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

摘要

We report on Monte Carlo simulations of the magnetization behavior of ～3 nm of (Fe_(0.26)Ni_(0.74))_(50)B_(50) and (Co_(0.25)Ni_(0.75))_(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.