and freezing within the gallium modification coinciding with disordered . Melting-freezing hysteresis for this modification was found to depend on temperatures of pre-warming. It was suggested that the freezing of this modification is driven by the amount of frozen crystallites of the phase with the highest onset of freezing. The melting broadening and the lowering of the phase transition temperatures for the confined gallium modifications were discussed.

X-ray studies of the melting and freezing phase transitions for gallium in a porous glass

We use x-ray powder diffraction to study the melting and freezing processes for gallium within a porous glass. The only modification to solid gallium was found above 20 K, in contrast with previous x-ray studies of confined gallium. The size of gallium crystallites remained nearly constant during cooling and warming, while both the melting and freezing processes were smeared. The size of confined gallium crystallites was estimated as 22 nm, which is significantly larger than the pore size. The reasons for the phase-transition broadening and hysteresis between freezing and melting are discussed on the basis of the results obtained. @S0163-1829~98!02942-7#

Magnetic, electrical properties and magnetoresistance of Cu substituted La0.7Sr0.3CoO3-δ crystals (0.35 ≤ δ ≤ 0.45)

Abstract The magnetic susceptibility, resistivity, and magnetoresistance of La 0.7 Sr 0.3 Co 1− x Cu x O (0≤ x ≤0.4 and 0.3≤ δ ≤0.4) compounds have been studied for applied field up to 50 kOe. Cuire temperature and saturation magnetization of the ferromagnetic perovskite La 0.7 Sr 0..3 Co 1− x Cu x O decreases, when cobalt is substituted by copper. In the doping, all coballtites exhibited ferromagnetic long-range ordered behaviors. If the content of copper exceeds 10%, the electronic properties of the material show the behavior of transformation from metallic to semiconductor. The semiconducting behavior is connected with the magnetic disorder, which show variable range hopping, logxa0 ρ ∼ T −1/4 . The magnetoresistance, which can be expressed as MR ∝ H q when H ≤10 kOe, is negative and exceeds 32%. The results are interpreted in terms of a double-exchange and super-exchange model.

Magnetic phase transitions in UPd2Ge2

Abstract Magnetic properties of UPd 2 Ge 2 have been studied at 4.2–300 K and in an applied field up to 55 kG. The magnetic state of the compound is different in three temperature regions. At 140 K UPd 2 Ge 2 transforms from para- to antiferromagnetic state, and in H ext = 0 the material is antiferromagnetic in the range 140-80 K. The strong field H ext > 14 kG can induce a ferromagnetic component in this region. Below 80 K a small ferromagnetic component appears spontaneously, and it is strongly enhanced by the applied field. In the range 80-20 K the magnetic state can be considered as a coexistence of antiferromagnetic and ferromagnetic states, or as a ferrimagnetic state. In high H ext the antiferromagnetic state can be suppressed and ferromagnetic phase dominates. Below 20 K the ferromagnetic component is small and the antiferromagnetic phase dominates.

Annealing effects on the physical properties of UAu2Si2

Abstract We have investigated the effects of annealing on the uranium intermetallic compound UAu 2 Si 2 . The physical properties of both the annealed and as-cast samples were studied from 5 to 300 K. Both the resistivity ϱ ( T ) and low-field magnetic susceptibility χ ( T ) of the as-cast sample show anomalies at 18 K and at 82 K; whereas the heat capacity C ( T ) shows only an anomaly at 20.5 K. The ϱ ( T ) and χ ( T ) of the annealed sample displays a low-temperature anomaly around 20 K; and a complex magnetic phase transition between 48 K and 55 K was observed. A sharp anomaly of C ( T ) in the annealed UAu 2 Si 2 can be seen at 21.5 K. The annealed sample shows Fermi-liquid behavior below 10 K with a coefficient of the quadratic term in the temperature dependence of the resistivity being 11.5 μΩ-cm K −2 . The low temperature electronic specific heat coefficient γ of the annealed sample is 92 mJ mol K −2 . We infer that UAu 2 Si 2 has heavy-fermion behavior with ferromagnetic ground state, which is an uncommon behavior in the uranium intermetallic compounds.

Magnetic and electrical properties of the oxide system of La0.8Sr0.2Co1-xCuxO3-δ

Abstract This work investigates the electrical resistivity, magnetization and magnetoresistance in polycrystalline La 0.8 Sr 0.2 Co 1− x Cu x O 3− δ compounds (0.1⩽x⩽0.4 and 0.4⩽δ⩽0.5) in the temperature range of 5–300xa0K. In the doping range, all cobaltites are semiconducting state, which displays variable range hopping, and ferromagnetic long-range order behaviors except for T>T c . Therefore, the magnetic phase transitions are independent of the electrical resistivity in the system. In addition, a decreasing Cu content causes an increasing of magnetic transition temperature T c . The large negative magnetoresistance effect is observed as well.