Interface exchange coupling in Co nanoparticles dispersed in a Mn matrix

Abstract

The structural and magnetic properties of 1.8 nm Co particles dispersed in a Mn matrix byco-depositing pre-formed mass-selected Co clusters with an atomic vapour ofMn onto a common substrate have been studied by using EXAFS (extendedx-ray absorption fine structure), XMCD (x-ray magnetic circular dichroism),magnetometry, and theoretical modelling. At low Co volume fraction (5%) Co@Mnshows a significant degree of alloying and the well-defined particles originallydeposited become centres of high Co concentration CoMn alloy that evolves from pureCo at the nanoparticle centre to the pure Mn matrix within a few nm. Eachinhomogeneity is a core–shell particle with a Co-rich ferromagnetic core in contact with aCo-depleted antiferromagnetic shell. The XMCD reveals that the Co momentlocalized on the Co atoms within the Co-rich cores is much smaller than theferromagnetic moment of the Co nanoparticles deposited at the same volumefraction in Ag. Electronic structure calculations indicate that the small magnitudeof the core Co moment can be understood only if significant alloying occurs.Monte Carlo modelling replicates the exchange bias (EB) behaviour observedat low temperature from magnetometry measurements. We ascribe EB to theinteraction between the ferromagnetic Co-rich cores and the antiferromagnetic Mn-richshells.