Spin-phonon coupling effects in transition-metal perovskites: A DFT + U and hybrid-functional study

Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO₃, SrMnO₃, BaMnO₃, LaCrO₃, LaFeO₃, and La₂(CrFe)O₆, are investigated using density-functional methods. The calculations are carried out both with a hybrid-functional Heyd-Scuseria-Ernzerhof (HSE) approach and with a DFT + U approach using a U that has been fitted to HSE calculations. The phonon frequency shifts obtained in going from the FM to the AFM spin configuration agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the AMnO₃ materials class with $A=$ Ca, Sr, and Ba, this frequency shift decreases as the A cation radius increases for the Γ phonons, while it increases for R-point phonons. In LaMO₃ with $M=$ Cr, Fe, and Cr/Fe, the phonon frequencies at Γ decrease as the spin order changes from AFM to FM for LaCrO₃ and LaFeO₃, but they increase for the double perovskite La₂(CrFe)O₆. We discuss these results and the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.