Magnetism in RECo2 compounds under high pressure

We review the results of experiments and density-functional-theory (DFT) calculations focused on the stability of Co magnetism in RECo2 (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 (μCo). These phenomena are qualitatively different for compounds with light RE (represented in our studies by NdCo2) and with heavy RE elements (HoCo2 and ErCo2). In the NdCo2-type compounds the μCo exists above TC, whereas for the latter ones it forms and orders only at TC due to the itinerant electron metamagnetism (IEM) assisted by ferromagnetically ordered RE sublattice. At moderate pressures P, TC decreases linearly with increasing P in all the three compounds at a comparable rate ∂ln TC/∂P. Magnetovolume effect measurements and μCo calculations for reduced lattice parameters also reveal a negative pressure effect on μCo. At higher pressures, the TC 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>Pc∼(4 GPa) is indicated by the vanishing ρ(T) drop at TC. A scenario is discussed assuming that for P>Pc the 3d band broadens and the projected Co-3d density of states in the vicinity of EF 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 ErCo2. An apparently different picture holds for NdCo2 in which the μCo is much less sensitive to pressures up to 8 GPa