Spin-phonon coupling in R2CoMnO6 (R = Pr, Nd, Sm) thin films under biaxial compressive strain

Abstract

The R2CoMnO6 (R = Pr, Nd, Sm) thin films on (001) LaAlO3 substrates were fabricated successfully by a simple polymer assisted deposition method, confirming by atomic force microscopy, X-ray diffraction, and Raman spectroscopy. The study on the magnetic properties indicates that all the films are B-site ordering and show a single ferromagnetic transition at Curie temperature (Tc) with decreasing temperature. Meanwhile, an anomalous phonon softening related to the spin-phonon coupling is observed at around Tc in all the temperature-dependent Raman spectra. With the decreases of R-site ionic radius (rR) and/or the in-plane biaxial compressive strain, both the turning point TR of the temperature-dependent Δω(T) and Curie temperature Tc decrease, revealing the existence of spin-phonon coupling and its intensity in the decreasing order of Pr2CoMnO6 > Nd2CoMnO6 > Sm2CoMnO6 accompanied with the Co-O-Mn bond angle decreasing and the Co(Mn)-O bond length elongating. It is suggested that the existence of the biaxial compressive strain may be helpful to form the B-site ordered phase and suppress the formation of the disordered phase by decreasing the oxygen vacancies. Both the chemical strain and the biaxial compressive strain are cooperating on the magnetic interaction and spin-phonon coupling in the system, which are of special interest for application.