Sign reversal of magnetoresistance in a perovskite nickelate by electron doping

We present low temperature resistivity and magnetotransport measurements conducted on pristine and electron doped ${\mathrm{SmNiO}}_3$ (SNO). The low temperature transport in both pristine and electron-doped SNO shows a Mott variable range hopping with a substantial decrease in localization length of carriers by one order in the case of doped samples. Undoped SNO films show a negative magnetoresistance (MR) at all temperatures characterized by spin fluctuations with the evolution of a positive cusp at low temperatures. In striking contrast, upon electron doping of the films via hydrogenation, we observe a crossover to a linear nonsaturating positive $\mathrm{MR}\sim 0.2\%$ at 50 K. The results signify the role of localization phenomena in tuning the magnetotransport response in doped nickelates. Ionic doping is therefore a promising approach to tune magnetotransport in correlated perovskites.

Figure 1

(a) Plot of ln $\left(\rho \right)$ vs $T^{-1/4}$ for SNO and HSNO. Black lines are linear fits indicating Mott VRH. (b) Comparison of $T_M$ vs $\rho$ for SNO and HSNO films with other oxide systems: SNO, HSNO (this work), ${\mathrm{Sm}}_{0.5}{\mathrm{Nd}}_{0.5}{\mathrm{NiO}}_3$ (SNNO) [34], ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_6$ [35], ${\mathrm{CaCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ (CCTO) [36], ${\mathrm{Nd}}_2{\mathrm{O}}_3$ [37], ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ [38]. (c), (d) CAFM images for SNO and HSNO over a region of $5\times 5\mu {\mathrm{m}}^2$. CAFM of the doped sample appears dark due to large suppression of electronic conductivity.

Figure 2

(a) Raman scan taken at various positions along the red line across the SNO/HSNO interface (shown in the inset). The inset shows the optical image of the electrode (Pt), SNO and HSNO regions. (b) Raman spectrum of pristine SNO (red) being compared with experimental data (blue line) from literature (Girardot et al. [45]). The vertical dotted lines (black) correspond to those predicted by theory.

Figure 3

(a) Magnetoresistance (in percent) vs $H$ for SNO film at various temperatures. (b) Magnetoresistance (in percent) vs $H$ for HSNO film at various temperatures. (c) Comparison of magnetoresistance (in percent) vs $H$ for SNO and HSNO film at $T=50$ K. (d) Magnetoresistance (in percent) (left $y$ axis) vs $T$ at $H=9$ T for SNO film. Crossover field ${\mathrm{H}}_m$ (right $y$ axis) vs $T$ for SNO film.

Figure 4

(a) Magnetoconductance vs $H$ for SNO film at various temperatures. Solid black lines are fits according to Eq. (6). (b) Temperature dependence of fit parameter $H_e$ (black squares). Red squares indicate theoretical predictions for $H_e$. (c) Magnetoresistance vs $H$ for HSNO film at various temperatures. Solid black lines are a guide to the eye. (d) Illustration of mechanisms leading to magnetoresistance in SNO (top panel) and HSNO (bottom panel) films.