Mn2+ induced structure evolution and dual-frequency microwave absorption of MnxFe3-xO4 hollow/porous spherical chains made by a one-pot solvothermal approach

High-quality MnxFe3-xO4 (0 <= x <= 1.09) hollow/porous spherical chains (H/PSCs) were prepared via a facile one-pot solvothermal approach. These chains were formed through a magnetic field induced-Oswald ripening mechanism. The external magnetic field induced nanocrystals to aggregate into microspheres along the [111] direction, and these microspheres further assembled into 1D H/PSCs. Characterization confirmed that an increase in the Mn2+/Fe3+ molar ratio decreased the crystal size, diameter, and aspect ratio as well as increased internal strain, lattice constant, Mn doping amount, and specific surface area. Consequently, saturation magnetization and coercivity decreased because of the crystal size and Mn2+ substitution. Mn2+ substitution induced a dual-frequency absorption at 2-18 GHz, in which the absorption band at lambda/4 decreased and that at 3 lambda/4 increased with increasing x. Compared with Fe3O4 solid spheres and hollow spheres, Mn0.746Fe2.254O4 H/PSCs exhibited a broader absorption band (R-L <= -20 dB) of 9.86 GHz (2.05-7.91 and 14-18 GHz, respectively). The enhanced absorption performance may be related to hollow and porous structures, oriented arrangement of nanocrystals, and Mn2+ substitution.