Magnetic properties of Fe nano-clusters stabilized at grain boundaries of Yb films

The temperature dependence of the magnetic properties of Fe nano-sized clusters in (5.0 at.-% Fe)Yb films, prepared by vapor co-deposition, has systematically been investigated using zero-field and in-field Mössbauer spectroscopy as well as AC and DC magnetization measurements. Room temperature Mössbauer spectra reveal two types of Fe clusters, with non-cubic symmetry, located at Yb grain boundaries. Spectra, taken at 4.2-K, show a complex distribution of magnetic hyperfine fields, which can be related to inhomogeneous inter-cluster interactions. In addition, zero-field and in-field Mössbauer experiments indicate that the interactions within the Fe-clusters are predominantly ferromagnetic-like and their average magnetic moments are about 80-μ_B. As suggested by Mössbauer and AC magnetization data, the Fe-clusters are superparamagnetic above ca. 20-K, while their magnetic behavior below this temperature is that of cluster-glass-like systems with weak inter-cluster interactions. The details of the freezing process as a function of frequency and magnetic field are discussed and magnetic anisotropy constants for these Fe clusters are estimated to be K_eff-∼-8-12-×-10^5-J m^-3.