Rare-Earth-Free Exchange-Coupled Nanocomposites Based on M-Type Hexaferrites

Efficient permanent magnets that are concomitantly economically viable are of paramount importance for allowing industrial stakeholders to maintain a growing and competitive advantage. This study provides a comprehensive overview of recent developments in the field of rare-earth-free nanocomposite permanent magnets based on hexaferrites. The basic phenomenology of exchange-spring-coupled nanocomposites, comprising hard and soft magnetic components, is thoroughly explained. The use of hexaferrites as a hard phase, serving as a viable alternative to rare-earth-based permanent magnets, is extensively discussed, taking economical, accessibility-related, and environmental aspects into consideration. State-of-the-Art architectures of hard–soft magnetic nanocomposites based on hexaferrites as the hard magnetic phase, ranging from typical nanocomposites to nanowire arrays and special core–shell-like morphologies, are explored in detail. The maximum energy product (BH)max, representing the quality indicator for permanent magnets, is investigated by taking into consideration various degrees of freedom, such as substitutions, geometry, size, shape, preparation, and processing conditions (annealing), volume fraction of magnetic phases, and interfaces. Promising strategies to overcome the present challenges (e.g., size control, coercivity–remanence trade-off, and optimization for large-scale production) are provided within the framework of future permanent magnet design.