Biomedical subjects

Mikio Takano

Publications and source records attributed to Mikio Takano.

8 recordsLinked to original sources

PubMed2005

Magnetic properties of isostructural BaCoP2O7, BaNiP2O7, and BaCuP2O7 studied with dc and ac magnetization and specific heat.

Magnetic properties of three isostructural compounds BaMP2O7 (M = Co, Ni, and Cu) were investigated by dc and ac magnetization and specific heat measurements. BaCuP2O7 was shown to be an excellent quasi-one-dimensional linear-chain Heisenberg antiferromagnet with an exchange constant (J/kB) of 103.8 K (Hamiltonian H = J Sigma SiS(i+1)) and a temperature for the long-range magnetic order (TN) of 0.81 K giving the ratio kBTN/J = 0.78%. BaCoP2O7 and BaNiP2O7 exhibited long-range antiferromagnetic order at TN = 10.4 and 10.1 K, respectively. BaCoP2O7 and BaNiP2O7 showed a large contribution of the short-range correlation above TN. BaNiP2O7 remained in the antiferromagnetic state up to 90 kOe at 2 K, whereas BaCoP2O7 demonstrated two metamagnetic phase transitions at about 52 and 71 kOe at 2 K if the magnetic field was parallel to the easy direction. BaMP2O7 melted incongruently at 1323 K (M = Co), 1344 K (M = Ni), and 1338 K (M = Cu).

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PubMed2005

Investigation of the crystal structure and the structural and magnetic properties of SrCu2(PO4)2.

SrCu2(PO4)2 was prepared by the solid-state method at 1153 K. Its structure was solved by direct methods in the space group Pccn (No. 56) with Z = 8 from synchrotron X-ray powder diffraction data measured at room temperature. Structure parameters were then refined by the Rietveld method to obtain the lattice parameters, a = 7.94217(8) A, b = 15.36918(14) A, and c = 10.37036(10) A. SrCu2(PO4)2 presents a new structure type and is built up from Sr2O16 and Cu1Cu2O8 units with Cu1...Cu2 = 3.256 A. The magnetic properties of SrCu2(PO4)2 were investigated by magnetic susceptibility, magnetization up to 65 T, Cu nuclear quadrupole resonance (NQR), electron-spin resonance, and specific heat measurements. With spin-dimer analysis, it was shown that the two strongest spin-exchange interactions between Cu sites result from the Cu1-O...O-Cu2 and Cu2-O...O-Cu2 super-superexchange paths with Cu1...Cu2 = 5.861 A and Cu2...Cu2 = 5.251 A, and the superexchange associated with the structural dimer Cu1Cu2O8 is negligible. The magnetic susceptibility data were analyzed in terms of a linear four-spin cluster model, Cu1-Cu2-Cu2-Cu1 with -2J(1)/kB = 82.4 K for Cu1-Cu2 and -2J(2)/k(B) = 59 K for Cu2-Cu2. A spin gap deduced from this model (Delta/kB = 63 K) is in agreement with that obtained from the Cu NQR data (Delta/kB = 65 K). A one-half magnetization plateau was observed between approximately 50 and 63 T at 1.3 K. Specific heat data show that SrCu2(PO4)2 does not undergo a long-range magnetic ordering down to 0.45 K. SrCu2(PO4)2 melts incongruently at 1189 K. We also report its vibrational properties studied with Raman spectroscopy.

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PubMed2005

Designed ferromagnetic, ferroelectric Bi(2)NiMnO(6).

A newly designed ferromagnetic, ferroelectric compound, Bi(2)NiMnO(6), was prepared by high-pressure synthesis at 6 GPa. The crystal structure, as determined by synchrotron X-ray powder diffraction, is a heavily distorted double perovskite with Ni(2+) and Mn(4+) ions ordered in a rock-salt configuration. The presence of 6s(2) lone pairs of Bi(3+) ions and the covalent Bi-O bonds give ferroelectric properties with T(CE) of 485 K, while -Ni(2+)-O-Mn(4+)-O-Ni(2+)- magnetic paths lead to a ferromagnetism with T(CM) of 140 K. This simple material design to distribute two magnetic elements with and without e(g) electrons on B sites of Bi- and Pb-based perovkites can be applied to other Bi(2)M(2+)M'(4+)O(6) and Pb(2)M(3+)M'(5+)O(6) systems to search for newer ferromagnetic ferroelectrics.

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PubMed2005

Low-dimensional ferromagnetic properties of SrCuV2O7 and BaCuV2O7.

The crystal structure of isostructural SrCuV(2)O(7) and BaCuV(2)O(7) consists of one-dimensional (1D) zigzag chains of Cu atoms with next-nearest-neighbor interaction. The main intrachain interaction was found to be ferromagnetic and estimated at 4.6 K (Hamiltonian H approximately -2J). SrCuV(2)O(7) and BaCuV(2)O(7) are new examples in the scanty family of 1D ferromagnets. Isothermal magnetization measurements at 0.08 K and specific heat data showed that MCuV(2)O(7) exhibits antiferromagnetic long-range ordering at T(N) = 1.36 K for SrCuV(2)O(7) and T(N) = 1.47 K for BaCuV(2)O(7). Spin-flop transitions were observed in the antiferromagnetic state at 0.08 K near 0.5 kOe in SrCuV(2)O(7) and 2 kOe in BaCuV(2)O(7). In air, SrCuV(2)O(7) and BaCuV(2)O(7) melted incongruently above 983 and 1018 K, respectively.

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PubMed2005

Effect of explicit cationic size and valence constraints on the phase stability of 1:2 B-site-ordered perovskite ruthenates.

The related parameters of cation size and valence that control the crystallization of Sr(3)CaRu(2)O(9) into a 1:2 B-site-ordered perovskite structure were explored by cationic substitution at the strontium and calcium sites and by the application of high pressure. At ambient pressures, Sr(3)MRu(2)O(9) stoichiometries yield multiphasic mixtures for M = Ni(2+), Mg(2+), and Y(3+), whereas pseudocubic perovskites result for M = Cu(2+) and Zn(2+). For A-site substitutions, an ordered perovskite structure results for Sr(3-x)Ca(x)CaRu(2)O(9), with 0 </= x </= 1.5. In contrast, Ba(2+) substitution for Sr(2+) is accompanied by a phase change to a hexagonal BaTiO(3) structure type. At high pressures and temperatures, a 1:2 B-site-ordered perovskite structure is stabilized for Sr(3-x)Ba(x)CaRu(2)O(9), with 0 </= x </= 3. The scarcity of B-site-ordered perovskite ruthenates at ambient pressure and the metastable nature of the high-pressure phases underscore the strict size and valence requirements that must be met by the constituent cations to achieve these uncommon ordered structures.

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PubMed2004

Pressure-induced buckling of spin ladder in SrCu2O3.

Pressure-induced structural phase transition of spin ladder compound SrCu2O3 was investigated by synchrotron X-ray powder diffraction with a diamond anvil cell (DAC). The change was characterized by a buckling of the Cu2O3 plane in the rung direction of the ladder. The structure of the high-pressure phase was found to be essentially the same as that of CaCu2O3. Application of an external pressure of 3.4 GPa therefore affected the structure in the same manner that the chemical (internal) pressure does.

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PubMed2003

Short-range and long-range magnetic ordering in SrCuP2O7 and PbCuP2O7.

Magnetic properties of SrCuP2O7 and PbCu(1-x)ZnxP2O7 (x=0, 0.1, and 0.5) were studied by magnetic susceptibility, chiT, and specific heat, Cp(T). Both data showed that magnetism of SrCuP2O7 and PbCuP2O7 can be described by the one-dimensional (1D) uniform chain model despite the structural features suggesting the presence of zigzag chains with next-nearest-neighbor interactions. The chiT data were fitted by the Bonner-Fisher curve (plus temperature independent and Curie-Weiss terms) with g=2.20 and J/kB=9.38 K for SrCuP2O7 and g=2.17 and J/kB=8.41 K for PbCuP2O7 (Hamiltonian H=J SigmaSiS(i+1)). Magnetic specific heat, Cm(T), exhibited one broad maximum due to short-range ordering and one sharp peak at TN=1.64 K for SrCuP2O7 and TN=1.15 K for PbCuP2O7 due to long-range antiferromagnetic ordering. The characteristic values of the broad maxima on the Cm(T) curves (Cmax and TC(max)) were in good agreement with the theoretical calculations for the uniform 1D S=1/2 Heisenberg chain. Magnetic properties of PbCu0.9Zn0.1P2O7 still obeyed the 1D uniform chain model but those of PbCu0.5Zn0.5P2O7 did not. In air, SrCuP2O7 was stable at least up to 1373 K while PbCuP2O7 melted incongruently above 1180 K.

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PubMed2002

Growth of Na-doped Ca(2)CuO(2)Cl(2) single crystals under high pressures of several GPa.

Single crystals of Na-doped Ca(2)CuO(2)Cl(2) have been grown for the first time by a flux method under high pressures of up to 5.5 GPa. By changing the Na-solubility limit through the applied pressure, the Na content x was successfully controlled without introducing appreciable compositional inhomogeneity within the millimeter-sized crystals. Structural and chemical characterization indicated that the crystals span the phase diagram continuously from the parent antiferromagnetic insulator to an underdoped high-temperature superconductor. Because of the well-defined cleavage plane and resulting high surface quality, these oxychloride single crystals will provide a unique opportunity to explore the electronic evolution of the high-temperature superconductors, using spectroscopic techniques such as scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy.

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