Magnetism in rare earth Co-2 compounds under high pressures

We have studied electrical resistivity anomalies connected with magnetism i n RECo2 (RE = Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambien t pressure the former two compounds exhibit a second order magnetic phase t ransition (SOMPT) at T-C,T- whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although T-C decreases with pressur e in all four compounds, the T-C(P) dependence for NdCo2 and TbCo2 differs considerably from this for HoCo2 and ErCo2. For the latter two, T-c vs P da ta deviate dramatically from the initial linear dependence above a critical pressure P-c to become almost pressure independent at higher pressures. We propose that this is reflecting the loss of Co metamagnetism that is also indicated by the vanishing resistivity drop at T-c for P > 4 GPa and by the change from a FOMPT to a SOMPT. A scenario is discussed assuming that for P > P-c the projected Co 3d density of states at E-F decreases. Hence the C o moment collapses because the RE-Co-RE exchange channel becomes ineffectiv e to induce the itinerant 3d electron metamagnetism. The localized RE momen ts, however, order at a "residual" T-C due to the persisting Ruderman-Kitte l-Kasuya-Yosida-type exchange interaction. For NdCo2 and TbCo2 the T-c valu es decrease with pressure by an exponential law but remain rather high in t he highest applied pressures. Results of first-principles electronic struct ure calculations using the full-potential linearized augmented plane wave m ethod are presented for HoCo2 and NdCo2 compounds, as well. (C) 2001 Americ an Institute of Physics.