Raman scattering investigation across the magnetic and metal-insulator transition in rare earth nickelateRNiO3(R=Sm, Nd) thin films

Abstract

We report a temperature-dependent Raman scattering investigation of thin-film rare earth nickelates ${\text{SmNiO}}_{3}$, ${\text{NdNiO}}_{3}$, and ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_{3}$ which present a metal-to-insulator (MI) transition at ${T}_{\text{MI}}$ and an antiferromagnetic-paramagnetic N\'eel transition at ${T}_{N}$. Our results provide evidence that all investigated samples present a structural phase transition at ${T}_{\text{MI}}$ but the Raman signature across ${T}_{\text{MI}}$ is significantly different for ${\text{NdNiO}}_{3}$ $({T}_{\text{MI}}={T}_{N})$ compared to ${\text{SmNiO}}_{3}$ and ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_{3}$ $({T}_{\text{MI}}\ensuremath{\ne}{T}_{N})$. It is namely observed that the paramagnetic-insulator phase $({T}_{N}lTl{T}_{\text{MI}})$ in ${\text{SmNiO}}_{3}$ and ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_{3}$ is characterized by a pronounced softening of one particular phonon band around $420\text{ }{\text{cm}}^{\ensuremath{-}1}$. This signature is unusual and points to an important and continuous change in the distortion of ${\text{NiO}}_{6}$ octahedra (thus the Ni-O bonding) which stabilizes upon cooling at the magnetic transition. The observed behavior might well be a general feature for all rare earth nickelates with ${T}_{\text{MI}}\ensuremath{\ne}{T}_{N}$ and illustrates intriguing coupling mechanism in the ${T}_{N}lTl{T}_{\text{MI}}$ regime.