Magnetism in RECo2 compounds under high pressure

We review the results of experiments and density-functional-theory (DFT) ca lculations focused on the stability of Co magnetism in RECo2 (RE = rare ear th element) compounds exposed to high pressures. In the experiments, we inv estigated pressure effects on the electrical resistivity and volume anomali es that are closely related to the formation and ordering of Co itinerant 3 d electron moments (mu (Co)). These phenomena are qualitatively different f or compounds with light RE (represented in our studies by NdCo2) and with h eavy RE elements (HoCo2 and ErCo2). In the NdCo2-type compounds the mu (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 ferromagn etically ordered RE sublattice. At moderate pressures P, T-C decreases line arly with increasing P in all the three compounds at a comparable rate part ial derivative ln T-C/partial derivativeP. Magnetovolume effect measurement s and mu (Co) calculations for reduced lattice parameters also reveal a neg ative pressure effect on mu (Co). At higher pressures, the Tc vs. P depende ncies observed for the two types of compounds show different trends. For th e heavy RE-based compounds suppression of the Co (meta)magnetism in P > P-c approximate to (4 GPa) is indicated by the vanishing rho (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 crit ically and so that RE-Co-RE exchange channel becomes ineffective to induce the metamagnetic splitting of the Co 3d majority and minority subbands. Thi s scenario is corroborated by the results of ab initio calculations for ErC o2. An apparently different picture holds for NdCo2 in which the pc, is muc h less sensitive to pressures up to 8 GPa. (C) 2001 Elsevier Science B.V. A ll rights reserved.