Single crystal Ca 3 Ru 2 O 7 shows a metallic antiferromagnetic phase intermediate between a first-order metal to nonmetal transition at T M 48 K and the antiferromagnetic ordering (Néel) temperature, T N 56 K. The metallic antiferromagnetic phase is predicted within various Mott-Hubbard models. Magnetization and electrical resistivity reveal strongly anisotropic metamagnetism in the nonmetallic antiferromagnetic phase. The charge and spin excitations are strongly coupled: The H-T phase diagrams determined by magnetization and magnetoresistivity are indistinguishable and reveal a multicritical point. The heat capacity of Ca 3 Ru 2 O 7 suggests it is a highly correlated electron system. [S0031-9007(97)02552-0]
The magnetic, transport, optical, and structural properties of quasi-one-dimensional BaIrO 3 show evidence for the simultaneous onset of electronic density wave formation and ferromagnetism at T c3 175 K: Two additional features in the chain direction dc conductivity show a sudden change to metallic behavior below T c2 80 K and then a Mott-like transition at T c1 26 K: Highly non-linear dc conductivity, optical gap formation at Ϸ9k B T c3 , additional phonon modes, and emergent X-ray satellite structure support density wave formation. Even at very high (30 T) fields the saturation Ir moment is very small, Ϸ0.04m B /Ir. ᭧ 2000 Elsevier Science Ltd. All rights reserved. Transition metal oxides (TMO) with low crystalline symmetry are known to exhibit electronic density wave formation [1][2][3]. However, to our knowledge, density wave formation has not yet been observed accompanying the onset of ferromagnetic order. However, the ferromagnetism at T c3 175 K in BaIrO 3 [4] appears to be accompanied by and possibly driven by a collective electronic excitation or at least partial gapping of the Fermi surface. This demonstrates once again the strong coupling between spin and charge in the heavy (4d-and 5d-based) TMOs [5][6][7]. BaIrO 3 has a highly anisotropic quasi-one-dimensional structure [8][9][10] and this gives rise, in our single crystal samples, to large anisotropy of r(T), the electrical resistivity, with the quasi-one-dimensional axis, the c-axis, having much lower resistivity. This kind of low-dimensional structure is necessary for the formation of an insulating charge density wave (CDW) ground state, which is a collective electron mode normally incommensurate with the underlying lattice for partially filled bands [3].Evidence for density wave formation comes from: (1) A discontinuous increase in the slope of r (T) vs. T at T c3 T C ; the Curie temperature-an abrupt transition to a more insulating phase. (Two additional features of r (T) along the c-axis, at T c2 80 K and T c1 26 K; mark a sudden return to "metallic" behavior (possibly a crossover from partial toward full gapping of the Fermi surface) and a well-defined Mott-like metal-insulator transition, respectively). (2) An abrupt feature in the non-linear conductivity showing negative differential resistivity. (3) Gap formation at about 1200 cm Ϫ1 in the electron excitation spectrum and a splitting of a phonon mode at 350 cm Ϫ1 , which appear for T Ͻ T c3 (This was determined by optical reflectivity studies in the far and near infrared.). (4) Additional satellite formation for T Ͻ T C3 in the X-ray diffraction spectrum.The structure of BaIrO 3 is monoclinic and consists of Ir 3 O 12 trimers of face-sharing IrO 6 octahedra which are vertex-linked to other trimeric clusters forming columns roughly parallel to the c-axis. These clusters form channels accommodating Ba ions. The space group is C2/m and the
We report a first-order phase transition at T M =357 K in single crystal Ca 2 RuO 4 , an isomorph to the superconductor Sr 2 RuO 4 . The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low temperature orthorhombic to a high temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at T M and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T N =110 K. PACS: 61.10 Nz, 71.30 +h,
The low-temperature properties of MFe4P» (M =La, Pr, Nd, arid Th) single crystals have been studied by means of electrical-resistivity, magnetization, specific-heat, and magnetoresistivity measurements. Superconductivity among these compounds is known to occur only in LaFe4P», which has a superconducting transition temperature T, of -4 K. The compounds PrFe4P» and NdFe4P» display features that suggest the occurrence of antiferromagnetic ordering below -6.2 K and ferromagnetic ordering below -2 K, respectively. Isothermal magnetization curves for PrFe4P» below 6 K reveal a spin-flop or metamagnetic transition.
Ca 2 RuO 4 , which has the single-layer tetragonal K 2 NiF 4 structure, shows nonmetallic behavior for TϽ300 K unlike its isostructural counterpart, Sr 2 RuO 4 , which is metallic for all TϽ1300 K and which undergoes a superconducting transition temperature below Tϭ1.35 K, possibly with p-wave spin pairing. Magnetization, electrical-resistivity, and heat-capacity data for single-crystal Ca 2 RuO 4 are presented. An antiferromagnetic transition is identified at T N ϭ110 K in all samples studied. The easy axis for magnetization is parallel to the a or b axis ͑in the Ru-O plane͒. Isothermal magnetization studies in fields to 30 T show a spin reorientation transition for Bϭ3.5 T at Tϭ105 K and a metamagnetic transition for TӶT N at about 9 T. The saturation magnetization even at 30 T, M sat Ϸ0.4 B /Ru, is less than expected for the Ru Sϭ1 moment (M sat Ϸ2.0 B /Ru͒. The electrical resistivity, (T), increases with decreasing temperature by eight orders of magnitude for 70ϽTϽ300 K and fits a variable-range hopping model including correlations. The magnetoresistivity below T N shows some dependence on spin orientation. In contrast to all the other Sr and Ca-based ruthenates, Ca 2 RuO 4 has a relatively small low-temperature electronic specific-heat coefficient ͑␥ϭ4 mJ/mole K 2 ͒, and unlike them, Ca 2 RuO 4 shows no anomaly in either (T) or d(T)/dT at T N . Some magnetization results are presented for lightly Sr-doped Ca 2 RuO 4 . The data are contrasted with those of metallic Sr 2 RuO 4 and discussed in terms of the weak spin disorder scattering in Ca 2 RuO 4 compared to the very strong coupling in the other Sr-and Ca-based ruthenates. ͓S0163-1829͑97͒50230-X͔
The new compound UFe4P12, which was found to be isostructural to superconducting LaFe4P12 and have a lattice constant of 7.7729 Å, is a semiconductor and shows ferromagnetic order below 3.15 K. CeFe4P12 is also a semiconductor, and its magnetic susceptibility is unusually small in comparison to LaFe4P12. The semiconducting behaviors of both UFe4P12 and CeFe4P12 seem anomalous and may arise from strong f-electron hybridization.
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