Rigid Exchange Coupling in Rare‐Earth‐Lean Amorphous Hard/Soft Nanocomposites

Abstract

considerably larger than for a single hard phase, since the strong coercivity H c of the hard phase is combined with the high saturation magnetization M s of the soft phase. [7] With sufficiently strong exchange coupling between the two magnetic phases, the soft phase moments will follow the hard phase during magnetization reversal. Such a rigidly exchange-coupled composite exhibits a magnetic hysteresis loop characteristic of a single ferromagnetic phase, [8] rather than a loop with drops, kinks, or shoulders from two superposed switching processes. [9] For most practical applications, it is also crucial that the Curie temperature T c of the composite material is well above room temperature, and that the overall M s value is large. A model for the interfacial exchangespring coupling between a soft and a hard phase was introduced by Goto et al. [10] According to that model, the two phases can reverse their moments coherently, that is, as a single component, only if the coupling is strong enough to extend from the interface through the entire thickness of the soft phase. Thus, for a given combination of soft and hard materials, rigid coupling is only possible up to a critical thickness of the soft phase. Therefore, strong interphase exchange coupling is usually manifested only for nanostructured materials, [11] and the fabrication method and resulting micro-and nanostructure have strong impact on the final magnetic properties of hard/soft nanocomposites and multilayers. [12,13] In crystalline heterostructures, the exchange coupling is often reduced at the interfaces due to structural transitions, defects, or both. [14-16] Multilayers and composites made using amorphous alloys have the advantage that undesirable effects from, for example, atomic steps at interfaces and other structural defects are avoided simply by the amorphous nature, resulting in smooth and continuous interfaces. [17,18] The alternative, to produce high quality epitaxial heterostructures of hard/soft magnets, is a much more cumbersome route toward smooth interfaces. [19] In addition, the epitaxial growth is limited to specific phases with matching lattice constants and specific magnetic properties. The magnetic properties of many amorphous building blocks can, on the other hand, be tuned almost continuously through their composition. [17,20] The sputter process allows industrial-scale production of amorphous multilayers with smooth interfaces, while recent advances in additive manufacturing of amorphous Febased materials open up new perspectives to create amorphous composite magnets of larger sizes and different shapes. [21,22] Electrification of vehicles and renewable energy is increasing the demand for permanent magnets, but the cost and scarcity of rare-earth metals is an obstacle. Creating nanocomposites of rigidly exchange-coupled hard and soft magnets, for which the magnetization reversal occurs as in a single magneticphase material, is a promising route toward rare-earth-lean permanent magnets with high energy products. The hard/soft exch...

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