Optimizing the magnetic properties of hard and soft materials for producing exchange spring permanent magnets

The exploitation of the exchange coupling between hard and soft magnetic materials has been proposed for enhancing the magnetic performances of rare-earth free permanent magnets, with the aim of extending their use to all applications where moderate energy product (35–100 kJ m⁻³) is required. Strontium hexaferrite (SFO)/spinel ferrite composites seem particularly promising to achieve this target, although the conditions to maximize the effect while using techniques easily scalable to industrial production have not yet been identified. Within this framework, the optimization of the structural, chemical, and magnetic properties of the two moieties before the coupling procedure is crucial to enhance the energy product of the final composite. Here we report the syntheses of both nanometric SFO with high coercivity (ca. 525 kA m⁻¹) and quasi-bulk saturation magnetization (68 Am² kg⁻¹) and a series of nanosized zinc-doped ferrite (Zn_xFe_3−xO₄, 0.0 ⩽ x ⩽ 0.4) through cheap, easily scalable and eco-friendly approaches. The structural and chemical stability of the two magnetic phases as a function of temperature were investigated up to 1100 °C, with the aim of finding the best compromise between preservation of the nanometric scale and magnetic properties. A very high-magnetization (106 Am² kg⁻¹) ferrite was obtained by annealing Zn_0.3Fe_2.7O₄ nanopowder at the highest investigated temperature. A preliminary attempt at coupling the two phases, starting from a mixture of the nanopowders, was performed through a classic annealing process in the temperature range 500 °C–1100 °C. The adopted procedure allowed for obtaining an exchange coupled composite at 1100 °C where the two phases are intimately and homogeneously mixed, with micrometric (0.3–5 μm) and nanometric (up to 50 nm) spinel ferrite particles. Despite these promising results, no enhancement of the energy product was found, highlighting the need for further experimental efforts to improve the coupling procedure. (paper)