Magnetostructural coupling behavior at the ferromagnetic transition in double-perovskite Sr2FeMoO6

The ordered double-perovskite Sr2FeMoO6 (SFMO) possesses remarkable room-temperature low-field colossal magnetoresistivity and transport properties which are related, at least in part, to combined structural and magnetic instabilities that are responsible for a cubic-tetragonal phase transition near 420 K. A formal strain analysis combined with measurements of elastic properties from resonant ultrasound spectroscopy reveal a system with weak biquadratic coupling between two order parameters belonging to Gamma(+)(4) and m Gamma(+)(4) of parent space group Fm (3) over barm. The observed softening of the shear modulus by similar to 50% is due to the classical effects of strain/order parameter coupling at an improper ferroelastic (Gamma(+)(4)) transition which is second order in character, while the ferromagnetic order parameter (M Gamma(+)(4)) couples only with volume strain. The influence of a third order parameter, for ordering of Fe and Mo on crystallographic B sites, is to change the strength of coupling between the Gamma(+)(4) order parameter and the tetragonal shear strain due to the influence of changes in local strain heterogeneity at a unit cell scale. High anelastic loss below the transition point reveals the presence of mobile ferroelastic twin walls which become pinned by oxygen vacancies in a temperature interval near 340 K. The twin walls must be both ferroelastic and ferromagnetic, but due to the weak coupling between the magnetic and structural order parameters it should be possible to pull them apart with a weak magnetic field. These insights into the role of strain coupling and relaxational effects in a system with only weak coupling between three order parameters allow rationalization and prediction of how static and dynamic properties of the material might be tuned in thin film form by choice of strain contrast with a substrate.