## 3 Results and Discussion  
As our nanoparticles are ferromagnetically ordered at temperatures above room temperature, single domain behavior is expected to be observed. The magnetization results as a function of temperature of (Fe\({}_{0.69}\)Co\({}_{0.31}\))B\({}_{0.4}\) particles measured under zero field cooling (ZFC) and field cooling (FC) conditions applying a field \(H\) = 50 Oe are plotted in Figure 2 for the dispersed and powder samples. Figure 1: Size distribution of the (Fe\({}_{0.69}\)Co\({}_{0.31}\))B\({}_{0.4}\) nanoparticles measured by light scattering method (bars). The solid line corresponds to the least-square fitting of the data with a log-normal distribution. In both systems, the observed behavior represents irreversibility, and consequently the blocking temperature (the temperature above which the system behaves as a superparamagnet) is above room temperature. This blocking temperature is unexpected because the usual energy barrier for 3 nm amorphous ferromagnetic nanoparticles is small, generally giving a superparamagnetic regime at room temperature. In our case, the coupling of the single domain magnetization of the core of the particle with the surface anisotropy of the surface spins via exchange interaction enhances the effective anisotropy. Taking into account the non-interacting nature of the particles, the FC magnetization curve presents an atypical shape, remaining quasi-constant with decreasing temperature (Figure 2(a)). This fact is probably evidence of intraparticle interactions or magnetic order frustration at the particle surface [16,17]. Monte Carlo simulations of non-interacting Fe-Co nanoparticles within the core-shell model satisfactorily reproduce the measured ZFC-FC magnetization curves [24]. In this model, a different orientation of the uniaxial surface anisotropy axes for Fe and Co magnetic moments is considered, producing frustration in the superficial magnetic order. This effect leads to the anomalous FC curve measured on our non-interacting nanoparticles. The increase of the magnetization at low temperatures is also reproduced and it is related to an improvement of the internal magnetic order of the particles, as it will be described later with the hysteresis loops results. On the other hand, the \(M(T)\) curve under FC condition of the powder sample, where strong interparticle interactions are present, behaves very differently, _i.e._, the magnetization decreases at low temperatures (Figure 2(b)). This curve  
Figure 2: Magnetization _vs._ temperature curves for (Fe\({}_{0.69}\)Co\({}_{0.31}\))B\({}_{0.4}\) dispersed (**a**) and powder (**b**) particles measured under zero-field-cooling (ZFC) and field-cooling (FC) conditions. shape is typical of spin-glass and cluster-glass systems where interactions frustrate the magnetic order, giving the observed magnetization curve [25, 26]. These interparticle interactions probably also frustrate the magnetic order of the particle surface spins, reinforcing the magnetization reduction effect. Magnetization loops, _M_(_H_), were performed at different temperatures. As an example, Figure 3(a) displays the hysteresis loops for the (Fe\({}_{0.69}\)Co\({}_{0.31}\))B\({}_{0.4}\) dispersed particles at three different temperatures. The different behavior of the dispersed and powder samples confirms the uniform dispersion of the nanoparticles in the polymer without dispersion of aggregates of particles. Note that the _M_(_H_) cycles have a negligibly coercive field (_H\({}_{C}\)_) for _T_\(\geq\) 100 K (see Figure 4). This fact is very irregular, taking into account the large irreversibility observed in the _M_(_T_) curves measured under ZFC-FC conditions, which is usually associated to open magnetization cycles in the blocked regime. At low temperatures, _H\({}_{C}\)_ grows abruptly, evidencing an increase of the effective anisotropy. On the other hand, the _M_(_H_) behavior of the interacting particles is very different (Figure 3(b)).