Impact of antiferromagnetism on the optical properties of rare-earth nickelates

We study the temperature dependence of the optical conductivity of rare-earth nickelate films of varying composition and strain close to the antiferromagnetic ordering temperature, $T_N$. Two prominent peaks at 0.6 and 1.3 eV, which are characteristic of the insulating phase, display a small but significant increase in intensity when the material passes from para- to antiferromagnetic. This observation indicates the presence of a positive feedback between antiferromagnetic (AF) and bond disproportionation (BD) order. By analyzing the temperature dependence near $T_N$, and using a Landau-type free-energy expression for BD and AF order, we infer that BD order is a necessary condition for the AF phase to appear, and that the antiferromagnetism contributes to stabilization of the bond disproportionation. This model also explains why hysteresis is particularly strong when the transition into the insulating state occurs simultaneously with antiferromagnetic order.

Figure 1

Optical conductivity spectra for selected temperatures of ${\mathrm{SmNiO}}_3$ on a ${\mathrm{LaAlO}}_3$(001) substrate, ${\mathrm{NdNiO}}_3$ on ${\mathrm{NdGaO}}_3$(101), and ${\mathrm{NdNiO}}_3$ on ${\mathrm{NdGaO}}_3$(110) [9].

Figure 2

Temperature dependence of the optical conductivity, ${\sigma }_1(\omega ,T)$, at selected photon energies for (top left) SNO/LAO-001 and (top right) NNO/NGO-101. The small jump at 150 K in the NNO/NGO-101 data is an experimental artifact and is ignored in the discussion. Middle panels: color plots of $d{\sigma }_1(\omega ,T)/dT$. Bottom panels: The optical conductivity spectra at the Néel temperature, ${\sigma }_1(\omega ,T_N)$ (red curves), and the antiferromagnetism induced contribution to the optical conductivity, ${\sigma }_A\left(\omega \right)$ (blue circles).

Figure 3

(a) Calculated phase diagram in the temperature/tolerance factor plane of ${\mathrm{RNiO}}_3$. Hysteresis limit curves enlargement centered on SNO/LAO-001 and on NNO/NGO-110 are shown in (b) and (c), respectively. (d)–(f) Temperature dependence of the AF ($m^2$) and metal insulator (${\mathrm{\Phi }}^2$) order parameters for three selected values of the tolerance factor, so as to match the experimental metal-insulator transition temperatures of (g) SNO/LAO-001, (h) NNO/NGO-101, and (i) NNO/NGO-110.

Figure 4

Temperature dependence of the ellipsometric parameters of the three samples, showing strong hysteresis for NNO/NGO-110 (right panel), and the absence of hysteresis for NNO/NGO-101 (central panel) and SNO/LAO-001 (left panel). Red curves are during warm-up, blue curves during cool-down.

Figure 5

Temperature derivative of the BD parameter ${\mathrm{\Phi }}^2$ (top panels) and the temperature derivative of the optical conductivities of SNO/LAO-001 and NNO/NGO-101 (bottom panels) measured at photon energies corresponding to the maximum of peaks $A$ (navy) and $B$ (red).