Magnetoelastic coupling and microstructure dynamics associated with spin-orbit coupling in the ferrimagnetic/ferroelastic ordered double perovskite Ba₂FeReO₆

Strain coupling and relaxation dynamics associated with the ferrimagnetic/ferroelastic phase transition at T-c approximate to 310 K in double perovskite Ba2FeReO6 with a high degree of Fe/Re order have been investigated with resonant ultrasound spectroscopy through the temperature interval similar to 5-600 K and with applied magnetic field of up to +/- 2 T. Strain analysis using diffraction data from the literature is consistent with a Landau model of the transition as Fm (3) over bar m1' -> I4/mm'm', improperly ferroelastic, driven by a magnetic order parameter with symmetry Gamma(+)(4). Ferroelastic shear strain of up to similar to 0.0015 arises from spin/orbit coupling and is smaller than is typical of coupling with octahedral tilting. It provides the underlying cause of softening of the shear modulus observed over an interval of similar to 100 K below T-c, though with order/disorder rather than displacive character for the transition. Hysteretic effects suggest that precursor microstructures and mixed magnetic/ferroelastic domains below T-c depend on the thermal history of the sample and can evolve on a timescale of hours and days at room temperature. Elasticity data collected as a function of external magnetic field reveal that poled samples are slightly softer than those with multiple magnetic domains at 4 K. At 300 K there is a time-dependent viscous component of the response to the field that relates to the bulk modulus and, hence, to volume changes associated with magnetic ordering. A loss peak seen similar to 20-50 K below T-c in AC magnetic measurements made at frequencies of 0.2-1 kHz yielded an activation energy similar to 0.4 eV and has been attributed to freezing of magnetic/ferroelastic domain walls. No equivalent loss peak was seen in the acoustic data measured at similar to 100-500 kHz, however, implying that these domain walls are mobile in response to a dynamic magnetic field on a timescale of similar to 10(-2)-10(-3) s but immobile in response to a dynamic stress field applied on a timescale of similar to 10(-5)-10(-6) s. Debye-like acoustic loss peaks at temperatures below similar to 100 K yielded activation energies of similar to 0.02-0.1 eV and are discussed in terms of pinning/freezing of polarons. Ba2FeReO6 is a material with magnetoelastic and magnetoelectric heterogeneities that might be tuned by choice of thermal history and cation order.