Magnetotransport and magnetism in granular EuS-Co and EuS-Ag nanocomposites prepared by mechanical alloying

Magnetic and magnetotransport properties have been studied in nanocomposites (EuS)-Co and (EuS)-Ag prepared by mechanical alloying. For (EuS)₇₀Co₃₀, the large negative magnetoresistance (~50%) found at 20 K, just above the Curie temperature (T_c) of EuS, arises from the suppression of spin fluctuation in EuS. Below T_c, the positive magnetoresistance (~15%), which is uncharacteristic of ferromagnetic semiconductors, is probably due to defect-induced magnetic fluctuation. On the other hand, (EuS)₇₀Ag₃₀, where nonmagnetic silver replaces cobalt, exhibits negative magnetoresistance below T_c, which is possibly related to the spin dependent scattering commonly seen in GMR materials. While (EuS)₇₀Co₃₀ shows semiconducting conductivity, (EuS)₇₀Ag₃₀ is metallic. Reentrant spin glass behavior has been observed in EuS-Ag system, where the rise of antiferromagnetic coupling is due to the defects and disorder caused by mechanical milling