Vasileios Alexandrakis

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 and 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, though synthetically the opposite may be more convenient. Here we review and 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 appear that are scientifically interesting and may even include topological configurations, these are not relevant to the problem of achieving high energy products. The optimal size and phase content are within the simple homogeneous exchange-spring regime and must be determined under the contradictory requirements of achieving homogeneous reversal and avoiding thermal fluctuations.

Bimetallic colloidal CoPt nanoalloys, with a very low platinum content, were successfully synthesized following a modified polyol approach. Powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and Transmission Electron Microscopy studies were performed to estimate the crystal structure, morphology and surface functionalization of the colloids respectively, while the room temperature magnetic properties were measured using Vibrating Sample Magnetometer. The particles exhibit excellent uniformity, with a narrow size distribution, and display strong room temperature hysteretic ferromagnetic behavior even in the as-made form. Upon annealing at elevated temperatures, progressive formation and co-existence of exchange coupled both chemically ordered and disordered phases, significantly enhance the room temperature coercivity.