Interplay between Disorder and Quantum and Thermal Fluctuations in Ferromagnetic Alloys -- New Systems

Previously$^{1}$ we addressed the effects of disorder on the ferromagnetic ordering temperature, T$_{C}$, in UCu$_{2}$Si$_{2-x}$Ge$_{x}$. In that work the measured non-monotonic variation of T$_{C}$ with disorder (as measured by the resistivity) could be explained within a model$^{2}$ of localized spins interacting with an electronic bath. This model predicts that, in some cases, T$_{C}$ can be \textit{enhanced} by the interplay between quantum and thermal fluctuations with disorder. We have extended this work in other ferromagnetic alloys, with both significantly larger as well as similar variations of T$_{C}$ with doping compared to the $<$10 {\%} variation of T$_{C}$ observed in UCu$_{2}$Si$_{2-x}$Ge$_{x}$. Resistivity, magnetic susceptibility, and specific heat will be presented, along with a comparison to the theory$^{2}$. $^{1}$M. B. Silva Neto, A. H. Castro Neto, D. Mixson, J. S. Kim, and G. R. Stewart, Phys. Rev. Lett. \underline {91}, 257206 (2003). $^{2}$M. B. Silva Neto and A. H. Castro Neto, Europhys. Lett. \underline {62}, 890 (2003).