Optimizing the Energy Product in Core-Shell Nanoparticle Magnets: General Guidelines and the FePt/CoFe System

The optimization of the energy product in permanent magnets presents a complicated multi-parametric problem that encompasses a large variety of intrinsic and microstructural properties. As both high remanent magnetization and coercivity are required, the main concern in optimizing a given material is often how to deal with the trade-off between these two properties. A promising approach is to combine high-anisotropy with high-magnetization phases in chemically synthesized magnetically hard-soft nanoparticles. The magnetization reversal in such systems has been studied by micromagnetics, but most of the solutions are given for a magnetically hard shell surrounding a magnetically soft core, although the inverse configuration may be more accessible from a fabrication perspective and can even help induce tetragonicity in phases such as CoFe. Here we summarize the basic general design rules for such systems, and we present specific calculations for the FePt/CoFe system. Though in larger particles complex reversal modes that are scientifically interesting occur, these are not relevant to the problem of achieving high energy products. Optimal energy products are achieved in small particles in the homogeneous exchange spring regime. Therefore, the optimal size and phase content must be determined under the contradictory requirements of achieving homogeneous reversal and avoiding thermal fluctuations.