Magnetic characterization of self-assembled nanostructures in cobalt ferrites using first-order reversal curve (FORC) analysis

Magnetic ceramics are important for numerous technologically relevant applications with a detailed understanding of structure, property, and processing inter-relationships playing a critical role in tailoring magnetic properties. Spinel ferrites are a particularly interesting class of magnetic ceramics of chemical formula AB2O4, with applications including biomedical hyperthermia and high frequency electrical power conversion. In this contribution, we seek to investigate a unique class of Co-ferrites in which spinodal decomposition can produce a ferrite nanocomposite with chemistry and stress state fluctuating within the interior of crystalline grains on the nm-scale, resulting in corresponding fluctuations of intrinsic magnetic properties as well as exchange and magnetostatic interactions. Structural and magnetic characterization of spinel ferrite samples are carried out (1) in the as-milled state prior to thermal processing, (2) after chemical and structural homogenization with a thermal calcination step, and (3) in the spinodal decomposed state following a subsequent annealing treatment within the Co-ferrite miscibility gap. Of note is the formation of a wasp-waisted hysteresis loop which emerges for the spinodal decomposed Co-ferrite sample, indicative of more complex magnetization reversal processes at relatively large applied fields than for homogeneous Co-ferrite samples of similar particle size and identical nominal chemistry. First order reversal curve (FORC) analysis is applied to further characterize the magnetization response, and a conventional interpretation of observed features in the FORC contrast is presented to discuss potential dominant magnetization mechanisms. The work described here represents the first application of FORC to spinodal decomposed magnetic ceramics and provides a strong foundation for future investigations seeking to quantitatively describe the impacts of nm-scale chemical, structural, and magnetic fluctuations on magnetization processes in ferrite spinel nanocomposite systems.