Anomalous Nernst effect in the ceramic and thin film samples of La0.7Sr0.3CoO3 perovskite

Abstract

We report the anomalous Nernst effect (ANE) in large-grain ceramics, nanogranular ceramics, and nanogranular thin films of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$, measured over the temperature range 5--300 K. The study is complemented with thermopower, resistivity, and magnetic measurements. The temperature-dependent ANE below the Curie temperature ${T}_{\mathrm{C}}$ (240--250 K) is analyzed with the help of longitudinal resistivity and Seebeck using a previously proposed formula derived by a combination of Onsager reciprocity, the Mott formula, and the relation between transverse and longitudinal resistivity, ${\ensuremath{\rho}}_{xy}\ensuremath{\propto}{\ensuremath{\rho}}_{xx}^{n}$. We observe a characteristic exponent n \ensuremath{\sim} 0.4 in agreement with the universal scaling for the bad-metal-type conduction regime. The nanogranular samples are characterized by higher resistivity, lower saturated magnetization, and a higher coercive field compared to large-grain ceramics. On the other hand, the magnitude of the ANE is independent of grain size. This observation likely insinuates that the characteristic length scale characterizing the ANE in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ is below the grain size of nanogranular samples \ensuremath{\sim}40 nm. Therefore, the ANE associated with the bad-metal regime is independent of barriers associated with grain surface, which are responsible for activated resistivity and lowered magnetization due to a magnetically ``dead'' layer. The observation that the advantage of the higher coercive field of nanogranular samples is not deteriorated by a lower ANE is important for possible applications in zero magnetic field.