T. Goko

Pressure effect on transport and magnetic properties of A2FeMoO6(A=Ba,Sr)

Abstract For double perovskite oxides Ba 2 FeMoO 6 (BFMO) and Sr 2 FeMoO 6 (SFMO), resistivity and magnetization have been measured under pressure and magnetic field. By applying pressure the magnetic transition temperature T C is largely and slightly enhanced for BFMO and SFMO, respectively. The different value of T C between BFMO and SFMO can be explained in terms of the chemical pressure effect. The saturation magnetization does not exhibit the pressure dependence in both of BFMO and SFMO. The resistivity of BFMO is drastically reduced by applying pressure, although the pressure effect on magnetoresistance is quite small, indicating that the components of conduction which contribute to the colossal magnetoresistance effect hardly depends on pressure.

μSR study of magnetic order in La2-xMxCu1-yZnyO4 (M = Sr, Ba)

Abstract Zero-field μSR measurements have been performed on Zn-free and Zn-substituted samples of La2−xSrxCuO4 (LSCO) and La2−xBaxCuO4 (LBCO) over a wide doping range. In LSCO and LBCO, around x∼ 1 8 , superconductivity is suppressed in the same doping range where magnetic order appears, suggesting that superconductivity competes with the magnetic order. Zn substitution induces or enhances the magnetic order for LSCO, but depresses the magnetic order for LBCO. The difference is attributed to the existence of stable static magnetic correlations in LBCO. In the doping range around x∼ 1 8 , the magnetic ordering temperature is almost independent of the doping, which indicates segregation of the spin and hole regions. The constant internal magnetic field observed around x∼ 1 8 is much smaller than that for La2CuO4, suggesting that the magnetic order around x∼ 1 8 is essentially different from the three-dimensional antiferromagnetic order in La2CuO4. The estimated magnetic volume fraction reaches 100% even in the superconducting region.

μSR study of magnetic order in LTO and LTT phase of La2-xMxCu1-yZnyO4 (M = Sr, Ba)

In order to investigate the origin of so-called 1/8 anomaly in La(Sr,Ba)-214, we have performed μSR experiments for the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structure phases in La2−xSrxCu1−yZnyO4 (LSCO) and La2−xBaxCu1−yZnyO4 (LBCO), and decided the magnetic order temperature Tm where μ-spin coherent rotation starts. 1% Zn-substitution depresses magnetic order in the LTT phase of LBCO while it induces or enhances magnetic order in the LTO phase of LSCO. With doping Tm rises first and keeps constant around x=1/8 and decreases in both Zn-substituted LSCO and LBCO. This implies that doped holes are never uniformly distributed on the CuO2 plane, suggesting the segregation of spin and hole around x=1/8.

Pressure Effect on Superconductivity and Magnetic Order in La2−xBaxCuO4 with x = 0.135

We have performed muon spin relaxation (μSR), resistivity and magnetic susceptibility measurements under high pressure for La2−xBaxCuO4 with x = 0.135. An oscillation component is observed in the μSR spectra at low temperatures under ambient pressure and pressure of 1.1 GPa. This result suggests that the low‐temperature tetragonal (LTT) structure is dispensable for the appearance of the magnetic order, since several experimental results indicate that the LTT structure vanishes at 1.1 GPa. The superconducting temperature Tc increases drastically with increasing pressure, while the magnetic ordering temperature Tm hardly depends on pressure. The suppression of Tc therefore has no correlation with Tm. However, by applying pressure, the magnetic ordering region is replaced by the superconducting region. We speculate that the suppression of Tc correlates with the magnetic volume fraction.

Muon Spin Relaxation Study of an Impurity Doped Antiferromagnetic Triangular Lattice with a 1D Ferromagnetic Chain: Ca3(Co1−xZnx)2O6

Measurements of magnetization and muon spin relaxation (μSR) have been carried out for the sample of Ca3 (Co1−xZnx)2O6 with a frustrated antiferromagnetic triangular lattice consisting of Ising ferromagnetic chains for the purpose of the investigation of Zn‐substitution effect on the frustrated system. The magnetic ordering temperature Tc1 determined from magnetization data is 22 K for x = 0.03, which is 3 K lower than that of the un‐doped sample. This implies the reduction of an inter‐chain interaction due to Zn‐substitution, In the μSR measurement in longitudinal fields, static magnetic order could not be observed even at 18 K below Tc1. By Zn‐substitution up to 3% in Ca3Co2O6, the magnetic frustration is not removed but rather enhanced due to the reduction of the exchange interaction in nearest and next nearest neighbors.

Effects of Pressure and Eu-Substitution on Superconductivity and Magnetism in RuSr2GdCu2O8

We have performed magnetization measurements under pressure in RuSr2Gd1−xEuxCu2O8 with various x. The magnetic transition temperature Tm, increases with increasing pressure in the samples with x=0.0 (RuGd1212) and 1.0 (RuEu1212) and increases with decreasing Eu concentration x (chemical pressure) in RuSr2Gd1−xEuxCu2O8. The increase of Tm is attributed to the enhancement of the Ru-Ru interaction due to the lattice contraction. The saturation magnetization Ms decreases with increasing pressure in the sample with x=1.0. It indicates that the Ru magnetic moments are canted, leading to a ferromagnetic net moment. The superconducting temperature Tc decreases in RuGd1212 and hardly changes in RuEu1212 with increasing pressure. However Tc decreases with increasing x in RuSr2Gd1−xEuxCu2O8. The former can be interpreted by the doping effect caused by pressure but the latter can not be explained in terms of the carrier change. It is suggested that larger Eu ion directly disturb the electronic states of the CuO2 planes.