Magnetism in rare earth Co2 compounds under high pressures

We have studied electrical resistivity anomalies connected with magnetism in RECo2 (RE = Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at T-C,T- whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although T-C decreases with pressure 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 data 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 Co moment collapses because the RE-Co-RE exchange channel becomes ineffective to induce the itinerant 3d electron metamagnetism. The localized RE moments, however, order at a "residual" T-C due to the persisting Ruderman-Kittel-Kasuya-Yosida-type exchange interaction. For NdCo2 and TbCo2 the T-c values decrease with pressure by an exponential law but remain rather high in the highest applied pressures. Results of first-principles electronic structure calculations using the full-potential linearized augmented plane wave method are presented for HoCo2 and NdCo2 compounds, as well. (C) 2001 American Institute of Physics.