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E. Takayama-Muromachi

Publications and source records attributed to E. Takayama-Muromachi.

At least 37 records · Page 2Linked to original sources
arXiv2008

Magnetic and charge transport properties of the Na-based Os oxide pyrochlore

Na-based osmium oxide pyrochlore was synthesized for the first time by an ion-exchange method. KOs2O6 was used as a host compound. Elelectron probe micro-analysis, synchrotron x-ray diffraction analysis, and thermo-gravimetric analysis confirmed its structure not as the beta-type but as the defect-type pyrochlore. The composition was identified as Na1.4Os2O6.H2O. Electrical resistivity, heat capacity, and magnetization measurements of the polycrystalline Na1.4Os2O6.H2O clarified absence of superconductivity above 2 K, being in contrast to what were found for the beta-pyrochlore AOs2O6 (A = Cs, Rb, K). Sommerfeld coefficient of 22 mJ K-2 mol-1 of Na1.4Os2O6.H2O was smallest among those of AOs2O6. A magnetic anomaly at ~57 K and possible associated magnetoresistance (+3.7 % at 2 K in 70 kOe) were found.

cond-mat.str-el↗
arXiv2008

Synthesis and superconducting properties of the iron oxyarsenide TbFeAsO0.85

The iron oxyarsenide TbFeAsO0.85 was synthesized by a high-pressure method. A high-quality polycrystalline sample was obtained and characterized by measurements of magnetic susceptibility and electrical resistivity. Bulk superconductivity with Tc of 42 K was clearly established without an F doping usually conducted to tune on superconductivity in the iron oxypnictide.

cond-mat.supr-con↗
arXiv2007

NaV2O4: a Quasi-1D Metallic Antiferromagnet with Half-Metallic Chains

NaV2O4 crystals were grown under high pressure using a NaCl flux, and the crystals were characterized with X-ray diffraction, electrical resistivity, heat capacity, and magnetization. The structure of NaV2O4 consists of double chains of edge-sharing VO6 octahedra. The resistivity is highly anisotropic, with the resistivity perpendicular to the chains more than 20 times greater than that parallel to the chains. Magnetically, the intrachain interactions are ferromagnetic and the interchain interactions are antiferromagnetic; 3D antiferromagnetic order is established at 140 K. First principles electronic structure calculations indicate that the chains are half metallic. Interestingly, the case of NaV2O4 seems to be a quasi-1D analogue of what was found for half-metallic materials.

cond-mat.str-el↗
arXiv2007

Effect of PVA doping on flux pinning in Bulk MgB2

The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA doping effects made two distinguishable contributions: first enhancement of Jc field performance and second an increase in Hc2 value, both because of carbon incorporation into MgB2 crystal lattice. The susceptibility measurement reveals that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped sample exhibited the Jc values greater than 10^5 A/cm2 at 5 & 10 K at low fields; which is almost 3 times higher than the pure one, while at high fields the Jc is increased by an order of magnitude in comparison to pure MgB2. From R(T)H measurements we found higher Tc values under magnetic field for doped sample; indicating an increase in Hc2. Also the magnetization measurements exhibited a significant enhancement in Hirr value. The improved performance of PVA doped MgB2 can be attributed to the substitution of carbon at boron site in parent MgB2 and the resulting impact on the carrier density and impurity scattering. The improved flux pinning behavior could easily be seen from reduced flux pinning force plots.

cond-mat.supr-con↗
arXiv2007

Room-temperature magnetocaloric effect in La0.7Sr0.3Mn1-xM'xO3 (M'=Al, Ti)

Magnetic entropy and adiabatic temperature changes in and above the room-temperature region has been measured for La0.7Sr0.3Mn1-xM'xO3 (M' = Al, Ti) by means of magnetization and heat capacity measurements in magnetic fields up to 6 T. The magnetocaloric effect becomes largest at the ferromagnetic ordering temperature Tc that is tuned to ~300 K by the substitution of Al or Ti for Mn. While the substitution of Al for Mn drastically reduces the entropy change, it extends considerably the working temperature span and improves the relative cooling power. The magnetocaloric effect seems to be only lightly affected by Ti substitution. Although manganites have been considered potential for magnetic refrigerants, the magnetocaloric effect in these materials is limited due to the existence of short-range ferromagnetic correlations above Tc.

cond-mat.mtrl-sci↗
arXiv2007

Inter comparison of the magneto transport of La2/3Ca1/3MnO3: Ag/In polycrystalline composites

In this article, we report the synthesis, magneto transport features, and magnetization of polycrystalline La2/3Ca1/3MnO3:Agx/Inx composites with x = 0, 0.1, 0.2, 0.3 and 0.4. In case of Ag the temperature coefficient of resistance (TCR) near ferromagnetic (FM) transition enhances significantly with addition of Ag. The FM transition temperature (TFM) is also increased slightly with Ag doping. Magneto-transport measurements revealed that magneto-resistance (MR) is found to be maximum near TFM. Very sharp TCR is seen near TFM with highest value of up to 15 % for Ag (0.4) sample, which is an order of magnitude higher than as for pristine sample and is the best value yet reported for any polycrystalline LCMO compound. Increased TCR, TFM and significant above room temperature MR of La2/3Ca1/3MnO3:Agx composites is explained on the basis of improved grains size. Interestingly the La2/3Ca1/3MnO3:Inx composites behaved exactly in opposite way resulting in decreased TFM, and TCR compared to pristine LCMO compound. In fact the grains morphology of LCMO:Inx composites is seen inferior to pristine LCMO which is opposite to the LCMO:Agx case.

cond-mat.str-el↗
arXiv2006

Impact of rare earth magnetic moment on ordering of Ru in Sr2RuREO6 (RE = Gd and Eu)

We report synthesis and magnetization of Sr2RuREO6 (RE/Ru-211O6) with RE = Gd and Eu. Both Gd,Eu/Ru-211O6 are formed in distorted perovskite-type single phase with monoclinic P21/n space group. Magnetization (M/T) measurements exhibited antiferromagnetic ordering of Ru moments for both Gd and Eu compounds at TN of around 30 K due to super-exchange Ru-O-O-Ru interaction. Interestingly enough, detailed M/H plots of Gd/Ru-211O6 exhibited, the development of a ferromagnetic (FM) component at 20 K. Further at below 10 K, the FM component disappears and rather an spin freezing field (HSF) of around 1000 Oe is seen at 5 K. In case of Eu/Ru-211O6, the basic antiferromagnetic ordering of Ru moments remains invariant down to 5 K, and the appearance of FM component is not seen. It seems that the high magnetic moment of 8mB for Gd3+ influences the AFM ordering of Ru in case of Gd/Ru-211O6.

cond-mat.str-el↗
arXiv2006

Magneto Transport of high TCR (temperature coefficient of resistance) La2/3Ca1/3MnO3: Ag Polycrystalline Composites

We report the synthesis, (micro)structural, magneto-transport and magnetization of polycrystalline La2/3Ca1/3MnO3:Agx composites with x = 0.0, 0.1, 0.2, 0.3 and 0.4. The temperature coefficient of resistance (TCR) near ferromagnetic (FM) transition is increased significantly with addition of Ag. The FM transition temperature (TFM) is also increased slightly with Ag addition. Magneto-transport measurements revealed that magneto-resistance MR is found to be maximum near TFM. Further the increased MR of up to 60% is seen above 300 K for higher silver added samples in an applied field of 7 Tesla. Sharp TCR is seen near TFM with highest value of up to 15 % for Ag (0.4) sample, which is an order of magnitude higher than as for present pristine sample and best value yet reported for any polycrystalline LCMO compound. Increased TCR, TFM and significant above room temperature MR of La2/3Ca1/3MnO3:Agx composites is explained on the basis of improved grains size and connectivity with silver addition in the matrix. Better coupled FM domains and nearly conducting grain boundaries give rise to improved physical properties of the La2/3Ca1/3MnO3 manganites.

cond-mat.str-el↗
arXiv2006

High pressure high temperature (HPHT) synthesis and magnetism of MSr2Y1.5Ce0.5Cu2O10 compounds with M = Ru, Mn, and Cr

The M-1222 compounds are synthesized in near single phase form. These compounds crystallise in I4/mmm Tetragonal structure. None of the studied compound is superconducting but they asre weakly magnetic. This is first report on synthesis of Cr and Mn -1222 compounds. These compounds are synthesized at ultra conditions of HPHT that is the 6 Gpa and 1400 C.

cond-mat.supr-con↗
arXiv2006

Physical property characterization of Fe-tube encapsulated and vacuum annealed bulk MgB2

We report phase formation, and detailed study of magnetization and resistivity under magnetic field of MgB2 polycrystalline bulk samples prepared by Fe-tube encapsulated and vacuum (10-5 torr) annealed (750 0C) route. Zero-field-cooled magnetic susceptibility (cZFC) measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 39 K (Tc). The measured magnetization loops of the samples, despite the presence of flux jumps, exhibited a stable current density (Jc) of around 2.4 x 105 A/cm2 in up to 2 T (Tesla) field and at temperatures (T) up to 10 K. The upper critical field is estimated from resistivity measurements in various fields and shows a typical value of 8 T at 21 K. Further, cFC measurements at an applied field of 0.1 T reveal paramagnetic Meissner effect (PME) that is briefly discussed.

cond-mat.supr-con↗
arXiv2006

Anomalous lattice expansion of RuSr2Eu1.5Ce0.5Cu2O10(Ru-1222) magneto superconductor: A low temperature X-ray diffraction study

This is the first report of the observation of the onset of excess volume and also of the strain along the a-axis near the magnetic ordering temperature in Ru-1222 superconductor, and indicates a coupling between the lattice and the magnetism in this system. Magnetization, magneto transport and thermoelectric power measurements being carried out on the same sample are also reported.

cond-mat.supr-con↗
arXiv2006

Fluxiod jumps coupled high critical current density of nano-Co3O4 doped MgB2

Polycrystalline MgB2 samples with 0, 2, 4 and 6 percent added nano-Co3O4 being synthesized by vacuum (10-5 Torr) annealing at 750 0C for three hours each are found to be nearly single phase with presence of only a small quantity of Mg/MgO in pristine sample and in addition the Co2O3 in doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in Zero-field-cooled (ZFC) magnetic susceptibility measurements with sizeable signal. The Field cooled (FC) measurements though having sharp transitions, but showed a very small signal, indicating high level of pinning centers in these samples. Further some of the doped samples exhibited Paramagnetic-Meissner-Effect (PME) in applied field of 5 Oe. The critical current density (Jc), being estimated by invoking Bean model for the pristine compound increase by nearly an order of magnitude for 2 and 4 percent nano-Co3O4 doping and later the same decreases sharply for 6 percent sample at nearly all studied temperatures and applied fields. Further the increased Jc (~ 10^8 A/cm2) is coupled with fluxiod jumps (T = 20 K and H = 1 T). Fluxiod Jumps are not seen in relatively low Jc pristine or 6 percent sample. This means the fluxiod-jumps are intrinsic only to the high Jc samples.

cond-mat.supr-con↗
arXiv2005

Superconducting phase diagram of NaxCoO2.yH2O

We synthesized Nax(H3O)zCoO2yH2O samples with various Na/H3O ratios but with the constant Co valence of s = +3.40, and measured their magnetic properties to draw phase diagrams of the system. The superconductivity is very sensitive to the Na/H3O ratio. With varying x under fixed s of +3.40, magnetically ordered phase appears in the intermediate range of x sandwiched by two separated superconducting phases, suggesting that the superconductivity is induced by moderately strong magnetic interactions. In the vicinity of the magnetic phase, transition from the superconducting state to the magnetically ordered state was induced by applying high magnetic field. This transition is of the second order, at least, above 1.8 K. The upper-critical field is expected to be much higher than the Pauli limit for a phase located far away from the magnetic phase regarding the Na/H3O ratio.

cond-mat.supr-con↗
arXiv2005

Valence and Na content dependences of superconductivity in NaxCoO2.yH2O

Various samples of sodium cobalt oxyhydrate with relatively large amounts of Na$^{+}$ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of a cobalt valence of $\sim$+3.48, which was compared with a previously obtained one of $\sim$+3.40. The superconductivity was significantly affected by the isovalent exchanger of Na$^{+}$ and H$_{3}$O$^{+}$, rather than by variation of Co valence, suggesting the presence of multiple kinds of Fermi surface. Furthermore, the high-field magnetic susceptibility measurements for one sample up to 30 T indicated an upper critical field much higher than the Pauli limit supporting the validity of the spin-triplet pairing mechanism.

cond-mat.supr-con↗
arXiv2005

Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O

The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in both the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K_{ab} decreases below 5 K, whereas K_c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.

cond-mat.supr-con↗
arXiv2005

New Misfit-Layered Cobalt Oxide (CaOH)1.14CoO2

We found a new cobalt oxide (CaOH)1.14CoO2 by utilizing the high-pressure technique. X-ray and electron diffraction studies revealed that the compound has layer structure which consists of CdI2-type CoO2 layers and rock-salt-type double CaOH atomic layers. The two subcells have incommensurate periodicity along the a-axis, resulting in modulated crystal structure due to the inter-subcell interaction. The structural modulation affects carrier conduction through the potential randomness. We found that the two-dimensional (2-D) variable-range hopping (VRH) regime with hole conduction is dominant at low temperature for this compound, and that the conduction mechanism undergoes crossover from the 2-D VRH regime to thermal activation-energy type one with increasing temperature. Based on the experimental results of resistivity, thermoelectric power, magnetic susceptibility and specific heat measurements, we suggested a possible electronic-band structure model to explain these results. The cobalt t2g-derivative band crosses Fermi energy level near the band edge, yielding small finite density of localized states at the Fermi level in the band. The observed resistivity, Seebeck coefficient, large Pauli paramagnetic component in the magnetic susceptibility and comparatively small Sommerfeld constant in the specific heat are principally attributed to the holes in the t2g-derivative band. We estimated the Wilson ratio to be about 2.8, suggesting the strong electron correlation realized in this compound.

cond-mat.mtrl-sci↗
arXiv2005

Metallic conductivity and a Ca substitution study of NaRh2O4 comprising a double chain system

The metallic compound NaRh2O4 forms a full range solid solution to the insulating phase CaRh2O4. At a Na concentration of 0.25 moles per formula unit, we found an unexpected contribution to the specific heat at low temperature [K. Yamaura et al. Chem. Mater. 17 (2005) 359]. To address this issue, specific heat and ac and dc magnetic susceptibilities were additionally measured under a variety of conditions for the Na0.25 sample. A new set of data clearly indicate the additional specific heat is magnetic in origin; however, the magnetic entropy is fairly small (~1 % of Schottky term for a simple splitting doublet), and there is no other evidence to suggest that a magnetic phase transition is responsible for the anomalous specific heat.

cond-mat.str-el↗