Fluctuating magnetic order in YBa$_2$Cu$_3$O$_7$ substituted with both Fe and Co: a $^{57}$Fe Mössbauer spectroscopy study | Ordre magnétique fluctuant dans YBa$_2$Cu$_3$O$_7$ où Cu est substitué partiellement par Fe et Co étudié par spectroscopie Mössbauer sur $^{57}$Fe

URL: http://www-spht.cea.fr/articles/s99/029We report a $^{57}$Fe Mössbauer study of superconducting samples of YBa$_2$Cu$_3$O$_7$ substituted with 1\% Fe and also, in addition, up to 4\% Co. Increasing the Co level has no influence on the $^{57}$Fe quadrupole interactions but strongly increases the saturated $^{57}$Fe hyperfine fields. For the sample with 4\% Co, where the increased hyperfine field leads to a reduced interference between the magnetic subspectra, we carry out a quantitative analysis of the thermal dependence of the fluctuation rate of the $^{57}$Fe hyperfine field. We compare the properties of the short range correlated magnetic order introduced by the Co/Fe as sensed by the perturbing $^{57}$Fe probe with those reported previously for the non-perturbing Mössbauer probe $^{170}$Yb in the same system