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

2021 
Abstract We report on temperature dependence of two-phonon Raman spectra in BiFeO 3 nanocrystals, above and below the Neel temperature T N using a resonant laser excitation line ( λ = 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 BiFeO 3 are correlated with antiferromagnetic ordering below T N .
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