Effect of iron-doping on spin-state transition and ferromagnetism in image cobalt oxides

Resistivity and DC magnetization measurements were performed for the polycrystalline image (image and 0.15) samples. The as-fabricated samples exhibit ferromagnetic (FM) transition and the transition temperature increases with increasing the Fe doping level. Annealing under the high oxygen pressure induces a spin-state transition of Co ions in the iron-free sample and such transition is reinforced with increasing the annealing oxygen pressure, while the annealing under high oxygen pressure suppresses the ferromagnetic ordering. Contrary to the case of the iron-free sample, no spin-state transition is induced by the annealing under high oxygen pressure for the Fe-doped samples, and the ferromagnetic transition temperature is nearly independent of the annealing procedures. The enhancement of the spin-state transition in the iron-free sample after annealing under high oxygen pressure should be attributed to the oxygen vacancies and the reduction of the cell volume. The suppression of the spin-state transition by the Fe doping is related to the enlargement of the cell volume. The enhancement of the ferromagnetism by the Fe-doping might arise from the ferromagnetic exchange interaction between image and LS image through oxygen atom (image–O–image).