Reduction of Ferromagnetic Exchange with Lattice Dilation in EuO

The reduction of the ferromagnetic exchange with lattice dilation has been studied in EuO films. Films deposited at high substrate temperatures (400°–500°C) show bulk-like properties and films deposited at lower substrate temperatures (below 350°C) show as much as a 1% room-temperature lattice dilation with a corresponding 14% blue shift (2.03–2.31 eV) in the room-temperature optical absorption band and 28% reduction (69°–50°K) in the Curie temperature. This lattice dilation causes a reduction in the cubic crystal field splitting of the 5d levels, and consequently a shift to higher energy of the 4f7→4f65t2g transition. The Eu2+–Eu2+ interatomic ferromagnetic exchange is reduced via the Goodenough model which involves overlap of a half-filled 4f level on one cation with an empty 5d level on a neighboring cation.