Progress and prospects of hard hexaferrites for permanent magnet applications

Permanent magnets (PMs) are key elements in technologies like electric mobility and the wind turbines that are pillars of the Green Transition. These technologies use high performing rare-earth (RE) magnets, mainly NdFeB ones. However, the China’s monopoly in all the RE magnets production chain and supply constitute a strong handicap for the Green Transition. This problem drives to the innovation and research in the ambit of PMs, in particular to develop new RE-free or lean PM [1-3]. On the other hand, hard hexaferrite magnets constitute nowadays the most produced PMs in the world, and they are in second position in economic importance after the RE PMs [3]. In fact, NdFeB magnets and Sr-hexaferrite ceramic ones dominate the PM market due to the high performances of the first (BHmax up to 400 kJm-3) and the lower cost and average performances ((BH)max up to 38 kJm-3) of the second. Even the big gap in the properties of these two materials, and driven the criticality of the RE supply, the fluctuations of the RE costs and by the low costs of the ceramic magnets, there is strong interest in the substitution of RE-magnets by ceramic ones [4,5]. In my presentation, I will illustrate different studies oriented to increase the magnetization and/or the coercive field of hard ferrites with the purpose to obtain ceramic magnets with better performances than commercial ones and hence to substitute RE PMs in particular applications. I will show that the increase of the coercive field of hexaferrites and Cobalt ferrite in nanoscale is possible thanks to the optimization of composition, size, composition and strain [6-8]. In addition, different types of novel hybrid ferrite materials have been synthesized with the aim to obtain high remnant magnetization[9-11]. These materials are typically composed by a Sr hexaferrite or Cobalt ferrite hard phase and a high magnetization phase and both moieties are coupled by exchange or dipolar mechanisms. Finally, I will discuss on the development of bulk magnets from single nanophase and hybrid composites illustrating also several cases of prototypes [12-14]. This research was developed in the framework of the PRIN 2022 PNRR project HyPerMag “HYbrid ferrite nanocomposites for novel Rare-earth free PERmanent MAGnets” that is financed by the European Union –NextGenerationEU.