  Spin-phonon coupling, the interaction of spins with surrounding lattice is a
key parameter to understand the underlying physics of multiferroics and
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
BaMnO3 are correlated through the spin-lattice coupling. Here, we report the
observation of an extensive renormalization of the Raman spectrum of 15R-BaMnO3
at 230 K, 280 K, and 330 K. Magnetic measurements reveal the presence of a
long-range and a short-range magnetic ordering in 15R-BaMnO3 at 230 K 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 ~ 280 K, possibly arising from a local lattice-distortion due to
the displacement of Mn-ions, that exhibits anomalous shift with temperature.
The origin of the observed renormalization and phonon anomalies in Raman
spectra are discussed based on the evidences 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-BaMnO3 thus suggesting that the optical phonons are strongly correlated to
its magnetoelectric properties.
