Magnetic study of nanocomposite films consisting of ultrafine Co particles embedded in a ferromagnetic AuCo alloy

Nowadays, the search for innovative nanocomposite systems consisting of at least two different magnetic phases is attracting remarkable attention. Indeed, the intimate mixing of the different phases at the nanoscale level may give rise to new materials showing unique properties. In particular, a fine tuning of their overall magnetic anisotropy may be obtained and, accordingly, of their magnetic hysteretic properties. In this framework, we present an in-depth study of the magnetic properties of a set of three AuCo films, ~ 30 nm thick, with different Au:Co concentration ratio (2:1, 1:1, 1:2). The samples were grown by magnetron sputtering co-deposition technique on naturally oxidized (100)-Si substrates, and it turns out that this method allows the alloying of Au and Co and the production of a bimetallic compound is achieved to a good extent. The samples mainly consist of a structurally disordered ferromagnetic alloy in which segregated Co particles (~ 2 nm in size) are dispersed and the two phases are finely intermixed. Magnetization measurements, performed with a SQUID magnetometer in the 6 K - 300 K temperature range, have pointed out a peculiar hysteretic behavior, especially well visible in samples Au1Co1 and Au1Co2, characterized by in-plane anisotropy and crossed branches in the loops measured along the hard magnetization direction (Fig. 1(a)). To elucidate the origin of this behavior, micromagnetic calculations have been carried out using the object-oriented micromagnetic framework (OOMMF) code. The calculations have been performed for a simplified system made of two exchange-coupled ferromagnetic phases: an AuCo matrix surrounding a Co cluster, i.e. an aggregate of smaller Co particles. Indeed, our experimental results suggest that the exchange coupling of the Co particles with the surrounding matrix may result in the formation of magnetic clusters. So our model emphasizes both the nanocomposite nature of the investigated samples and the role of interparticle magnetic interactions. The main features of the hysteretic behavior are qualitatively well reproduced provided that the two phases have almost orthogonal magnetic anisotropy axes. We hypothesize a dominant magnetoelastic character of the anisotropy in both phases and we discuss how this requirement can be plausibly fulfilled.