Anomalous anharmonic phonon dynamics and exchange bias enhancement in La1-xNdxCrO3: role of rare-earth substitution

This study explores the influence of Nd doping on the magnetic and vibrational properties of LaCrO3, with a focus on exchange bias (EB) behavior and the temperature-dependent evolution of Raman spectra. The doping introduces mixed valences, with the coexistence of 〖 Nd〗^(2+)and Nd^(3+) as well as〖 Cr〗^(3+) and Cr^(4+). The exchange bias (EB) phenomenon is observed in the doped samples and shows a significant increase with higher doping levels. The primary origin of this behavior is attributed to the Nd3+-O2--Cr3+ super-exchange interaction. Additionally, the double-exchange interactions, arising from the presence of mixed-valence states, further enhance this EB effect. Raman spectroscopy reveals mode mixing in the low wavenumber region at intermediate Nd doping levels, which prevents a crossover of modes with the same symmetry. At low temperatures, the spin-phonon coupling is observed along with an anomalous anharmonic behavior, attributed to three-phonon and four-phonon processes contributing with opposite signs. A comparative study on cation substitution at different lattice sites, including the NdCr0.5Ga0.5O3 sample, reveals that the observed anomaly likely originates from perturbations introduced at the rare-earth (A-site) position. The results suggest that the magnetic nature of rare-earth ions plays a dominant role in governing spin-phonon coupling, whereas transition-metal (B-site) ions have a comparatively minor influence.