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

C. Girardot; J. Kreisel; S. Pignard; N. Caillault; F. Weiss

doi:10.1103/PhysRevB.78.104101

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

C. Girardot, J. Kreisel, S. Pignard, N. Caillault, F. Weiss

DOI 10.1103/PhysRevB.78.104101 · Physical Review B

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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 Néel 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 behavior might well be a general feature for all rare earth nickelates with TMI≠TN and illustrates intriguing coupling mechanism in the TN<T<TMI regime.