Strain coupling and acoustic attenuation associated with glassy magnetic phase transitions in the disordered double perovskite La2FeMnO6

Abstract

Elastic and anelastic anomalies in a ceramic sample of ${\mathrm{La}}_{2}{\mathrm{FeMnO}}_{6}$ have been characterized by resonant ultrasound spectroscopy in order to understand the strength and form of magnetoelastic coupling that accompanies the glassy magnetic transitions of a double perovskite with no long-range order of the B-site cations. The first transition, to a cluster glass below $\ensuremath{\sim}280\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, does not appear to involve any significant coupling with strain. The second glassy transition, near 55 K, appears to conform to Vogel-Fulcher dynamics in which magnetic dissipation and acoustic loss peaks arise from freezing driven by interactions between ferromagnetic clusters, with an activation energy of $\ensuremath{\sim}0.03\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and time constant ${\ensuremath{\tau}}_{\mathrm{o}}\ensuremath{\sim}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. The magnetoelastic coupling mechanism appears to involve local spin states with strain relaxation enhanced by changes in local electronic structure. Mediation of the coupling via strain also ensures that local heterogeneity in the strain state, such as at ferroelastic twin walls, will contribute to the magnetic heterogeneity of these materials.