Structural and magneto-dielectric anomalies in Mn-doped Bi₂Fe₄O₉ nanograins

We attempt to modify the magnetoelectric properties of antiferromagnetic Bi₂Fe₄O₉ nanograins by substituting multivalent Mn ions at Fe sites. The doped Bi₂Fe₄O₉ nanograins are synthesized by a hydrothermal method with a different Mn content x (x is the mole ratio of Mn to Mn plus Fe; x = 0.00, 0.02, 0.04, 0.06, and 0.10). Among them, the x = 0.04 sample shows the anomalous decrease of lattice parameters a and b, the lowest antiferromagnetic transition temperature, and the largest saturated magnetization. Besides, the magneto-dielectric effect (Δɛ/ɛ ∼ −4.9%) and the magneto-striction constant (λ ∼ −1.1 × 10⁻⁴) of the x = 0.04 sample are also the largest compared with others (220 K and 0.6 T). Combined with Raman scattering and x-ray photoelectron measurement, it is considered that the substitution site of Mn ions varies with an increase of doping amount x, which also leads to the change of valence state of Mn ions and the lattice structure, and further affects the strength of magnetic interaction and spin-lattice coupling.