Magnetoelastic coupling in bulk and nanoscale MnO

Phonons are exquisitely sensitive to finite length scale effects because they are intimately connected to charge, structure, and magnetism, and a quantitative analysis of their behavior can reveal microscopic aspects of spin-lattice interaction. To investigate these effects in a model correlated oxide, we measured the infrared vibrational properties of nanoscale MnO and compared the results to those of a single crystal. A charge and bonding analysis reveals that Born effective charge, local effective charge, total polarizability, and the force constant are overall lower in the nanoparticles compared to the bulk. Several of these quantities split through the Néel transition due to magnetoelastic interactions. We find that the spin-lattice coupling drops from ~7 cm^-1 in the single crystal to <1 cm ^-1 in the nanoparticles. These results are important for understanding finite length scale effects in simple binary oxides and the more complicated functional oxides that emanate from this parent compound.