## 3 Results and Discussion
This magnetic disorder originates in the broken exchange bonds (which reduce the effective exchange interaction in the shell), and in the surface anisotropy, that inevitably lead to surface spin misalignment with respect to the ordered core spins [16, 17, 28]. Such spin disorder propagates from the surface to the core, giving a shell region with paramagnetic regime. In this case, the saturation magnetization measured originates in the small ordered core, also giving a negligibly small \(H_{C}\) field, because it originates only in the shape anisotropy (the sample is amorphous and we assume that the particles are not spherical, rather probably having an ellipsoidal shape). From the \(M_{S}\) value, an average 2 nm diameter for the magnetically ordered core has been estimated. At intermediate temperatures, 70 K \(<T<\) 300 K, a change in \(M_{S}\) occurs, suggesting a modification of the surface contribution to the magnetization. With decreasing temperature, magnetic correlation between surface spins develops, giving rise to exchange-coupled ferromagnetic clusters in the shell, contributing to the total magnetization value. Additionally, these clusters are strongly influenced by the surface anisotropy, with the easy axis perpendicular (parallel) to the surface for the Fe (Co) spins, giving a disordered surface-spin state [18, 19]. Due to the large surface anisotropy energy, the ferromagnetically ordered regions in the shell \"freeze\" in a disordered state, coupled to the ferromagnetic core. Because of this coupling, the freezing of the surface spins actually exerts a magnetic frustration on the core moment, giving a cluster-glass character to the whole particle, displaying anomalous closed magnetization loops with \(H_{C}\sim\) 0. This cluster-glass behavior avoids aligning the spins in a complete ferromagnetic order, giving the observed \(M_{S}\) value. At low temperatures, \(T<\) 70 K, an additional change in the magnetic order of the non-interacting particles is observed. The saturation magnetization value grows, evidencing the full order of the spins, yielding larger \(M_{S}\) values than that measured in the bulk alloy. This effect is probably due to a modification in the conduction electron band, which is confined to metallic nanoparticles. In correspondence to the saturation magnetization, the coercive field shows an abrupt increase in this low temperature range. Actually, below 70 K, the subsequent growing of the ferromagnetic regions in the shell aligned in the field direction, allowing their exchange coupling with the ferromagnetic core of the nanoparticles, making its reversal more difficult, and thus strongly increasing the effective anisotropy of the whole particle [18, 19]. This enhancement in the magnetic order is also evidenced in the increase of the FC magnetization at low temperatures measured at \(H=50\) Oe [16, 18, 19]. An additional confirmation of the internal magnetic order change below \(T=70\) K is given by the \(M(T)\) curve measured at high fields. In Figure 6 the measured \(M(T)\) curve is plotted, cooling the sample down to \(T=100\) K, reflecting the magnetization value originated by the internal magnetic order of the spins corresponding to this range of temperature. On the other hand, on cooling the sample down to \(T=5\) K a different magnetic order is favored, characteristic of low temperatures, where the spins are fully aligned. Performing the experiment warming the sample, the measured \(M(T)\) curve has larger values than the previous experiment, corresponding to the low-temperature ordered state. This low-temperature magnetic order remains even at room temperature because it is stabilized by the applied magnetic field. These results strongly contrast with the \(M_{S}(T)\) values obtained from the magnetization loops. In the loops, where a previous zero-field-cooling is performed, the internal magnetic order is frozen at each characteristic temperature and then the magnetization is recorded applying the magnetic field.