Magnetization enhancement at low temperature due to surface ordering in Fe–Ni–B amorphous nanoparticles

We report magnetization measurements up to 5 T as a function of temperature from 5 to 300 K in an amorphous Fe–Ni–B system nanoparticle (average diameter 3 nm). These nanoparticles were dispersed in a non-magnetic polymer matrix to minimize interparticle interactions and the results were contrasted with a concentrated powder sample. The saturation magnetization shows a sudden increase below 40 K. This behavior is observed in both the dispersed and the concentrated systems, being noticeably more pronounced in the first. This result can be explained considering that the large magnetic contribution at low T is due to the magnetic ordering of the particle surface.