Ferromagnetism induced by lattice volume expansion and amorphization in EuTiO₃ thin films

Lattice volume expansion or amorphization renders EuTiO₃ ferromagnetic, although the stable phase of crystalline EuTiO₃ is an antiferromagnet. The lattice volume expansion is induced into the crystalline EuTiO₃ thin film by utilizing the lattice mismatch between the thin film and a substrate. The magnetization at low temperatures monotonically increases with an increase in lattice volume for the crystalline EuTiO ₃ thin film, coincident with the results of calculations based on the hybrid Hartree-Fock density functional approach. The ferromagnetic interaction between Eu²⁺ ions is enhanced by the amorphization as well; the amorphous EuTiO₃ thin film becomes a ferromagnet, and the Curie temperature is higher for amorphous Eu₂TiO₄ than for its crystalline counterpart. The phenomenon, that is, the volume expansion- and amophization-induced ferromagnetism, is explained in terms of the competition between ferromagnetic and antiferromagnetic interactions among Eu²⁺ ions.