EFFECT OF SN SUBSTITUTION FOR CU ON THE MAGNETIC AND SUPERCONDUCTING PROPERTIES OF LA2-XSRX(CU1-YSNY)O4+DELTA(0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.15)

We have investigated the effect of Sn doping on the magnetic and superconducting properties of La2-xSrx(Cu1-ySny)O4+delta with 0 less-than-or-equal-to x less-than-or-equal-to 0.15 and 0 less-than-or-equal-to y less-than-or-equal-to 0.01. Measurements of the Sn-119 Mossbauer isomer shift of samples doped with y = 0.005 (Sn-119) show that Sn is tetravalent (Sn4+) in the whole concentration range 0 less-than-or-equal-to x less-than-or-equal-to 0.15. On the other hand, the electrical resistivity, the Neel temperature (T(N)), and the superconducting temperature (T(c)) are found to be affected by increasing the Sn content. In the antiferromagnetic state (x = 0,0.008), we find that T(N) is slightly affected by increasing Sn content. In the superconducting state (x = 0.065, 0.10, and 0.15), T(c) is strongly suppressed with increasing Sn content. It is shown that the variation of T(N) upon Sn doping depends on the competition between two mechanisms: (1) magnetic disorder due to the nonmagnetic impurity which induces an uncompensated local magnetic moment on the CuO2 lattice and thereby tends to decrease T(N); (2) reduction (compensation) of the number of the free O- holes by the excess electrons introduced by Sn4+ which leads to stabilizing the antiferromagnetic order and increasing T(N). We further show that the suppression of T(c) with increasing Sn content can be explained by the same magnetic mechanism (1) responsible for decreasing T(N).