Tuning the magnetic anisotropy of Co nanoparticles by metal capping

Abstract

The magnetic anisotropy of Co clusters with diameters ranging from 1.1 nm to 4.5 nm turns out to be significantly larger than in bulk and strongly increasing with decreasing cluster size. The dominating role of the surface can be used to modify the anisotropy by changing the electronic properties of the matrix surrounding the clusters. We find that capping the clusters by a metallic (Cu and Au) layer significantly enhances the anisotropy, thus also stabilizing the magnetization against thermal fluctuations. The observed anisotropy enhancement is attributed to the bonding of the Co 3d electrons to the conduction band of the capping layer, which depends on the electronic band structures of both metals.

X‐ray magnetic circular dichroism and reflection anisotropy spectroscopy Kerr effect studies of capped magnetic nanowires

“…It is well known that an induced polarisation of Au capping layers occurs due to the interaction with the magnetic Co [19]. This observation, combined with the much reduced coercivity from either uncapped 4-atom wide wires or 1.3ML of Co on Pt(997) [17], is indicative that the Au capping has indeed greatly modified the magnetic anisotropy of the system, as has been observed previously in capped Co spherical clusters [4,20,21]. However, unlike the previous observations which were for spherical particles, the capping can be seen to induce a preferred orientation for the easy axis of magnetization .…”

Morphology and capping effects in the magnetic and magneto-optical properties of nanoparticulate Co films

“…This change may be due either to the slight polarization of Au in the Co/ Au interface region or to the enhancement of the Co anisotropy, as observed in continuous and nanoparticulate films covered by Au. 21,22 A very slight decrease of the saturation field can be observed in the nanoparticulate Co films grown at 300 and 600°C. In the case of the polar loops shown in the right column of Fig.…”

Structural and magnetic properties of amorphous Co-W alloyed nanoparticles

“…11) are similar to those for bare and Cu, Ag, and Au-capped Co nanoparticles. 10,11,14 At lower temperatures, FC and ZFC for each sample separate, and the ZFC curve shows a maximum at a blocking temperature T B . The values of T B for the Co-W NPs are given in Table III.…”

“…In these granular systems, we have been able to control the cluster size, 10 the average number of neighbors to a given cluster, 8 and even the surface anisotropy by capping the cobalt nanoparticles with noble metals such as Cu, Ag, and Au. 11 These studies have enabled us to perform quantitative determination of parameters such as magnetic anisotropy 12,13 or dipolar interactions. 8 The capping metal does not diffuse on Co, so the Co particle retains its integrity as a nearly spherical crystalline particle.…”