| Summary: | Two series of B-site ordered, double-perovskite A<sub>2</sub>CoMnO<sub>6</sub> and A<sub>2</sub>NiMnO<sub>6</sub> (A = La, Pr, Nd, Sm, Gd) epitaxial films with thickness d ~ 100 nm were grown on SrTiO<sub>3</sub>(111) substrates via metalorganic aerosol deposition. Polarization and temperature-dependent Raman spectroscopy were carried out in order to determine the spin-phonon coupling constant, λ, and the impact of the A-site cation radius on the phonon properties. The reduction of the A-site cation radius from La<sup>3+</sup> down to Gd<sup>3+</sup> systematically shifts the Raman modes to lower wavenumbers, and decreases the magnetization-induced softening of the Ag breathing mode, described by the spin-phonon coupling constant, λ, which changes from λ = 1.42 cm<sup>−1</sup> (La<sub>2</sub>CoMnO<sub>6</sub>) and λ = 1.53 cm<sup>−1</sup> (La<sub>2</sub>NiMnO<sub>6</sub>) down to λ = 0.58 cm<sup>−1</sup> (Gd<sub>2</sub>CoMnO<sub>6</sub>) and λ = 0.44 cm<sup>−1</sup> (Gd<sub>2</sub>NiMnO<sub>6</sub>). A similar effect of the A-cation radius was established for the c-lattice parameter and Curie temperature, TC, in this series of double-perovskite films. Our observations directly demonstrate a strong impact of the lattice structure on the ferromagnetic superexchange interaction in double perovskites. Moreover, the A<sub>2</sub>CoMnO<sub>6</sub> and A<sub>2</sub>NiMnO<sub>6</sub> series exhibit very similar behavior of spin-phonon coupling due to the only moderate difference of Co<sup>2+</sup> and Ni<sup>2+</sup> cation size.
|
|---|