Magnetoelastic coupling and multiferroic ferroelastic/magnetic phase transitions in the perovskite KMnF₃

The magnetic, elastic, and anelastic behavior of single-crystal KMnF₃ have been investigated by superconducting quantum interference device (SQUID) magnetometry and resonant ultrasound spectroscopy (RUS) through the sequence of phase transitions: phase I, <i>Pm</i>̅3<i>m</i> →(<i>T</i>_c₃ = 185 K) → phase II, <i>I</i>4<i>/mcm</i> →(T_c₂ =T_N = 87 K) → phase III, antiferromagnetic, <i>Cmcm</i> → (T_c₃ = 82 K) → phase IV, canted ferromagnet, Pnma. It is concluded that observed changes in the elastic properties can be explained simply in terms of strain/order parameter coupling for the octahedral tilting transitions. There appears to be no evidence in the present data or in data from the literature for coupling between the magnetic order parameter and shear strains. Any coupling between the magnetic and structural transitions is therefore weak, probably occurring only biquadratically through a small common volume strain. The combined data show unambiguously that, for the crystal used, the Néel point and the structural transition at 87 K are coincident. In other crystals, with slightly different stoichiometries and defect contents, this need not be the case, however, and the overlap of transition temperatures in KMnF₃ is essentially accidental. Strong acoustic dissipation at ∼0.1–1 MHz in the stability field of phase II is attributed to the local mobility of transformation twin walls under externally applied stress. A Debye-like loss peak near 130 K is attributed to pinning of at least some twin walls by defects, but relatively high levels of acoustic dissipation below this freezing temperature imply that some of the twin walls remain mobile due to weak pinning or the absence of any pinning. Acoustic losses continue in the stability field of phase III <i>(Cmcm)</i> but diminish substantially in the stability field of phase IV <i>(Pnma)</i>, implying quite different twin mobilities in the different structure types. Overlap of the structural and magnetic instabilities in KMnF₃ opens up possibilities for manipulation of ferroelastic twinning by application of a magnetic field and for creation of materials in which the ferroelastic twin walls have quite different magnetic properties from the matrix in which they lie