Raman scattering investigation across the magnetic and MI transition in rare earth nickelate RNiO3 (R = Sm, Nd) thin films

TL;DR

This study uses temperature-dependent Raman scattering to explore structural and magnetic phase transitions in rare earth nickelate thin films, revealing distinct phonon behaviors linked to their metal-insulator and magnetic transitions.

Contribution

It provides new insights into the coupling between structural distortions and magnetic transitions in RNiO3 thin films, highlighting differences based on the relation between TMI and TN.

Findings

Structural phase transition at TMI observed in all samples.

Unusual phonon softening around 420 cm-1 in certain phases.

Coupling mechanism between structural and magnetic transitions elucidated.

Abstract

We report a temperature-dependent Raman scattering investigation of thin film rare earth nickelates SmNiO3, NdNiO3 and Sm0.60Nd0.40NiO3, which present a metal-to-insulator (MI) transition at TMI and an antiferromagnetic-paramagnetic Neel transition at TN. Our results provide evidence that all investigated samples present a structural phase transition at TMI but the Raman signature across TMI is significantly different for NdNiO3 (TMI = TN) compared to SmNiO3 and Sm0.60Nd0.40NiO3 (TMI =/ TN). It is namely observed that the paramagnetic-insulator phase (TN < T < TMI) in SmNiO3 and Sm0.60Nd0.40NiO3 is characterized by a pronounced softening of one particular phonon band around 420 cm-1. This signature is unusual and points to an important and continuous change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which stabilizes upon cooling at the magnetic transition. The observed behaviour might well be a general feature for all rare earth nickelates with TMI =/ TN and illustrates intriguing coupling mechanism in the TMI > T > TN regime.