Plasma Synthesis of AlNiCo Core/Shell Nanopowders

Aluminum-nickel-cobalt (AlNiCo) permanent magnets revolutionized the permanent magnet industry in 1920’s but has not been thoroughly studied after the development of powerful rare-earth permanent magnets. As compared to rare-earth magnets, AlNiCo magnets have high remanent force (Br), better corrosion resistance and higher Curie temperature but lack high coercivity. If the coercivity is improved, BHmax value may increase up to values that would potentially fill the gap in between rare-earth and rare-earth free permanent magnets. The alloy stands as a good candidate to be produced with novel methods and investigated with the latest analysis techniques and it is now well established that magnetic behavior is strongly dependent on the size of the nanoparticles. In this work, magnetic properties of micron-size ball-milled powders and radio frequency (RF) plasma synthesized core-shell nanopowders are compared. Scanning transmission electron microscopy (S/TEM) analysis of the plasma synthesized nanoparticles showed a core/shell structure in which AlNiCo powders were encapsulated with magnetic Fe3O4. The magnetic measurements indicated that coercivity increased significantly with decreasing particle size, particularly for the nanopowders encapsulated with hematite (Fe3O4) layer. It is likely that this oxide coating contributes to the increase in coercivity due to effective exchange coupling between the core and the shell structure while serving as a protective layer against room temperature oxidation. The magnetic behavior of AlNiCo powders with respect to size and core-shell structure formation will be discused in details in conjunction with TEM, S/TEM, VSM and XRD results