Yoshihide Kimishima

Magnetic properties of NiO nanoparticles

Abstract NiO nanoparticles were produced by annealing Ni(OH)2 monolayer-nanoclusters above 973 K in air. Their diameters were estimated to be between 2 and 6 nm from X-ray diffraction patterns. In the magnetization measurements superparamagnetic or ferromagnetic behaviors were observed and characteristic properties of NiO nanoparticles were confirmed.

Superconducting Properties of CdCNi3

CdCNi 3 is a newly synthesized superconductor with T c =2.5–3.2 K. The crystal has the same perovskite-type structure as MgCNi 3 but the lattice constant is ∼1% larger than that of MgCNi 3 . For th...

New Antiperovskite-Type Superconductor ZnNyNi3

We have synthesized a new superconductor ZnN y Ni 3 with T c ∼3 K. The crystal structure is of the same antiperovskite-type as MgCNi 3 and CdCNi 3 . As far as we know, this is the third superconducting material in the Ni-based antiperovskite series and the first antiperovskite nitride superconducting material. For this new superconductor, the lower critical field H c1 (0), upper critical field H c2 (0), coherence length ξ(0), penetration depth λ(0), Ginzburg–Landau parameter κ(0), electronic specific heat coefficient γ, and Debye temperature Θ D have been experimentally determined.

Structural, magnetic and thermal characterizations of Fe2O3 nanoparticle systems

X-ray, magnetic and differential thermal analysis and thermogravimetric (DTA-TG) measurements of Fe2O3 nanoparticles surrounded by amorphous SiO2were carried out. The mass loss above 370 K could be attributed to the dehydration. The broadened exothermic peak around 900 K was observed by the DTA analysis. Considering the results of the X-ray and magnetic analyses, this anomaly was interpreted as due to the g- to a-transition in the present Fe2O3nanoparticle system. The broadness of the peak and thus the gradual progress of the transformation would be attributed to the stress caused by the amorphous SiO2 network surrounding extremely small particles.

S-doping effects on pinning property of Bi2212 single crystals

Abstract Sulfur doping effects on pinning property were studied for Bi2212 single crystals prepared by self-flux method. The M – H curves of S-doped crystal showed the enlargement of magnetic hysteresis at 77 K, which means the increasing of pinning force in this system. Zero field cooled M – H curves were analyzed by the improved Kim’s critical state model with equilibrium magnetization M eq . For the pure and doped crystals at 77 K, superconductive volume fraction were 0.72 and 0.04, and the cross-over field B 0 were 0.59 and 80 mT, respectively. The penetration field μ 0 H p were of the same orders of magnitude between 0.6 and 0.8 mT for both of the samples. These results show that the S-doping effects suppress the decay of J c as described by the Bean model.

Magnetization analysis of sintered Bi2223 using an extended critical state model

The field dependence of the magnetization was measured for sintered Bi2223 high Tc superconductor under a field amplitude of 5?T from 10 to 90?K. These data were analysed using our critical state model involving an equilibrium magnetization Meq and exponential decay of Jc. Some magnetic parameters including the equilibrium critical current density were determined for the intra-grain?and inter-grain parts. The temperature dependences of the intra-grain?and inter-grain Jc were also discussed. It was revealed that single vortex pinning was dominant in the intra-grain part, and flux creep caused by collective pinning was confirmed in the inter-grain part.

Comments on magnetically estimated Jc for MgB2 by critical state models

Abstract A reliable method is proposed for the estimation of J c eq ( H ) from irreversible magnetization data for a new type-II superconductor MgB 2 . In this case J c eq is the critical current density at the inner sample surface and H is the external field. The equation | J c ( H )|= ΔM ( H )/[ a {1− a /3 b }], often used for a 2 a ×2 b orthorhombic column geometry, is revealed to be incorrect near and below the full penetration field H p . Parameters obtained for the critical state suggest that flux bundle creeping is dominant in this superconductor.

Magnetic and Structural Studies of Ni(OH)2 Monolayered Nanoclusters

Chemical, magnetic and structural analysis have been carried out for the precipitates obtained by mixing NiCl26H2O and Na2SiO3 mH2O (m=0 or 9) aqueous solutions. The formation of Ni(OH)2 monolayered clusters with diameter of 2–3 nm was confirmed in the above precipitates. The exchange interaction in these Ni(OH)2 monolayered nanoclusters was also discussed on the basis of molecular field approximation.

Magnetic phase transitions in K2CucMn1−cF4 mixed crystals

The magnetic properties of insulating quasi-two-dimensional mixed ferromagentic (F) and antiferromagnetic (AF) system K 2 Cu c Mn 1- c F 4 were studied by d.c. and a.c. magnetic susceptibility (χ) measurements, and phase diagram of this system was obtained. AF-region lies in the Cu concentration range of 0≦ c <0.5, and F region does in 0.8< c ≦1. In the intermediate range (0.5< c <0.8), spin-glass behaviors were observed in a.c.χ and d.c.χ below 1 K. The relatively low spin-glass transition temperature was intepreted to result from the characteristic of quasi-two-dimensional nature of this system. The random-field effects were observed in the AF region, and T N ( c )-, T c ( c )-lines were discussed in relation to the percolation theory and orbital ordering of Cu 2+ ions.

The Magnetic Behaviors of Quasi-Two Dimensional Antiferromagnet (CH 3 NH 3 ) 2 CuBr 4

The temperature dependences of magnetic susceptibility and specific heat were measured for (CH 3 NH 3 ) 2 CuBr 4 . The phase transition to the antiferromagnetically ordered state was observed at 15.8 K. From the angle and field dependence of magnetic susceptibility, it was revealed that the preferred axis for the spins was c -axis in this compound. The origin of the anisotropy along c -axis was discussed qualitatively.