First-principles study of the electronic structure and exchange interactions in bcc europium

Magnetic properties of the europium metal in a bcc structure are studied from first principles using a two-step approach. First, the electronic structure of a ferromagnetic state is calculated in the local spin-density approximation (LSDA) to the density-functional theory whereby the highly localized $4f$ orbitals are treated as part of the atomic core. This description leads to an equilibrium lattice constant that compares well with experiment, in contrast to the standard LSDA which yields a significantly smaller atomic volume. In the second step, parameters of an effective Heisenberg Hamiltonian are derived from the self-consistent electronic structure and they are used to determine the magnetic ground state and to estimate the magnetic transition temperature. The calculated pairwise exchange interactions tend to couple the local magnetic moments of the nearest neighbors ferromagnetically. However, the interaction parameters exhibit a slow oscillatory decay as a function of the interatomic distance which makes them fully compatible with an observed spin-spiral ground state. The resulting wave vector of the spiral as well as the Néel temperature are in fair agreement with measured values.