Raman scattering investigation across the magnetic and metal-insulator transition in rare earth nickelate $R{\text{NiO}}_3$ ($R=\text{Sm}$, Nd) thin films

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 Néel 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 ${\text{NdNiO}}_3$ $\left(T_{\text{MI}}=T_N\right)$ compared to ${\text{SmNiO}}_3$ and ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_3$ $\left(T_{\text{MI}}\ne T_N\right)$. It is namely observed that the paramagnetic-insulator phase $\left(T_N<T<T_{\text{MI}}\right)$ 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 $420{\text{cm}}^{-1}$. 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 $T_{\text{MI}}\ne T_N$ and illustrates intriguing coupling mechanism in the $T_N<T<T_{\text{MI}}$ regime.

Figure 1

(Color online) Resistivity vs temperature for different rare earth nickelate thin films [${\text{SmNiO}}_3$ (SNO, ●/○), ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_3$ (SNNO, $▶/▷$), and ${\text{NdNiO}}_3$ (NNO, $★/☆$)]. The full and open signals correspond to heating and cooling, respectively. ${\rho }_{\text{MI}}$ is the resistivity at $T_{\text{MI}}$.

Figure 2

(Color online) Comparison of room-temperature Raman spectra for ${\text{SmNiO}}_3$, ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_3$, and ${\text{NdNiO}}_3$. The arrows indicate Raman bands from the ${\text{LaAlO}}_3$ substrate.

Figure 3

(Color online) Selected temperature-dependent Raman spectra for ${\text{SmNiO}}_3$ (top), ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_3$ (middle), and ${\text{NdNiO}}_3$ (bottom). The displayed spectra have been obtained by heating from $-180°\text{C}$ with a step of $5°\text{C}$; for simplicity only every second spectrum is shown. The red spectra correspond to MI-transition temperatures (deduced from Fig. 1), while the blue spectra correspond to magnetic Néel-transition temperatures (bulk literature data). Note that $T_{\text{MI}}$ measured by the electrical four-probe technique (in red) is by roughly $20°\text{C}$ higher than the observed anomalies in the Raman spectra (see text).

Figure 4

(Color online) Temperature-dependent evolution of the band position in the Raman spectra of (a) ${\text{SmNiO}}_3$ and (b) ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_3$. Lines and colors are guides to the eye to emphasize spectral changes which in turn are evidences for a structural phase transition at $T_{\text{MI}}$ (see text).

Figure 5

(Color online) Temperature-dependent evolution of the band showing a significant softening for (a) ${\text{SmNiO}}_3$ and (b) ${\text{Sm}}_{0.60}{\text{Nd}}_{0.40}{\text{NiO}}_3$. Lines and colors are guides to the eye to underline spectral changes at the magnetic $\left(T_N\right)$ and electronic $\left(T_{\text{MI}}\right)$ phase transitions, see text for more details. $T_N$ corresponds to literature results on bulk samples, while $T_{\text{MI}}$ corresponds to the measured metal-insulator phase transition on physically the same films which have been investigated by Raman scattering.