Structural memory effects in M-type hexaferrite magnets

M-type hexaferrites are a category of magnetic materials distinguished by their unique crystal structure and enhanced magnetic properties, which render them particularly suitable for applications in magnetic recording, microwave devices, and permanent magnets. M-type hexaferrites exhibit remarkable tunability in their magnetic properties through exposure to controlled gaseous atmospheres, including hydrogen, nitrogen, methane, and carbon-based gases, under heat treatment. These processes induce decomposition and partial reduction, enhancing saturation magnetization while reducing coercivity. Recalcination restores the hexaferrite structure, refining grain size and achieving superior magnetic properties. Interestingly, the recovery of the hexagonal structure occurs consistently across different hexaferrites (barium or strontium hexaferrite), reductant atmospheres (hydrogen, nitrogen, methane, or carbon), and techniques (heat treatment or mechanical milling). This reduction recombination process highlights a robust memory effect inherent in hexaferrites, offering opportunities for developing advanced materials with optimized magnetic properties. This review examines the mechanisms and methodologies of gas heat treatments and mechanical alloying, emphasizing their advantages over traditional approaches such as ion doping and wet chemical synthesis. It also identifies challenges and opportunities for leveraging these methods to engineer versatile magnetic materials for diverse applications in data storage, recording technologies, and beyond.