Abstract
We report the anomalous Nernst effect (ANE) in large-grain ceramics, nanogranular ceramics, and nanogranular thin films of La0.7Sr0.3CoO3, 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-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, rho(xy) proportional to rho(n)(xx). We observe a characteristic exponent n similar to 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 La0.7Sr0.3CoO3 is below the grain size of nanogranular samples similar to 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.