Intrinsic magnetic properties of Bi_xCo_2−xMnO₄ spinels obtained by short-time etching

Abstract: We report the structural and magnetic properties of Co₂MnO₄, partially substituted by Bi at the octahedral site. Bismuth enhances ferromagnetism due to a decrease of the Co²⁺–Co²⁺ antiferromagnetic interactions and an increase of the Mn³⁺–Mn⁴⁺exchanges. Spurious phases (magnetic and/or nonmagnetic oxides) can easily form because of the large differences between the ionic radii of Bi³⁺ and Co³⁺, hiding or altering the intrinsic physical properties of the main Bi_xCo_2−xMnO₄ phase. An easy way to eliminate the secondary phases is using acid reagents. Short-time etching of Bi_0.1Co_1.9MnO₄ using nitric acid was successfully used, keeping most of the properties of the initial compound, with no alteration of the crystallographic structure. Final stoichiometry was respected (∼Bi_0.08Co_1.82MnO₄), meaning that the material after etching definitely contains bismuth elements in its structure and the observed properties are intrinsic to the oxide spinel. Additional experiments were performed as a function of the synthesis conditions, showing that an optimal pH value of 7 allowed the best magnetic response of the non-doped material.