Elastic and anelastic relaxations accompanying magnetic ordering and spin-flop transitions in hematite, Fe2O3

Hematite, Fe2O3, provides in principle a model system for multiferroic (ferromagnetic/ferroelastic) behavior at low levels of strain coupling. The elastic and anelastic behavior associated with magnetic phase transitions in a natural polycrystalline sample have therefore been studied by resonant ultrasound spectroscopy (RUS) in the temperature range from 11 to 1072 K. Small changes in softening and attenuation are interpreted in terms of weak but significant coupling of symmetry-breaking and non-symmetry-breaking strains with magnetic order parameters in the structural sequence R (3) over bar c1' -> C2/c -> R (3) over barc. The R (3) over bar c1' -> C2/c transition at T-N = 946 +/- 1 K is an example of a multiferroic transition which has both ferromagnetic (from canting of antiferromagnetically ordered spin moments) and ferroelastic (rhombohedral -> monoclinic) character. By analogy with the improper ferroelastic transition in Pb-3(PO4)(2), W and W' ferroelastic twin walls which are also 60 degrees and 120 degrees magnetic domain walls should develop. These have been tentatively identified from microstructures reported in the literature. The very low attenuation in the stability field of the C2/c structure in the polycrystalline sample used in the present study, in comparison with the strong acoustic dissipation reported for single crystal samples, implies, however, that the individual grains each consist of a single ferroelastic domain or that the twin walls are strongly pinned by grain boundaries. This absence of attenuation allows an intrinsic loss mechanism associated with the transition point to be seen and interpreted in terms of local coupling of shear strains with fluctuations which have relaxation times in the vicinity of similar to 10(-8) s. The first order C2/c -> R (3) over barc (Morin) transition occurs through a temperature interval of coexisting phases but the absence of an acoustic loss peak suggests that the relaxation time for interface motion is short in comparison with the time scale of the applied stress (at similar to 0.1-1 MHz). Below the Morin transition a pattern of attenuation which resembles that seen below ferroelastic transitions has been found, even though the ideal low temperature structure cannot contain ferroelastic twins. This loss behavior is tentatively ascribed to the presence of local ferromagnetically ordered defect regions which are coupled locally to shear strains.