Magnetic Rare Earth Ordering and Superconductivity in (Tl,Pb)-1212 System

• Main Authors: Chiou-Chu Lai, 賴秋助
• Other Authors: Prof. Bi-Shiow Chiou;Prof. Huan-Chiu Ku
• Format: Others
• Language: en_US
• Published: 1993
• Online Access: http://ndltd.ncl.edu.tw/handle/30409319435227629874

博士 === 國立交通大學 === 電子研究所 === 81 === Low temperature magnetic study for the insulating TlBa2RCu2O7-y (R=Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu and Y) system with the TlBa2CaCu2O7-y type (1212) structure are reported. For tlBa2EuCu2O7 compound, anomalous temperature dependent of magnetic susceptibility can be described by a nonmagnetic Eu3+ ground state (J = 0) with closely spaced magnetic excited state (J > 0). No rare earth except, Pr and Gd, magnetic transitions above 2 K are observed form magnetic rare earth compounds TlBa2 RCu2O7 (R = Nd, Sm, Dy = 0.94 K, Ho, Er, Tm, Yb). Magnetic susceptibility heat capacity, neutron and x-ray diffraction measurements indicate an anomalously high antiferromagnetic Neel temperature TN(Pr) = 8K with an onset around 10 K for TlBa2 PrCu2O7 as compared to TN(Gd) = 2.2 K for TlBa2GdCu2O7. Detail neutron diffraction data show complex magnetic behavior around TN(Pr). In comparing with the anomalous Pr ordering in PrBa2Cu3 O7 system, charge carrier, and different ion size of alkali earth ion substitution effect are considered. For that, TlSr2 RCu2O7 (R = Pr and Gd) and (Pb,Cu)Sr2RCu2O7 (R = Pr and Gd) compounds are synthesized. In this 1212 system , charge carrier has minor effect on TN, but Sr substitution for Ba significantly reduced the magnetic Pr ordering temperature. The Rietveld x-ray refinement results show that Pr-O bond length is enlarged as Ba is replaced by Sr , alothough the c-axis parameter become shoter. These results strongly support that the hybridization of Pr(4f) and O(2p) orbitals are the mechanism of Pr ordering and superconductivity supression in PrBa2Cu3O7-y. The strong dipolar interaction of Gd-Gd ions maintains the TN(Gd) near 2.2 K not affected by the structure change and any charge carrier effect. This near constant ordering temperature in the double-CuO2 layer Gd containing cuprates suggests that the interaction is highly anisotropic (quasi-2D). A broad heat capacity shoulder below TN(Gd) indicates strong spin fluctuation of the large Gd spins