Magnetism in rare earth-silicon and rare earth-manganese intermetallic alloys

A comprehensive and systematic investigation of the magnetic, electrical and thermal properties of polycrystalline RSi{dollar}sb2{dollar} and RMn{dollar}sb2{dollar} (R = Y and La - Yb) intermetallics has been carried out at temperatures ranging from 1.8 K to room temperature using superconducting quantum interference device (SQUID) magnetometry, ac-susceptibility, resistivity, magnetoresistance and thermal expansion measurements. Our data indicate that while disilicides of Pr, Er and Tm order ferromagnetically, those with R = Nd, Eu - Ho show antiferromagnetic order with 39 K (EuSi{dollar}sb2{dollar} & GdSi{dollar}sb2{dollar}) being the highest transition temperature. All disilicides show Curie-Weiss behavior in the paramagnetic regime and the effective moments are found to be close to the free ion values. Additionally we have observed second transitions in some of the RSi{dollar}sb2{dollar} compounds, the nature of which is not known at present. While CeSi{dollar}sb2{dollar} and YbSi{dollar}sb2{dollar} have finite moments, there is no evidence of magnetic order down to 1.8 K. However, YbSi{dollar}sb2{dollar} is found to be a valence fluctuating system. In the ordered state all antiferromagnets have a T{dollar}sp3{dollar} dependence of resistivity and a positive magnetoresistance whereas all ferromagnets exhibit a T{dollar}sp2{dollar} dependence with large negative magnetoresistance. These observations are found to be consistent with existing theories.; The magnetism in case of RMn{dollar}sb2{dollar} compounds is manifested in the instability of the Mn moment. We have determined that R = Y and Pr - Sm compounds show antiferromagnetic order with transition temperatures that differ significantly from the de Gennes rule. Compounds with R = Gd - Tm order ferromagnetically and their transition temperatures agree fairly well with the de Gennes rule. Our thermal expansion data shows that the transitions in case of R = Y and Pr - Tb are first order, accompanied by large magnetovolume effect and those for R = Dy - Tm are second order. In YMn{dollar}sb2{dollar} the transition is believed to be associated with collapse of the Mn moment, which, through resistivity and magnetoresistance measurements is found to be localized below T{dollar}sb{lcub}rm N{rcub}{dollar} and itinerant above T{dollar}sb{lcub}rm N{rcub}{dollar}. We believe that magnetic order for light rare earth compounds is brought about by antiferromagnetic coupling between Mn moments while magnetism in heavy rare earth compounds is driven primarily by the rare earth species and the Mn moments are found to be very small or negligible. GdMn{dollar}sb2{dollar} and TbMn{dollar}sb2{dollar} are intermediate cases and it appears that the Mn and R sublattices order simultaneously in these systems. The resistivity data in case of Pr - Sm compounds does not show a simple temperature dependence and their magnetoresistance varies from positive to negative depending on the field and temperature. We do observe a T{dollar}sp2{dollar} dependence of resistivity and large negative magnetoresistance in case of Dy - Tm compounds and this is expected in a localized ferromagnet.