Rare earth size dependence of Nernst thermopower in ferromagnetic perovskites: R_0.6Sr_0.4CoO₃ (R = La, Pr, and Nd)

When a semiconductor or metal is subjected to a mutually perpendicular temperature gradient and an external magnetic field, an electric voltage is induced in the direction perpendicular to both of them and this phenomenon is known as the Nernst effect. Here, we discuss the Nernst thermopower (S_xy) in the ferromagnetic cobaltite perovskites R_0.6Sr_0.4CoO₃ with rare earth cations (R = La, Pr and Nd) of different ionic radii. In addition to S_xy, longitudinal thermopower (S_xx) and resistivity in zero magnetic fields were also measured. In each of these samples, the Nernst thermopower (S_xy) increases rapidly around the ferromagnetic Curie temperature (T_C), and goes through a maximum value at a certain temperature before decreasing at low temperatures. The field dependence of S_xy hysteresis resembling magnetization suggests anomalous Nernst contribution. The maximum value of S_xy = 0.27 μV/K for H = 2 kOe is found in the sample possessing the largest ionic radius (R = La) in this series. The field-dependent Nernst and Hall effect measurements were extended up to 5 T and down to 25 K for Nd_0.6Sr_0.4CoO₃ which shows ferrimagnetic interaction between Nd and Co sublattices. Our work guides future studies to be focused on the La-based cobaltite to realize higher S_xy value through optimizing composition and synthesis methods.