Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

Spin-phonon coupling, the interaction of spins with a surrounding lattice, is a key parameter to understand the underlying physics of multiferroics and to engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal $\mathrm{BaMn}{\mathrm{O}}_{3}$ are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of $15R\text{\ensuremath{-}}\mathrm{BaMn}{\mathrm{O}}_{3}$ at 230, 280, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in $15R\text{\ensuremath{-}}\mathrm{BaMn}{\mathrm{O}}_{3}$ at 230 and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below \ensuremath{\sim} 280 K, possibly arising from a local lattice distortion due to the displacement of Mn ions, that exhibits an anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidence from temperature- and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific-heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in $15R\text{\ensuremath{-}}\mathrm{BaMn}{\mathrm{O}}_{3}$, thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.