Structure–property trends in a hollandite multiferroic by Fe doping: structural, magnetic and dielectric characterization of nanocrystalline BaMn3−xFexTi4O14+δ

Abstract

BaMn₃Ti₄O_14+δ (δ = 0.25, BMT-134), a recently discovered single-phase multiferroic complex oxide was doped with varying concentrations of Fe in order to assess the effect on magnetic and dielectric behavior. The novel compound BaMn_3−xFe_xTi₄O_14+δ (BMFT) was prepared via a sol–gel chemical solution processing method. Four substituted variations were synthesized: BaMn_3−xFe_xTi₄O_14+δ (BMFT) with x = 1, 1.5, 2, and 2.25. The approach afforded a nanocrystalline product, with control over structure and morphology, while successfully allowing increasing amounts of iron to be incorporated into the structure. All variants belong to the same hollandite I4/m crystal class as the parent compound, confirmed by powder XRD. Electron microscopy (TEM, EDS and SEM) provided characterization of the microstructure and elemental composition. Mössbauer spectroscopy was used to probe the local chemical environment of Fe present within the nanocrystals, to indicate oxidation state and bonding geometry. The BMFT material system was characterized as a function of temperature and applied magnetic field: M–T and M–H curves were obtained (by MPMS) and allowed the determination of Curie/Weiss constants through fitting and low temperature hysteresis respectively. The Weiss constant becomes increasingly positive as Fe is added until an observed inflection point, indicative of an antiferromagnetic to paramagnetic short-range interaction transition. The frequency dependent dielectric permittivity of the series of BMFT compounds with differing Fe concentration, was obtained on MIM structures prepared from pressed pellets with a nanostructured morphology. The results show how Fe concentration can strongly influence dielectric behavior. Dielectric constants ranging from 100–1600 (low frequencies) to 150–200 (high frequencies) were observed, together with a decrease in the AC conductivity with increasing Fe.