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 similar to 3 nm of (Fe0.26Ni0.74)(50)B-50 and (Co0.25Ni0.75)(65)B-35 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., H-c(T) and M-r(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.