Electrochemical Tuning of Magnetism in Ordered Mesoporous Transition-Metal Ferrite Films for Micromagnetic Actuation

Controlling magnetism in situ and in a reversible manner using battery redox chemistry is a new concept in the wider field of magnetoelectrics, and electrochemical lithium tuning of electrode materials is one such approach. Here we show that the general idea of combining nanomagnetism and electrochemical energy storage concepts can be applied to ordered mesoporous spinel ferrite thin films. Via an evaporation-induced self-assembly method, a series of cobalt and nickel ferrites (CoFe2O4, Coo.,Ni0.5Fe2O4, and NiFe2O4) were prepared and characterized by electron microscopy, grazing incidence small- and wide-angle X-ray scattering, 'Fe Mossbauer spectroscopy, and magnetometry. The dip-coated films after heating in air to temperatures beyond 600 degrees C are single phase with partially inverted spinel structures and exhibit 15-20 nm pores of oblate spheroidal shape arranged on a cubic lattice. Magnetic measurements clearly show stable ferrimagnetic ordering at room temperature, especially for the cobalt-based materials. More importantly, when using the different transition-metal ferrites as insertion electrodes in lithium-ion cells and carefully controlling the cutoff potentials, they allow for the robust tuning of magnetization without compromising both the lattice and pore structure. As a potential application area, micromagnetic actuation technology is considered.