Seven-Nanometer Hexagonal Close Packed Cobalt Nanocrystals for High-Temperature Magnetic Applications through a Novel Annealing Process

Abstract

Seven-nanometer cobalt nanocrystals are synthesized by colloidal chemistry. Gentle annealing induces a direct structural transition from a low crystalline state to the hexagonal close packed (hcp) phase without changing the size, size distribution, and the lauric acid passivating layer. The hcp structured nanocrystals can be easily redispersed in solvent for further application and processing. We found that the magnetization at saturation and the magnetic anisotropy are strongly modified through the annealing process. Monolayer self-assembly of the hcp cobalt nanocrystals is obtained, and due to the dipolar interaction, ferromagnetic behavior close to room temperature has been observed. This work demonstrates a novel approach for obtaining small size hcp structured cobalt magnetic nanocrystals for many technological applications.

A New Intrinsic Magnetic Property Due to the Mesoscopic Ordering of Cobalt Nanocrystals in Supra-Crystals

“…We see that for both samples the T B has increased significantly (see Table 1) and we attribute this to an increase in the anisotropy of the Co nanocrystals, arising from the change in crystallinity from fcc to hcp nanocrystals. This is consistent with the behavior observed for 2D and diluted assemblies of Co nanocrystals [9] where a 3-fold increase in K a was found after the fcc-hcp transition. After annealing, the width of the supracrystal normalized ZFC peak remains unchanged w.r.t.…”

“…As illustrated by the electronic diffraction pattern (inset, Fig. 3a), annealing drastically improves the nanocrystal crystallinity from a poorly crystallized fcc structure to a monocrystalline hcp structure [9]. No oxidation is detected.…”

“…This enables interesting comparisons to be drawn between mesoscopic and atomic magnetic systems and magnetic behavior ranging from superparamagnetism to spinglass-like can be observed [7][8]. Moreover, increasing the magnetocrystalline anisotropy of the nanocrystals in an assembly (for example by annealing [9][10]) can optimize the interactions in the system leading to magnetic transitions close to room temperature, which are desirable for applications. We have developed synthetic and deposition procedures which allow us to fabricate, with the same population of nanocrystals, either disordered or long-range fcc ordered 3D assemblies [6].…”

“…The normalized ZFC curves of the annealed disordered and ordered samples are now superimposed. For both samples there is a significant increase in saturation magnetization (M s ) after annealing due to the formation of pure hcp nanocrystals [9]. (NB, due to the very low mass of these 3D assemblies with respect to their substrates, accurate determination of the Co mass was not possible and therefore only relative and not absolute values of M s can be determined.)…”

From the Co Nanocrystals to Their Self-Organizations: Towards Ferromagnetism at Room Temperature

“…The ZFC magnetisation increases with increasing temperature up to a maximum, T B , above which the behaviour is paramagnetic [47]. In this case, T B (130 K) is significantly larger than that observed for a dilute system of Co nanocrystals of the similar size [54] due to the strong magnetic dipolar interactions between the nanocrystals in the supracrystal. The FC magnetisation curve is nearly temperature independent below T B , characteristic of spin glass behaviour [47].…”