Energy product and coercivity of a rare-earth-free multilayer FeCo/FePt exchange spring magnet

Abstract

Using the full potential linearized augmented plane wave (FLAPW) method, we have explored the energy product and the coercivity field of rare-earth-free FeCo/FePt(001) multilayered exchange spring magnet systems. We have considered 5 and 7 monolayers (ML) of a FePt hard layer and 3, 5, 7, and 9 ML of a FeCo soft layer. The FeCo soft layers are found to show close to half metallic features, while the FePt hard layers manifest conventional metallic behavior. A giant perpendicular magnetocrystalline anisotropy energy (E_MCA) is observed. For instance, an E_MCA of 27.24 meV/cell is found in FeCo(9 ML)/FePt(7 ML) multilayer structure. The energy product almost linearly increases with increasing FeCo thickness, while the coercivity filed shows the opposite behavior. Interestingly, we have obtained that the multilayer structures display very large energy product and coercivity field. For example, FeCo(9 ML)/FePt(5 ML) multilayer has an energy product of about 82 MGOe and a coercivity field of about 130 KOe. Moreover, we find that the multilayer system may show enhanced coercivity field compared with that found in FeCo/FePt bilayer film structures, while the energy product is comparable to that observed in bilayer films. Therefore, our results may imply that the FeCo/FePt multilayer can be employed as a potential rare- earth-free permanent magnet material.