arXiv subjects

E. Takayama-Muromachi

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

At least 73 records · Page 4Linked to original sources

arXiv2003

Investigation of the ferromagnetic transition in the correlated 4d perovskites SrRu$_{1-x}$Rh$_x$O$_3$

The solid-solution SrRu$_{1-x}$Rh$_x$O$_3$ ($0\le x \le1$) is a variable-electron-configuration system forming in the nearly-cubic-perovskite basis, ranging from the ferromagnetic 4$d^4$ to the enhanced paramagnetic 4$d^5$. Polycrystalline single-phase samples were obtained over the whole composition range by a high-pressure-heating technique, followed by measurements of magnetic susceptibility, magnetization, specific heat, thermopower, and electrical resistivity. The ferromagnetic order in long range is gradually suppressed by the Rh substitution and vanishes at $x \sim 0.6$. The electronic term of specific-heat shows unusual behavior near the critical Rh concentration; the feature does not match even qualitatively with what was reported for the related perovskites (Sr,Ca)RuO$_3$. Furthermore, another anomaly in the specific heat was observed at $x \sim 0.9$.

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arXiv2003

The role of the water molecules in novel superconductor, Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O

In order to investigate the role of the water molecules in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O, we synthesized superconducting Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O and nonsuperconducting Na$_{0.35}$CoO$_{2}\cdot$0.7H$_{2}$O, and measured their normal-state magnetic susceptibilities. The susceptibility of Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O has an enhancement below ~150 K probably caused by ferromagnetic fluctuation, whereas no such enhancement was observed in Na$_{0.35}$CoO$_{2}\cdot$0.7H$_{2}$O. The water molecules in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O may work to shield random coulomb potential of the Na ions with smoother potential at the CoO$_{2}$ layer. This effect may account for the appearance of superconductivity in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O.

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arXiv2003

Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O

We report on a Raman scattering study on the superconducting cobaltite $Na_xCoO_2\cdot yH_2O$ as function of Na content and hydration (x$\approx$1/3, 3/4 and y$\approx$0, 2/3, 4/3). The observed phonon scattering and scattering continua are analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of one or two $H_2O$ layers, releases a part of this strain. Our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the $a_{1g}$ level from the other $t_{2g}$ states of $Co^{4+}$ and superconductivity.

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arXiv2003

Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na$_x$CoO$_2$$\cdot y$H$_2$O

Co NQR studies were performed in recently discovered superconductor Na$_x$CoO$_2$$\cdot y$H$_2$O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na$_x$CoO$_2$ were examined, SC bilayer-hydrate sample with $T_c \sim 4.7$ K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate $1/T_1$ in the SC sample, it was found that the coherence peak is absent just below $T_c$ and that $1/T_1$ is proportional to temperature far below $T_c$. These results, which are in qualitatively agreement with the previous result by Fujimoto {\it et al.}, suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, $1/T_1T$ of the SC sample shows gradual increase below 100K down to $T_c$, whereas $1/T_1T$ of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between $1/T_1T$ and $χ_{\rm bulk}$ in the SC sample, the increase of $1/T_1T$ is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.

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arXiv2003

Superconductivity of up to 80 K for Tb-123 - TbSr2Cu2.7Mo0.3O7

The Tb-123 phase has been synthesized in single-phase form with the composition TbSr2Cu2.7Mo0.3O7 by solid-state reaction route and its phase purity is confirmed by neutron powder diffraction experiments. As-synthesized sample does not show superconductivity down to 5 K. An unusually high antiferromagnetic ordering temperature (TN) of around 7 K is seen for Tb. After 120-atm-O2 post-annealing, bulk superconductivity is achieved in the compound at around 30 K, without any significant effect on TN. To achieve higher oxygen content and higher Tc, the as-synthesized sample is subjected to high-pressure oxygenation (HPO) carried out in a closed cell at 5 GPa and 400 oC in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction of larger than 10 % at 5 K. Our observation of superconductivity at 80 K is the highest Tc to-date among the Tb-123 compounds.

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arXiv2003

Spin Glass Behavior in the Ru-1222 and Ru-1212 rutheno-cuprate families: a comparative study

Strong evidences of a spin glass phase on polycrystalline RuSr2Ln1.5Ce0.5Cu2O10-x (LnRu-1222, Ln = Y, Ho and Dy) is provided by ac susceptibility and dc magnetization measurements. The fingerprint of a spin glass transition, the frequency-dependent cusp in ac susceptibility vs. T measurements at low magnetic field, is observed for all LnRu-1222 samples. The strong supression of the cusp in the ac susceptibility measurement in the presence of magnetic fields as low as 500 Oe, and the results of thermoremanent magnetization (TRM) and isothermal remanent magnetization (IRM) measurements are also evidences of a spin glass behavior. By comparing these results with measurements on a YRu-1212 sample we conclude that the glassy behavior is characteristic of the Ru-1222 phase, not being observed for the studied YRu-1212 sample. We suggest that the spin glass transition occur only in the Ru-1222 phase possibly due to the larger number of oxygen vacancies present in these compounds.

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arXiv2003

Magnetism and Superconductivity in RuM1.5Ce0.5Sr2Cu2O10 (M=Eu and Y)

RuY1.5Ce0.5Sr2Cu2O10, is not superconducting (SC) and orders magnetically at TM =152 K. The Ru moment at 5K is 0.75 mB. Ferromagnetic hysteresis loops are observed below Tirr= 100 K. At 100 K, the remanent magnetization (Mrem) and the coercive field (HC) become zero. Surprisingly, at Tirr <T< TM a reappearance of antiferromagnetic (AFM) hysteresis loops are observed with a peak at 120 K for both Mrem and HC. The paramagnetic constants are: Peff =2.06 mB (greater than the 1.73 mB expected for low spin Ru5+) and q= 136 K which agrees well with TM. Substitution of Zn (2.5% atm.) for Cu in RuEu1.5Ce0.5Sr2Cu2O10 (TC=38 K), suppresses TC but does not affect the magnetic features such as: TM and Tirr, Mrem and HC. This indicates that both SC and magnetic states are practically decoupled. We argue that the magnetic structure of this system becomes (i) AFM ordered at TM. (ii) Due to the tilting of the RuO6 octahedra away from the crystallographic c axis, at Tirr < TM, weak-ferromagnetic (W-FM) order is induced by the canting of the Ru moments. (iii) At TC (TM/TC~4), the specific materials (M=Eu, Gd) become SC. Below TC, both SC and W-FM states coexist.

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arXiv2003

Magnetization study of RuSr2Y1.5Ce 0.5Cu2O10 (Ru-1222)

We have studied the magnetic properties of the non-superconducting RuSr2Ln1.5Ce0.5Cu2O10 (Ln=Y, Dy and Ho, Ru-1222) compounds synthesized under high pressure (6 Gpa) at elevated temperature. The materials become magnetically ordered at TM =152(2) K regardless of Ln. The wide ferromagnetic-like hysteresis loops which open at 5 K, close themselves around Tirr= 90-100 K and the remanent magnetizations (Mrem) and the coercive fields (HC) become zero. Surprisingly, at Tirr<T< TM a reappearance of the Mrem and HC (with a peak at 120-130 K) is observed for all three samples studied. For the non-magnetic Ln=Y compound, the extracted saturation moment at 5 K and the effective paramagnetic moment are is 0.75 and 2.05 mB /Ru, values which are close to the expected 1 mB and 1.73 mB respectively, for the low-spin state of Ru5+. We argue that the Ru-1222 system becomes (i) anti-ferromagnetically (AFM) ordered at TM. In this range a metamagnetic transition is induced by the applied field (ii). At Tirr < TM, weak-ferromagnetism (W-FM) is induced by the canting of the Ru moments.

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arXiv2002

Magneto-superconductivity of 100-atm O2-annealed RuSr2Gd1.5Ce0.5Cu2O10

Studied 100-atm O2-annealed RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) compound crystallized in a tetragonal I4/mmm space group crystal structure. Thermo-gravemetric (TG) analysis of the compound showed the release of oxygen and breaking to metallic constituents in two distinct steps at around 350 and 500 0C. The DC magnetization data (M vs. T) revealed magnetic transition at 100 K followed by superconducting transition at 40 K. Low field M vs. H hysteresis loop showed a lower critical field (Hc1) value of around 25 Oe. Ferromagnetic component is evidenced at 5, 10, 20 and 40 K. Near saturation field of above 5 Tesla is observed at 5 K. Zero-field returning moment (Mr) and zero-moment coercive field (Hc) values at 5 K are 0.35mB and 250 Oe. The resistance vs. temperature (R vs. T) behaviour of the sample confirmed superconductivity at around 43 K. Superconductivity transition (Tc) is broadened under magnetic field with strong granularity like steps.

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arXiv2002

Synthesis of RuSr2(Ln3/4Ce1/4)2Cu2O10 magneto-superconducting compounds with Ln = Ho, Y and Dy by high-pressure high-temperature (HPHT) technique

The samples of composition RuSr2(Ln3/4Ce1/4)2Cu2O10 with Ln = Ho, Y and Dy. being synthesized by high pressure high temperature (6Gpa, 1200 0C) solid state synthesis route do crystallize in space group I4/mmm. These samples exhibit magnetic transitions at ~150 K with significant branching of zfc (zero-field-cooled) and fc (field-cooled) magnetization and a sharp cusp in zfc at ~ 100 K, followed by a superconducting transition at lower temperatures. All the compounds show typical ferromagnetic hysteresis loops in magnetic moment (M) versus field (H) magnetization at 5 K. Near saturation of the moments at 5 K is achieved in above 5 T (Tesla) applied fields with a value of ~ 0.80 muB. At low temperatures (5 K) these compounds exhibit both superconductivity and ferromagnetism. To our knowledge these are first successfully synthesized Ru-1222 compounds with various lanthanides including Y, Dy, and Ho. The results are compared with widely reported Gd/Ru-1222 compound.

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arXiv2002

Cation intermixing and ordering phenomenon in MO layer of MSr2YCu2Oz (M1212) compounds with M = Fe, Co, Al, and Ga: A neutron powder diffraction study

Various MSr2YCu2Oz (M1212 with M = Fe, Co, Al and Ga) compounds have been synthesized through a solid state reaction route. All these compounds are derived from the basic CuBa2YCu2O7 structure. Rietveld structural refinement of the room-temperature neutron diffraction data for Fe-1212 reveals that nearly half of Fe remains at the M site, while the other half goes to the Cu site in the CuO2 plane. For the M = Co, Al and Ga compounds, the basic unit cell gets doubled due to two distinct M sites in the M-O layer and the space group Ima2, without any cation intermixing is found more appropriate. For the M = Fe sample, magnetic ordering of Fe spins is seen from magnetization data below 70 K. In the case of the M = Co compound, a shallow down-turn in magnetic susceptibility is seen below 150 K, indicating low dimensional magnetic characteristics of the CoO layer. The compounds with M = Al and Ga show paramagnetic behaviour in their magnetization data down to 5 K. The oxygen content, established for each compound based on oxygen occupancies, refined from the neutron diffraction data comes close to 7.0 for each.

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arXiv2002

Spin Glass Behavior in RuSr2Gd1.5Ce0.5Cu2O10

The dynamics of the magnetic properties of polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O2 planes below Tc ~ 40 K. The exact nature of Ru spins magnetic ordering is still debated and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac susceptibility vs. T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin glass with magnetically interacting clusters. Such interpretation is supported by themoremanaent magnetization (TRM) measurements at T = 60 K. TRM relaxations are well described by a stretched exponential relation, and present significant ageing effects.

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arXiv2002

Possible phase separated bulk superconductivity in Ru0.9Sr2YCu2.1O7.9

The Ru0.9Sr2YCu2.1O7.9 sample being synthesized by high-pressure high-temperature solid-state reaction underwent (weak) ferromagnetic transition at 150 K followed by superconducting transition at 30 K. It showed clear Meissner signal in the field-cooled process up to external magnetic field of a few hundred Oe (~300 Oe). The magnetic susceptibility data and magnetic hysteresis data could be explained assuming that the sample was a macroscopic mixture of a superconductor and a (weak) ferromagnet. Any anomalous behaviour implying the coexistence of superconductivity and magnetism on a microscopic scale was not observed.

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arXiv2002

Electronic properties of the novel 4d metallic oxide SrRhO3

The novel 4d perovskite compound SrRhO3 was investigated by isovalent doping studies. The solubility limits of Ca and Ba onto Sr-site were below 80% and 20%, respectively. Although SrRhO3 was chemically compressed, approximately 5.7% by the Ca doping, no significant influence was observed on the magnetic and electrical properties.

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arXiv2002

Crystal structure, electronic, and magnetic properties of the bilayered rhodium oxide Sr3Rh2O7

The bilayered rhodium oxide Sr3Rh2O7 was synthesized by high-pressure and high-temperature heating techniques. The single-phase polycrystalline sample of Sr3Rh2O7 was characterized by measurements of magnetic susceptibility, electrical resistivity, specific heat, and thermopower. The structural characteristics were investigated by powder neutron diffraction study. The rhodium oxide Sr3Rh2O7 [Bbcb, a = 5.4744(8) A, b = 5.4716(9) A, c = 20.875(2) A] is isostructural to the metamagnetic metal Sr3Ru2O7, with five 4d electrons per Rh, which is electronically equivalent to the hypothetic bilayered ruthenium oxide, where one electron per Ru is doped into the Ru-327 unit. The present data show the rhodium oxide Sr3Rh2O7 to be metallic with enhanced paramagnetism, similar to Sr3Ru2O7. However, neither manifest contributions from spin fluctuations nor any traces of a metamagnetic transition were found within the studied range from 2 K to 390 K below 70 kOe.

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arXiv2002

Systematic trends in the electronic structure parameters of 4d transition metal oxides SrMO_{3} (M = Zr, Mo, Ru, and Rh)

We investigated the electronic structures of the perovskite-type 4$d$ transition metal oxides Sr$M$O$_3$ ($M$ = Zr, Mo, Ru, and Rh) using their optical conductivity spectra $σ(ω)$. The interband transitions in $σ(ω)$ are assigned, and some important physical parameters, such as on-site Coulomb repulsion energy $U$, charge transfer energy $Δ_{pd}$, and crystal field splitting $10Dq$, are estimated. It is observed that $Δ_{pd}$ and 10$Dq$ decrease systematically with the increase in the atomic number of the 4$d$ transition metal. Compared to the case of 3$d$ transition metal oxides, the magnitudes of $Δ_{pd}$ and 10$Dq$ are larger, but those of $U$ are smaller. These behaviors can be explained by the more extended nature of the orbitals in the 4$d$ transition metal oxides.

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arXiv2001

Synthesis, Crystal Structure and Magnetic Properties of the Linear-Chain Cobalt Oxide Sr5Pb3CoO12

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 390 K). The cobalt oxide was found to be analogous to the copper oxide Sr5Pb3CoO12, which is comprised of magnetic-linear chains at inter-chain distance of 10 A. Although the cobalt oxide chains (mu_eff of 3.64 mu_B per Co) are substantially antiferromagnetic (theta_W = -38.8 K), neither low-dimensional magnetism nor long-range ordering has been found; a local-structure disorder in the chains might impact on the magnetism. This compound is highly electrically insulating.

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arXiv2001

Crystal Structure and Magnetism of the Linear-Chain Copper Oxides Sr5Pb3-xBixCuO12

The title quasi-1D copper oxides (0=< x =<0.4) were investigated by neutron diffraction and magnetic susceptibility studies. Polyhedral CuO4 units in the compounds were found to comprise linear-chains at inter-chain distance of approximately 10 A. The parent chain compound (x = 0), however, shows less anisotropic magnetic behavior above 2 K, although it is of substantially antiferromagnetic (mu_{eff}= 1.85 mu_{B} and Theta_{W} = -46.4 K) spin-chain system. A magnetic cusp gradually appears at about 100 K in T vs chi with the Bi substitution. The cusp (x = 0.4) is fairly characterized by and therefore suggests the spin gap nature at Delta/k_{B} ~ 80 K. The chain compounds hold electrically insulating in the composition range.

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