Coupled phonons, magnetic excitations, and ferroelectricity in AlFeO3: Raman and first-principles studies

Abstract

We determine the nature of coupled phonons and magnetic excitations in AlFeO 3 using inelastic light scattering from 5 to 315 K covering a spectral range from 100 to 2200 cm − 1 and complementary first-principles density functional theory–based calculations. A strong spin–phonon coupling and magnetic ordering–induced phonon renormalization are evident in (1) anomalous temperature dependence of many modes with frequencies below 850 cm − 1 , particularly near the magnetic transition temperature T c ≈ 250 K, and (2) distinct changes in band positions of high-frequency Raman bands between 1100 and 1800 cm − 1 ; in particular, a broad mode near 1250 cm − 1 appears only below T c , attributed to the two-magnon Raman scattering. We also observe weak anomalies in the mode frequencies ∼100 K due to a magnetically driven ferroelectric phase transition. Understanding of these experimental observations has been possible on the basis of first-principles calculations of the phonons’ spectrum and their coupling with spins.