Magnetism in RECo2 compounds under high pressure

Abstract We review the results of experiments and density-functional-theory (DFT) calculations focused on the stability of Co magnetism in RECo 2 (RE=rare earth element) compounds exposed to high pressures. In the experiments, we investigated pressure effects on the electrical resistivity and volume anomalies that are closely related to the formation and ordering of Co itinerant 3d electron moments ( μ Co ). These phenomena are qualitatively different for compounds with light RE (represented in our studies by NdCo 2 ) and with heavy RE elements (HoCo 2 and ErCo 2 ). In the NdCo 2 -type compounds the μ Co exists above T C , whereas for the latter ones it forms and orders only at T C due to the itinerant electron metamagnetism (IEM) assisted by ferromagnetically ordered RE sublattice. At moderate pressures P , T C decreases linearly with increasing P in all the three compounds at a comparable rate ∂ln T C /∂ P . Magnetovolume effect measurements and μ Co calculations for reduced lattice parameters also reveal a negative pressure effect on μ Co . At higher pressures, the T C vs. P dependencies observed for the two types of compounds show different trends. For the heavy RE-based compounds suppression of the Co (meta)magnetism in P > P c ≈(4 GPa) is indicated by the vanishing ρ ( T ) drop at T C . A scenario is discussed assuming that for P > P c the 3d band broadens and the projected Co-3d density of states in the vicinity of E F decreases critically and so that RE–Co–RE exchange channel becomes ineffective to induce the metamagnetic splitting of the Co 3d majority and minority subbands. This scenario is corroborated by the results of ab initio calculations for ErCo 2 . An apparently different picture holds for NdCo 2 in which the μ Co is much less sensitive to pressures up to 8 GPa.