Structural and Magnetic Properties of (R_<1-x>Y_x)_<1.85>Ce_<0.15>CuO_4(R = Pr, Nd, Sm and Eu)
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(R_<1-x>Y_x)_<1.85>Ce_<0.15>CuO_4(R = Pr, Nd, Sm and Eu) have been investigated by means of X-ray-diffraction and magnetic-susceptibility measurements. Basically, the lattice parameters a, c, and V decrease linearly with decreasing ionic radius of R in R_<1.85>Ce_<0.15>CuO_4(R = Pr, Nd, Sm and Eu), and a pronounced change of the slope at R = Eu is observed. This suggests that a structural distortion occurs in the R_<1.85>Ce_<0.15>CuO_4 series, and the boundary may lie between Eu_<1.85>Ce_<0.15>CuO_4 and Gd_<1.85>Ce_<0.15>CuO_4. Lattice parameters a, c, and the unit-cell volume V for (R_<1-x>Y_x)_<1.85>Ce_<0.15>CuO_4(R = Pr, Nd, Sm and Eu) decrease with increasing Y concentration x. It is found that the critical concentration x_c, at which the superconductivity disappears is 0.25, 0.2, 0.15, and 0.0125 for R = Pr, Nd, Sm and Eu in (R_<1-x>Y_x)_<1.85>Ce_<0.15>CuO_4, respectively. In (Eu_<1-x>Y_x)_<1.85>Ce_<0.15>CuO_4, the magnetic anomaly is observed at x = 0.025, and the magnitude of the anomaly increases with increasing Y concentration x. Thus the substitution of Y with small ionic radius for Eu plays an important role on the suppression of superconductivity in (Eu_<1-x>Y_x)_<1.85>Ce_<0.15>CuO_4. This provides clear evidence that the absence of superconductivity in Gd_<1.85>Ce_<0.15>CuO_4 is due to weak ferromagnetism in the Cu-O planes resulting from the too small ionic radius of Gd, which induces a lattice distortion in the T' structure.
- 社団法人応用物理学会の論文
- 2000-06-15
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