Y. Xin

Magnetocaloric effect in La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ bulk materials

The dependence of magnetization on the applied magnetic field and temperature was measured carefully near their Curie temperatures for two perovskite manganite samples: La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ. It is suggested by the results that these materials can be utilized as both the thermal storage (passive regeneration) and as the working material (active regeneration) in an active magnetic regenerative refrigerator with very large temperature span, for their significant entropy change upon the application of a magnetic field and the easily tuned Curie temperatures.The dependence of magnetization on the applied magnetic field and temperature was measured carefully near their Curie temperatures for two perovskite manganite samples: La{sub 0.67}Ca{sub 0.33}MnO{sub {delta}} and La{sub 0.60}Y{sub 0.07}Ca{sub 0.33}MnO{sub {delta}}. It is suggested by the results that these materials can be utilized as both the thermal storage (passive regeneration) and as the working material (active regeneration) in an active magnetic regenerative refrigerator with very large temperature span, for their significant entropy change upon the application of a magnetic field and the easily tuned Curie temperatures. {copyright} {ital 1996 American Institute of Physics.}

Synthesis and neutron powder diffraction study of the superconductor HgBa2Ca2Cu3O8 + δ by Tl substitution

Substitution of T1 for Hg was performed in the Hg based 1223 phase HgBa2Ca2Cu3Os + ~ ( Tc = 135 K), resulting in an increase of the superconducting transition temperature to 138 K for samples with a nominal composition of Hgo.sTlo.2Ba2Ca2Cu3Os ÷ ~. The crystal structure of this solid solution has been investigated by neutron powder diffraction techniques at room temperature and at 10 K. The compound has the same crystal structure as Hg-1223 with the space group symmetry P4/mmm and lattice parameters a = 3.8489 ( 1 ), c = 15.816 ( 1 ) ,~. Rietveld analysis results indicate that Hg is partially replaced by T1, and the oxygen content, & is 0.33. The lattice-parameter changes resulting from the T1 substitution are too small to account for the T~ change by mimicking the effect of pressure. No phase transition occurs down to 10 K.

Magnetic properties and giant magnetoresistance in La0.67Ca0.33MnOx bulk material

The magnetic and transport properties of a perovskite‐type crystal sample with a nominal composition La0.67Ca 0.33MnOx have been investigated in the temperature range from 5 to 300 K with applied magnetic fields up to 5 T. Magnetization data obtained in the zero‐field‐cooled process exhibit the magnetic behavior with a Neel temperature, TN∼50 K and Curie temperature, TC∼300 K. The interesting feature in the temperature dependence of magnetoresistance is the appearance of a peak at T∼50 K, with value Δρ/ρ=[ρ(0)−ρ(5 T)]/ρ(5 T)=1300% at the Neel temperature, rather than at Curie temperature.

Thick film of HgBa2Ca2Cu3O8+δ via the sol-gel technique

Abstract We have prepared superconducting HgBa 2 Ca 2 Cu 3 O 8+δ thick films on polycrystalline MgO substrate via the sol-gel technique together with a slow annealing method. A precursor film of Ba 2 Ca 2 Cu 3 O 7 was annealed with a bulk mixture of unreacted HgBa 2 Ca 2 Cu 3 O 8+δ and a precursor, Ba 2 Ca 2 Cu 3 O 7 , pellet that was employed to control the HgO vapor pressure. Our films show an onset superconducting transition temperature of 140 K and a zero resistance temperature of 130 K after oxygen annealing. The high T c thick films are consistently reproducible.

Effects of fluorine substitution in superconducting Tl2Ba2CuO6±δ

Abstract The fluorine substituted thallium based 2 2 0 1 phase samples were fabricated with starting compositions 0.95Tl 2 O 3 + 0.1SnO 2 + (2− x )BaO 2 + x BaF 2 + CuO. Mossbauer and X-ray spectra were used as a study of the role played by fluorine. All samples show a near 2 2 0 1 pure phase. The best fluorine substituted sample has T c (onset) of 100 K and the T c (R = 0) of 97 K. The Mossbauer absorption area varies, parallel to T c , with the fluorine content and reaches a maximum absorption near x = 0.6, which indicates that fluorine could have replaced oxygen in the crystal lattice sites and thus does not rule out the presence of a phonon superconducting mechanism.

Fluorine doped high Tc cuprate TlSr2CaCu2O7

Superconducting samples with the starting composition of 12 Tl2O3+ 2SrO + (1 − p)CaO + pCaF2 + 2CuO, where p = 14, 12, and 1, were prepared. The participation of fluorine in the starting composition converts the 1 2 1 2 structured non-superconducting TlSr2CaCu2O7 to a superconductor. The doping of F appears to reduce the hole carrier concentration of the parent material. This experimental result may provide the possible evidence of anion substitution in high Tc superconductors.

Thick films of the Tl-based superconductor fabricated via the sol-gel technique

Abstract Superconducting thick films of Tl-based cuprate were grown via the sol-gel technique with a two-step method: deposition of a Tl-free precursor film followed by Tl-vapor annealing. A zero resistance superconducting transition temperature ( T c ) of 120 K on polycrystalline MgO substrates for Tl-1223 films and of 124 K on (100) LaAlO 3 substrates and polycrystalline MgO substrates for Tl-2223 films was obtained. The intragranular critical current density at 5 K of these films is greater than 10 6 A/cm 2 for Tl-1223 and Tl-2223. The microstructure of these films was examined with X-ray diffraction and scanning electron microscopy.

Quantum relaxation of vortices in a sintered Tl-2223 high-Tc superconductor

Abstract The time relaxation of the zero-field cooled magnetization of a sintered Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) high-T c superconductor (HTSC) (T co ≅ 110 K) has been measured as a function of temperature at low (50 Oe) and moderate applied magnetic fields (3 and 6 kOe). It is found that the two-dimensional vortices relax thermally and quantum mechanically at high and low temperatures, respectively, for all applied magnetic fields. It is also found that the crossover temperature between the thermal and the quantum regime changes from 2.2 K to 10 K, as the applied field decreases.

Flux noise, vortex glass transition, and irreversibility line in ceramic Tl2Ba2Ca2Cu3O10−δ high-Tc superconductors

Abstract The flux noise and irreversibility line in ceramic Tl 2 Ba 2 Ca 2 Cu 3 O 10− δ high- T c superconductors have been measured by a DC SQUID, and the vortex glass transition has been detected by the standard four-terminal I vs. V measurement. In the low magnetic field region (0 H ≤1000 Oe), it is found that the flux-noise peak coincides with the vortex glass phase transition temperature T g . However, there is an obvious difference in temperature between the irreversibility temperature T irr and T g . T irr is always higher than T g . Our experimental results indicate that the irreversibility line is a dynamical boundary rather than a phase boundary.

Experimental evidence of quantum tunneling of 2D vortices up to 10 K in bulk Tl2Ba2Ca2Cu3O10 superconductor

Abstract Low-temperature magnetic relaxation measurements have been performed in a sintered Tl 2 Ba 2 Ca 2 Cu 3 O 10 bulk high- T c superconductor, with an applied magnetic field of 50 Oe. A temperature-independent normalized magnetic relaxation rate below 10 K is observed. The transition from the thermally activated vortex motion regime to the quantum tunneling regime is found to be very sharp, indicating the absence of dissipation in this magnetic field. The observed behaviour is explained in terms of single 2D vortex motion in the Cu-O layers.