Surface anisotropy and surface-core interaction in Co-Ni-B and Fe-Ni-B dispersed amorphous nanoparticles.

We report on Monte Carlo simulations of the magnetization behavior of ~3 nm of (Fe0.26Ni0.74)50B50 and (Co0.25Ni0.75)65B35 ferromagnetic amorphous nanoparticles dispersed in a polymer matrix. Simulations based on a core-shell model, describing the nanoparticles as consisting of a ferromagnetically ordered core and a disordered surface shell, reproduce satisfactorily the significant features observed in hysteresis cycles, i.e., Hc(T) and Mr(T) curves. The results provide clear evidence of the major role played at low temperature by surface anisotropy and surface-core exchange interaction in determining some anomalies of the hysteresis cycles and the large increase in effective particle anisotropy at low temperature.