Ferrimagnetic cluster formation due to oxygen vacancies in CaFe₂O_4−δ

Abstract

CaFe₂O₄ possesses an intriguing crystal structure characterized by the presence of FeO6 octahedra that share both edges and corners, and featuring zigzag Fe chains that are assembled in a honeycomb tube network. The nominal CaFe₂O₄ has been identified to show a long-range antiferromagnetic spin ordering of Néel temperature T N ∼ 184 K. From dc and ac magnetic susceptibility measurements on both polycrystalline and single-crystal samples, a random distribution of ferrimagnetically ordered clusters of Fe spins is proposed to exist in the oxygen deficient CaFe₂O_4−δ . The ferrimagnetic ordering is proposed, coming from the oxygen vacancy-induced Fe³⁺ to Fe²⁺ conversion for the antiferromagnetically coupled spins of Fe²⁺ ions in the low-spin state and the Fe³⁺ ions in the high-spin state, which leads to an incomplete cancellation of staggered magnetization below T N . Current model reasonably explains the inconsistencies found in the literature regarding the persistent ferromagnetic component for CaFe2O4 , having a confirmed antiferromagnetic long-range spin ordering from neutron diffraction studies. We calculate the antiferromagnetic spin structure and the parameters of a Heisenberg Hamiltonian via spin-polarized density functional theory, obtaining results that are consistent, to a very high degree, with our experimental results for ac and dc magnetic susceptibility.