Origin of the negative volume magnetostriction of the intermetallic compound GdAl2

Spontaneous volume magnetostriction of the intermetallic compound GdAl2 with a cubic C15 Laves structure is studied from first principles using the local spin-density approximation combined optionally with an open-core treatment of the atomic-like Gd-4f orbitals. Local magnetic moments and equilibrium lattice parameters are evaluated for two spin structures, namely for the ferromagnetic ground state and for a state with randomly oriented Gd moments. The resulting spontaneous volume magnetostriction compares well with experimentally observed values; its negative sign however cannot be ascribed to a variation of the size of the local Gd moment but has its origin in the volume dependence of exchange interactions between the Gd moments. The Curie temperature and its pressure dependence are derived from the calculated pair exchange interactions as well; their values are found in reasonable agreement with experiment supporting thus the above explanation of the anomalous magnetoelastic behavior of the GdAl2 compound.