Magnetization enhancement at low T due to shell ordering in Fe-Ni-B amourphous nanoparticles

E. DE BIASI; R. D. ZYSLER; C. A. RAMOS; H. ROMERO

San Carlos de Bariloche, Argentina

Congreso; Fifth Latin American Workshop on Magnetism, Magnetic Materials and theirs Applications; 2001

We report magnetization measurement up to 5 Teslas as a function temperature (T) from 5 K up to 300 K in an amourphous Fe-Ni-B nanoparticle (aprox. 3 nm diameter) system. These nanoparticles were dispersed in a non magnetic polimer matrix so that their interparticle interactions could be neglected (R. D. Zyslser et at. J. Magn. Magn. Mat. 221, 37 (2000)). The results are contrasted with a concetrated powder sample (PS). Magnetizationes measurements show a sudden increase of the saturation value below 40 K. This behaviour is observed in the dispersed sample system (DS) and also in the undispersed PS. Being noticeably more pronounced in the firts one. This result can be explained considering that the large magnetic contribution at low T is due to the particle surface magnetic ordering along its ferromagnetic core. High T susceptibility (X) data show large difference between the DS and PS: while the first one shows  a Curie-like behavior, X(T) in the PS remains constant, wihch we interpret as the surface frustration due to the interactions.