Spin-manipulated phonon dynamics during magnetic phase transitions in triangular lattice antiferromagnet CuCr_1-xMg_xO₂ semiconductor films

Raman scattering and infrared reflectance spectra of CuCr_1-xMg_xO₂ films (x = 0.03, 0.06 and 0.09) in the temperature range of 5-300 K have been studied, combined with first-principles calculations. The abnormal redshift of the E_g Raman mode center with decreasing temperature below 100 K for the lightly doped film (x = 0.03) is ascribed to the Cr 3d-O 2p-Cu 3d interaction. Strong disturbance of the local spin fluctuation at Cr sites in heavily Mg-doped films (x = 0.06 and 0.09) drives the E_g mode center to a normal blueshift with decreasing temperature. With further decreasing temperature, the out-of-plane structure increases the internal spin-charge coupling. It enhances the spin-flip splitting and brings back the abnormal redshift of the E_g Raman mode center. A similar but more obvious trend can be found from temperature-dependent E_u infrared mode center shifts. Two successive magnetic transitions were observed at the corresponding Néel temperatures T_N1 around 24.7 K and T_N2 around 23.0 K, as manifested by magnetoresistance measurements. The interesting phenomena of phonon dynamics are suggested to be manipulated by the spin structures during the magnetic transitions.