Tuning the magnetic properties of sandwiched hexaferrite/cobalt ferrite exchange-coupled nanocomposites obtained in high-boiling organic solvent

Abstract

Exchange-coupled magnetic nanocomposites present significant potential for advanced permanent magnets; however, scalable syntheses that maintain crystallographically coherent interfaces remain challenging. In this study, colloidal Al-substituted strontium hexaferrite nanoplates with average dimensions of 48 nm × 6 nm were covered with epitaxial cobalt ferrite nanolayers via the thermolysis of metal acetylacetonates in hexadecane. By simply adjusting the precursor concentration, we create sandwich-like CoFe₂O₄/Sr_0.95Fe_11.5Al_0.5O₁₉/CoFe₂O₄ particles with cobalt ferrite content ranging from 7 wt% to 58 wt%. The results from TEM investigations and theoretical calculations of the energy surface of the interface between CoFe₂O₄ and Sr_0.95Fe_11.5Al_0.5O₁₉ confirm the existence of a coherent {001} Sr_0.95Fe_11.5Al_0.5O₁₉ ‖ {111} CoFe₂O₄ interface. Magnetic measurements confirm that the composite particles behave as a single magnetic phase, exhibiting efficient exchange coupling. Magnetic properties reveal a continuous transition from hexaferrite-dominated magnetic behavior to cobalt ferrite-like characteristics as the proportion of the latter increases. This suggests the potential for precise control over the final magnetic properties of the nanocomposite. The proposed synthetic route is gram-scale and yields non-aggregated, uniformly covered nanomagnets with optimal structural and spin coupling between the constituent phases.