## 3 Results and Discussion
In this case, the maximum applied magnetic field \(H=10\) kOe is not enough to change the magnetic order at the corresponding temperature, but is still enough to stabilize the low-temperature order in the \(M(T)\) experiment. Figure 6: Magnetization _vs._ temperature measured, applying a field of \(H=10\) kOe: cooling the sample down to \(T=5\) K with the field (open triangles), cooling the sample in zero field down to \(T=100\) K (open circles), and cooling applying the field down to \(T=100\) K (solid circles). It is quite noticeable that the enhancement of the saturation magnetization decreasing temperature, related to the magnetic moment of the particles, is extremely dependent on composition. Figure 7 shows the saturation magnetization _vs._ composition for the dispersed samples at \(T\) = 300 K, 80 K, and 5 K near the \(x\) = 0.69 composition. In order to compare our results of magnetization in nanoparticles, we have also plotted the saturation magnetization for the bulk alloy. From the figure, it is evident that the magnetization enhancement takes effect only in a small range of composition, in opposition to the bulk materials where a broad maximum is observed. Moreover, magnetization increases with decreasing temperature for the nanoparticles at composition near \(x\) = 0.69 are much larger than the changes of _M_\({}_{S}\) for the other compositions studied. Probably, in the first case, the larger magnetic moment allows the complete ferromagnetic order in each particle at low temperature, yielding these saturation magnetization values. On the other hand, the powder samples have a very different behavior. As previously mentioned, the _M_(_T_) measurements under ZFC and FC conditions of (Fe\({}_{0.69}\)Co\({}_{0.31}\))B\({}_{0.4}\) composition reveal the presence of strong interactions between particles, suggesting the possibility of frustration in the magnetic order and cluster-glass behavior. Additionally, the _M_(_H_) curves are very different with respect to the non-interacting sample, confirming our description of the interparticle interaction role. The magnetization loops of the powder samples present a small but non-zero coercive field, which increases smoothly as the temperature decreases (Figures 3(b) and 4). Also, the saturation magnetization is very different from the non-interacting case: the interacting particles show a _M_\({}_{S}\) value close to the one at room temperature, approximately constant in temperature (Figure 5). The different features of both samples can be understood, considering that the interparticle interactions in the powder samples strongly affect the surface spins avoiding the growing of ferromagnetic ordered regions in the shell. In the dispersed sample case, where interparticle interactions are negligible, the spins in the shell may align along the core direction and enhance particle saturation magnetization. Figure 7: Saturation magnetization _vs._ composition for (Fe\({}_{x}\)Co\({}_{1-x}\))\({}_{0.6}\)B\({}_{0.4}\) dispersed nanoparticles at \(T\) = 5 K, 80K, and 300 K. The dashed line is the _M_\({}_{S}\) bulk alloy value from [1]. The solid lines are only a guide to the eye. Nevertheless, intra-particle interactions are present in the \"non-interacting\" (dispersed) particles, suggested by the anomalous behavior of the magnetization loop shape, the coercive field and the saturation magnetization. Evidently, the contrast observed between the two samples with (Fe\({}_{0.69}\)Co\({}_{0.31}\))B\({}_{0.4}\) composition (where the atomic magnetic moment is a maximum) is also present in the \(M_{S}\) values as a function of composition, where the maximum of \(M_{S}\) in the powder sample is also observed at \(x=0.69\), but this value is approximately constant in temperature.