Ching-Shuei Wur

A single-molecular magnet: [Mn12O12(O2CCH2Br)16(H2O)4]

Abstract The synthesis, X-ray crystal structure and magnetic properties of a new dodecanuclear manganese complex [ Mn 12 O 12 ( O 2 CCH 2 Br ) 16 ( H 2 O ) 4 ] · 4 CH 2 Cl 2 ( 2 ) · 4 CH 2 Cl 2 are reported. Complex 2 · 4 CH 2 Cl 2 crystallizes in tetragonal space group I41/a with the following cell dimensions at 293(2) K: a=b=26.697(4) A, c=13.246(4) A, and Z=4. The results of DC and AC magnetic susceptibility studies are described. DC measurements show the presence of hysteresis loops, and the out-of-phase component of AC magnetic susceptibilities has a frequency dependent maximum. Step-like features are seen on hysteresis loops and quantum mechanical tunneling could be the origin of these unusual relaxations.

Nuclear magnetic resonance, resistance and acoustic studies of the melting-freezing phase transition of gallium in Vycor glass

The melting-freezing phase transition of gallium confined within Vycor glass was studied by NMR, resistance and acoustic techniques. A single although broadened 71Ga NMR line corresponding to melted gallium was observed in contrast to lineshapes found until now for liquid gallium in porous matrices. A difference between results obtained using the three methods was explained by formation of various confined solid gallium modifications. A depression of the freezing and melting phase transition temperatures and a pronounced hysteresis in the melting-freezing processes were found and are discussed. Heterogeneous nucleation was suggested to explain the dependence of crystallization on temperatures of pre-warming. Irreversible melting was observed for the second gallium modification.

Superconductivity of gallium in porous glass

Abstract Superconductivity of gallium embedded in the porous glass with average pore size of 4 nm was studied using a SQUID magnetometer. Hysteretic behavior at both high and low magnetic fields was observed. The quantitative model of the superconducting properties of the porous glass with gallium has been developed on the bases of the measurements performed. The phase diagram in the H − T plane, including the irreversibility line, was obtained.

Superconductivity of gallium in confined geometries

Abstract Superconductivity of gallium embedded in the porous glass with average pore size of 4 nm was studied for the first time using a SQUID magnetometer. A sharp superconducting phase transition was found at 6.4 K. Hysteretic behavior at high and low magnetic fields was observed. The phase diagram in the H-T plane was obtained. The first order phase transition in the vortex system was observed. A qualitative model of the superconducting properties of the porous glass with gallium has been developed.

Freezing and melting of gallium in porous glass

Abstract We report an investigation of the melting and the freezing of conducting materials confined in porous glass by electrical resistance measurements. For gallium in porous glass, heat-capacity measurements show a broadened latent-heat peak associated with the melting at about 230 K much lower than the bulk melting point 302.9 K. The resistance exhibits a clear freezing-melting hysteresis. For gallium in porous glass, in contrast to the freezing and the melting transitions the superconducting transition is extremely sharp and does not show any hysteresis during the cooling and warming process.

Magnetic ordering in UPd2Ge2

Abstract We present magnetic, electric and thermal properties of UPd2Ge2. In UPd2Ge2, a paramagnetic to antiferromagnetic transition is observed at 140 K. Below 100 K, the magnetization oscillates with temperature and the hysteresis loops exhibit ferromagnetic properties. The magnetic properties of UPd2Ge2 can be understood by incommensurate phases.

Spin-glass-like behaviour in UPd2Ge2

The magnetic susceptibility, hysteresis curves, ZFC and FC magnetic moments and magnetoresistance measurements indicate the appearance of a spin glass state in UPd2Ge2 below 87 K, and the existence of a ferromagnetic phase below 80 K. From these studies and taking into account the neutron diffraction data a mixed spin glass-ferro-antiferromagnetic state is suggested in the low-temperature region.

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.

Ferroelastic phase transition in crystalline K3Na(CrO4)2: Acoustic studies

Ultrasonic studies of the temperature behavior of the velocity and damping of sound for the xx and zz longitudinal and yx and zx transverse waves in K3Na(CrO4)2 have been carried out in the temperature interval 185–295 K, which includes the region of the ferroelastic phase transition. The acoustic parameters for both shear and longitudinal waves were found to have anomalies in the region of the phase transition with a Curie temperature of 235.5 K. A theoretical analysis of the softening of the elastic moduli c44 and c66 was performed on the basis of the Landau expansion in terms of the strain tensor components ɛ4 and (ɛ2-ɛ1)/2 considered as the linearly coupled primary and secondary order parameter, respectively. The absolute values of the elastic moduli c11, c33, c44, c66, c12, and c14 at 295 K were calculated.