Hajime Shimizu

Structural and superconducting properties of R1−xCaxBa2Cu3O6+δ (R=Y, Er, Gd, Eu; 0<δ<1)

Abstract The influence of oxygen content δ on the superconducting critical temperature T c and the crystal structure of R 1− x Ca x Ba 2 Cu 3 O 6+ δ (R=Y, Er, Gd, Eu; x =0.0–0.2) has been studied by resistivity and X-ray powder diffraction measurements. All as-grown compounds with x =0.2 have δ ∼0.83 and are in the orthorhombic phase. Their optimum T c s are around 85 K. Both the compounds with R=Y and Eu show the plateau in T c vs. δ ( T c,pl ) at 60 K in the tetoragonal phase. For the compounds with R=Gd and Er the plateau appeared in the tetragonal phase at 20 and 40 K, respectively. The value of T c,pl for R=Gd increases from 20 to 60 K with decreasing Ca content x . It is shown that the values of T c,pl can be correlated with the c -axis lattice parameters. Furthermore, we provide the correspondence of the increase in T c above 60 K with orthorombicity over the present kinds of R.

Influence of Fe, Co, Ni and Zn substitution on critical temperature and structural change of GdBa2Cu3O7

Abstract Differences in T c -suppression and structural change between GdBa 2 Cu 3 O 7 and YBa 2 Cu 3 O 7 with substitution of Fe, Co, Ni, and Zn for Cu are investigated. Substitution effects of these elements on T c and lattice structure are different in Gd- and Y-compounds. With Fe- and Co-substitution, T c s of Gd-compounds are suppressed more than those of Y-compounds at the same concentration of dopant. Ni-substitution for 10% of Cu atom leads to T c ∽35 K in the Gd-compound. Furthermore, by substitution of Ni and Zn with x > 0.1, an orthorhombic to tetragonal phase transition is brought about in the Gd-compound, although oxygen content does not change in both compounds. T c -suppression and structural change in Gd-compounds are discussed.

Er3+-EPR study of Er-substituted YBa2Cu3Oy

Abstract Er 3+ -EPR measurements are carried out on the Er-doped YBa 2 Cu 3 O y . The Er 3+ -EPR linewidth consists of the sum of residual linewidth and linewidths corresponding to the Korringa and the Orbach processes. The electronic and magnetic properties of CuO 2 plane are discussed based on the analysis of these linewidths.

Antiferromagnetic coupling between Fe and Cu moments in superconductor YBa2(Cu1−xFex)3Oy

Abstract Superconductivity and magnetic properties are investigated in YBa 2 (Cu 1−x Fe x ) 3 O y with x =0−0.19 and y =6−7 through DC magnetic susceptibility and electrical resistivity. With increase of Fe contents and oxygen deficiency, T c decreases and, furthermore, their normal-state conductivity also decreases markedly, of which temperature dependence is approximated by In T , suggesting a weakly localized state in two dimensional systems. The susceptibility of all samples obeys the modified Curie-Weiss law. The effective Bohr magneton number P eff per Fe calculated from Curie constants decreases from 5.5 to 3.1 with an increase of Fe concentration up to x =0.19. Oxygen deficiency depresses these P eff to a minimum value of about 1.5 but more deficiency such as y =6 raises them up to about 3.0. These results indicate that Cu spins are weakly frozen around Fe atoms through weak localization due to Fe impurity and oxygen deficiency, and that their moments antiferromagnetically couple with the Fe moment locally without long range magnetic order.

Properties of the CuO2-plane in YBa2Cu3Oy (6<y<7) probed by Er-EPR

Abstract In order to study the change in magnetic and electronic properties of the CuO 2 -plane in YBa 2 Cu 3 O y due to hole-doping, Er 3+ -EPR measurements have been performed on Er-doped YBa 2 Cu 3 O y (6 y 3+ -spin embedded in Y-site consists of residual linewidth and linewidths responsible for the Korringa and the Orbach relaxation processes. Analysis of the linewidth due to the Korringa process indicates that the number of conduction carriers in the CuO 2 -plane saturates in y >6.4 and does not show any anomaly around y =6.7, where it is well known as the boundary for the 60 K plateau in T c . From the analysis of linewidth due to the Orbach process, we have obtained that the strength of orbit–lattice coupling of Er 3+ -spin changes noticeably around y =6.7. Magnetic and electronic properties of the CuO 2 -plane and the 60 K plateau have been investigated by using these analytical results.

Superconductivity and Magnetic Properties of YBa2(Cu1-xFex)3O7-y

Superconductivity and magnetic properties of YBa 2 (Cu 1- x Fe x ) 3 O 7- y with x =0-0.1 are investigated by the measurements of static magnetic susceptibility, electrical resistance, weight change and lattice parameters. Substitution of Fe atoms for Cu atoms causes orthorhombic to tetragonal phase transition at about x =0.035 and depresses critical temperature T c from 95 K to 32 K at x =0.1. The effective magnetic moment per Fe, about 5.5 µ B below x =0.01 decreases to 3.6 µ B at x =0.1. The removal of oxygen atoms in the specimens of superconducting tetragonal phase with x =0.04 by quenching brings about remarkable expansion of c -axis and decreases T c from 74 K to below 4.2 K. The effective moment per Fe was found to decrease from 4.4 µ B to 3.3 µ B with increase of oxygen deficiency due to quenching. Causes of reduced iron effective moments due to increase of iron concentration and oxygen deficiency are discussed in consideration of superexchange interaction between Fe and Cu spins.

Magnetic and electronic properties of La2−xSrxCuO4 as studied by EPR of ER spin probes

Abstract In order to study the change in magnetic and electronic properties of the CuO 2 plane due to hole doping and the role of Er ion in the T c -suppression due to the rare earth substitution, Er 3+ -EPR measurements have been performed on Er-doped La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7 compounds. It has been obtained that the EPR linewidth of Er 3+ spin in these compounds consists of residual linewidth and linewidths responsible for the Korringa and the Orbach relaxation processes. The change in the magnetic property of La 2− x Sr x CuO 4 due to hole doping has been studied by analysis of the residual linewidth. From analysis of the linewidths caused by the Korringa and the Orbach processes, the effect of Er substitution on the T c suppression in La 2− x Sr x CuO 4 is discussed in contrast to that in YBa 2 Cu 3 O 7 compound.

Gd3+-EPR study of GdBa2(Cu1−xMx)3O7(M=Fe, Co, Ni, Zn)

Abstract Gd 3+ -EPR measurements were carried out for samples of GdBa 2 (Cu 1− x M x ) 3 O 7 (M=Fe, Co, Ni, Zn). The Gd 3+ -linewidths of Fe- and Co-doped samples increase remarkably with decreasing temperature. However, this low-temperature rise is suppressed in Ni- and Zn-doped samples. This suggest that Gd-Gd correlation is disturbed by Ni spin in the Ni-doped sample and Zn-induced spin in the Zn-doped sample, respectively. On the other hand, the linewidth of the Ni-doped sample increases when temperature and Ni concentration increase. The increasing part of linewidth can be explained by the increase of relaxation rate of the Ni moment to the lattice via Ni-spin-lattice coupling. The increase of linewidth is, however, not observed in the Zn-doped sample. This is interpreted as the weak spin-lattice coupling of induced Cu moment.

Multiple Magnetic Transitions in RuSr2R1.4Ce0.6Cu2O10 (R=Gd, Eu) from Magnetization and ESR Spectroscopy

We studied magnetization and ESR on RuSr 2 R 1.4 Ce 0.6 Cu 2 O 10 (Ru1222; R = Gd, Eu). A ferromagnetic hysteresis in M – H curves is observed below T M = 180 K. The saturation moment is smaller than that for the expected Ru 5+ . These results suggest that Ru1222 is weak ferromagnetic. Interestingly, an enhancement in M observed at 100 K is more substantial than that observed at T M . ESR measurements confirm that this anomalous behavior originates from a change in the Ru moment.

Magnetic Properties of Superconducting and Nonsuperconducting GdBa2(Cu1-xMx)3O7 (M=Fe, Co, Ni: x=0-0.19)

Magnetization measurements were carried out for the samples of superconducting and nonsuperconducting GdBa 2 (Cu 1- x M x ) 3 O 7 (M=Fe, Co, Ni: x =0-0.19). From the magnetization curve at 1.3 K below Gd-ordering temperature T N , a Gd-spin flipping field, a molecular field and an anisotropy field were determined to be 1.5 T, 3.0 T and 0.4 T, respectively, and decrease of their saturation magnetizations due to increase of dopants was also observed. Furthermore, it was found that T N of metal doped samples decrease with increasing the concentrations of dopants. These results indicate that magnetic moments of doped metal atoms couple antiferromagnetically with Gd spins and this antiferromagnetic interaction reduces T N of Gd spins.