Magnetization andCu63NMR studies on granular FeCu alloys

Abstract

Nanostructured FeCu granular alloys (Fe $\ensuremath{\sim}1%--20%$) have been prepared by borohydride reduction and characterized by energy dispersive spectroscopy, x-ray, and transmission electron microscopy studies. Study of zero-field-cooling--field-cooling magnetization yields blocking temperature distribution of magnetic fine particles system in all samples. At low Fe concentration, the magnetizations of the samples combine superparamagnetism and paramagnetism near room temperature and small hystereses at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. High Fe content alloys are almost entirely ferromagnetic even at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. However, in all these samples, the observation of the $^{63}\mathrm{Cu}$ nuclear magnetic resonance signal at all temperatures $4--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ confirms the existence of a paramagnetic component having $\ensuremath{\sim}0.02--0.04\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Fe in Cu. The temperature dependence behavior of Fe contribution to $^{63}\mathrm{Cu}$ NMR linewidth and the Knight shift indicate that the paramagnetic component might exhibit a Kondo temperature of $\ensuremath{\sim}24\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ that is significantly higher than that obtained in bulk dilute FeCu alloys.