7 - From soft chemistry to 2D and 3D nanowire arrays with hard magnetic properties and permanent magnet applications

Abstract

The polyol method and the organometallic chemistry route are two liquid-phase processes which have proven to be well suited for the synthesis of high aspect ratio ferromagnetic nanoparticles with an excellent crystallinity. Cobalt nanowires and nanorods with a mean diameter in the range 5–40 nm and exhibiting very high magnetic anisotropy and more complex anisotropic particles such capped nanowires or core-shell nanowires and nanorods can be prepared by these methods. The organometallic route was also extended to the growth of 2D hexagonal arrays of cobalt nanowires of diameter in the range 5–10 nm, combining chemical reduction and epitaxy. The particles prepared by the liquid-phase process are model systems for studying the magnetism of anisotropic objects as well as the interactions between magnetic nanoparticles. Several micromagnetic studies were carried out to describe the effect of the particle shape and crystallinity on the coercivity of elongated cobalt nanoparticles. These studies provided useful guidelines for particle optimization to reach the best possible properties for permanent magnet applications. The excellent properties in terms of magnetization (Ms = 1.7 T) and coercivity (μ0HC = 0.3–1 T) of cobalt nanorods make them candidates for the fabrication of permanent magnets and for applications in the field of magnetic recording. 3D permanent magnets with an energy product, (BH)max, of 65 and 165 kJ m−3 were prepared by alignment and consolidation of cobalt nanorods. The bottom-up approach appears very promising to prepare new hard magnetic materials with BHmax that can fill the gap between ferrites and rare-earth-based magnets.