Anomalous magnetism in Al doped La2CoMnO6 ceramics

Abstract

La2CoMn1-xAlxO6 (0 ≤ × ≤ 1.00) samples were synthesized by sol–gel method, and their crystal structure and magnetic properties were investigated. X-ray diffraction reveals a structural transition from monoclinic (space group P21/n) to rhombohedral phase (space group R-3c) with Al content increasing. As envisionedandexpected, the remanent magnetization Mr and ferromagnetic (FM) transition temperature TC dramatically decrease with the increment of Al doping, which is due to the destruction of the long-range Co2+-O-Mn4+ FM super-exchange interaction induced by Al ion doping. However, abnormally increase of coercive field HC and effective magnetic moment μeff appear for × = 0.25. The results of infrared spectrum, Raman spectrum, and X-ray photoelectron spectroscopy illustrate that this anomaly can be attributed to the spin-state transition of Co3+ ions from intermediate spin (IS) to high spin (HS) state with × increasing to 0.25, rather than the valence changes of the magnetic ions. This spin-state transition is probably due to the increase of crystal symmetry with high disorder and the suppression of Jahn-Teller distortion. The suppressing Jahn-Teller distortion favors the stability of the HS state of Co3+ ions.