The room temperature inflection of magnetism and anomalous thermoelectric power in lacunar compounds of La0.85-xBixK0.15MnO3

Abstract

This study focuses on the effects of the substitution of a nonmagnetic ion on the structure, magnetism, and thermoelectric power of binary La_0.85-xBi_xK_0.15MnO₃ (x = 0, 0.15, 0.3) manganites synthesized via a ceramic route. Structural analysis reveals a dual crystallographic phase in the system, which results due to charge disproportionation. The consequent segregation of Mn₃O₄ grains is revealed via field-emission scanning electron microscopy images. Magnetically, the specimens have a second-order phase transition nature, and a Curie temperature (T_C) is interestingly observed at room temperature (∼300 K) in the x = 0 compound. The variation of T_C is discussed in light of a combined contribution from a lacuna and the 6s² lone pair of bismuth. The presence of a Griffiths singularity in all specimens is realized due to ferromagnetic clusters appearing in the paramagnetic matrix. The generation of Mn²⁺ ions due to charge disproportionation and consequent changes observed in the magnetic and thermoelectric power behaviour are discussed considering multiple hopping and direct hopping between Mn³⁺, Mn²⁺, and Mn⁴⁺ ions. A phenomenological model is exploited to obtain the thermomagnetic parameters. The magnon drag and phonon drag largely decide the thermoelectric power in the low-temperature ferromagnetic region, whereas the small polaron hopping of carriers contributes to the TEP in the high-temperature paramagnetic region. A colossal TEP value of -800 μV K^-1 is observed at around 14 K for the x = 0.3 sample.