Origins of the Appearance of Ferromagnetic State and Colossal Magnetoresistance in Cobaltites

Resumo

The crystal structure and magnetotransport properties of stoichiometric and anion-deficient Ba²⁺‑alloyed cobaltites with a perovskite structure have been studied. It was shown that the development of ferromagnetic state is related to the presence of cobalt ions mainly with single e_g electron (intermediate spin IS state), whereas the existence of two e_g electrons (high-spin HS state) leads to the antiferromagnetic state. The covalent component of the chemical bond stabilizes an electron configuration that is close to the intermediate spin state. The colossal magnetoresistance appears at either concentration or temperature boundary corresponding to the coexistence of ferromagnetic and antiferromagnetic phases or clusters and results from the filed-induced spin transition from the HS/LS mixture to the IS state. Cobalt ions in the IS state are responsible for the ferromagnetism and high resistivity.