Short range spin-spin correlation, spin-phonon coupling and isostructural phase transition in hetero-tri-spin 3d-5d-4f double perovskite Sm2CoIrO6

The low temperature vibrational properties of Ir-based double perovskite Sm2CoIrO6 (SMCO) has been investigated by Raman scattering to gain an insight into the mechanism of the isostructural phase transition present at -104 K under ambient atmospheric pressure. The temperature dependent Raman study also reveals the development of short range magnetic correlation above the long range magnetic ordering. From the comprehensive temperature dependent study, it is found that the system before entering the state of long range ferrimagnetic ordering, undergoes from a state of conventional paramagantism to a state of correlated paramagnetism. The interaction of the phonons with the underlying magnetic degrees of freedom has been studied for the most prominent mode in terms of Fano asymmetric parameter. The strong spin-phonon coupling below the global ordering temperature results in a significant softening of the phonon modes involving stretching and/or vibration of the (Co/Ir)O-6 octahedra.