Ultrathin cobalt nanowires with high energy product

Uniform and single crystalline hexagonal close-packed (hcp) Co nanowires (NWs) have been synthesized by optimizing the precursor concentration and cleaning process in a hydrothermal process¹. The TEM image indicates that the prepared Co NWs have uniform size distribution with a mean diameter and length of 8 nm and 150 nm, respectively. As the radius of the Co NWs is smaller than coherent radius R_coh = 3.65 l_ex, where l_ex is the exchange length, the ultrathin Co NWs exhibit the coherent magnetization reversal based on the Stoner-Wohlfarth mode. As a result, a pronounced coercivity of 11.4 kOe is achieved in the aligned Co NW assemblies. The high aspect-ratio, good crystallinity, uniform morphology and desired orientation of Co NW assemblies result in a record high energy product of 56.1 MGOe which is close to the theoretical value. The compaction of the prealigned NWs assemblies at various pressure results in packing density in the range from 5.3 to 7.3 g/cm³ and energy product as high as 20 MGOe. The obtained nanowire based permanent magnets with well controlled aspect ratios exhibited energy products in the intermediate range between Alnico and rare-earth containing magnets.

1 K. Gandha, K. Elkins, N. Poudyal, X. Liu, and J.P. Liu, Sci. Rep. 4, 5345 (2014).