Anomalous magnetism in Al doped La2CoMnO6 ceramics

La2CoMn1-xAlxO6 (0 <= x <= 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 P2(1)/n) to rhombohedral phase (space group R-3c) with Al content increasing. As envisioned and expected, the remanent magnetization M-r and ferromagnetic (FM) transition temperature T-C 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 H-C and effective magnetic moment mu(eff) appear for x = 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 x 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.