Unvailing the spin-phonon coupling in nanocrystalline BiFeO3 by resonant two-phonon Raman active modes

We report on temperature dependence of two-phonon Raman spectra in BiFeO3 nanocrystals, above and below the Neel temperature T-N using a resonant laser excitation line (lambda = 532 nm). Two-phonon modes exhibited anomalous frequency hardening and deviation from the anharmonic decay below T-N. Such behavior strongly supported the existence of spin-two-phonon interaction, because these modes are known to be very sensitive to the antiferromagnetic ordering. Within the mean-field theory for the nearest-neighbor interaction, the linear relationship between spin-spin correlation function and observed two-phonon frequency shift below T-N was obtained. This approach enabled to quantify the spin-phonon interaction by spin-phonon coupling strength for both two-phonon modes and justified the application of mean-field approach. Magnetic measurements revealed the coexistence of antiferromagnetic and weak ferromagnetic phases below T-N, which were found non competitive, additionally supporting the mean-field approach from which we deduced that the two-phonon modes in BiFeO3 are correlated with antiferromagnetic ordering below T-N.