The superexchange interactions in mixed Co–Fe ludwigite

The crystal structure, cation distribution and exchange interactions in the Co2.25Fe0.75O2BO3 ludwigite have been explored through X-ray diffraction, electrical resistivity, ac-susceptibility and Mossbauer effect measurements. The crystal data have shown that iron atoms occupy the most symmetric crystallographic sites Fe4 and Fe2. The complex magnetic behavior with two magnetic transitions near 70 and 115 K at low temperatures was found. The Mossbauer data have displayed the trivalent iron states only. The values of superexchange energies have been estimated for Co3O2BO3 and Co2.25Fe0.75O2BO3 yielding a significant role of frustrations in the ludwigite magnetic system. Variable range Mott hopping conductivity law was proved to be valid in the wide temperature region, pointing out a localized character of charge carriers rather than collective.

Research Highlights
► The high quality single crystals of Co2.25Fe0.75O2BO3 ludwigite were synthesized.
► The orthorhombic crystal symmetry with Pbam space group was found.
► The Fe3+ ions predominantly occupy the octahedral positions 2 and 4.
► The complex magnetic behavior and magnetic transitions at 70 and 115 K were found.
► The values of superexchange energies have been estimated.