Atomic-scale structure clarification of the planar Z phase and its influence on the magnetic properties in Sm(CoFeCuZr)_z permanent magnets

2:17-type SmCo-based permanent magnets with special cellular structure composed of 2:17R, 1:5H and planar Z phase have exhibited great potential for high temperature application. However, there are still disputes on the atomic structure of the planar phase and its influence on magnetic properties. Here using advanced aberration corrected high-angle annular dark field scanning transmission electron microscopy we found that most of the planar Z phases possess SmZr₄Co₁₃–H (P-6m2) or SmZr₂Co₉–2H (P-6m2) structure instead of SmZr₂Co₉–3R (R-3m) structure. This experimental observation is corroborated well with our calculation of the formation enthalpy as well as Gibbs energy that SmZr₄Co₁₃–H and SmZr₂Co₉–2H are more thermodynamically stable. Moreover, we found that the transition from the matrix to the planar phase includes both Sm-Co co-displacement along the basal plane as well as Zr replacement for Sm 1a and Co 2c sites. The calculated magnetocrystalline anisotropy of the planar phase under relaxed state exhibits easy axis of magnetization, while that of the planar phase under strain state inside the magnets transforms to easy plane of magnetization. Micromagnetic simulation indicates that the magnetocrystalline anisotropy of the planar phase determines the initial nucleation process of reversal magnetization inside the 2:17R cells, which influences significantly on the coercivity. Tuning the magnetocrystalline anisotropy of the ultrathin planar phase inside the magnets to easy axis will benefit the achievement of higher coercivity. Our investigation may provide insights for developing SmCo-based permanent magnets with higher performance from atomic scale perspective.