Synthesis and magnetic properties of core-shell structured (NiCo)O(AFM)-NiCo(FM) magnetic nanoparticles

We have succeeded in the synthesis of nanometer size monodispersed Ni-Co metal particles through the modified-polyol process. Ni-Co metal particles of about 40 and 100 nm with Co-rich core and Ni-rich shell were produced by the manipulation of the reaction kinetics through a suitable selection of hydroxyl ion concentration in the ethylene glycol-Ni-Co system. Furthermore, core-shell structured (NiCo)O(AFM)-NiCo(FM) was produced by oxidizing particles at 533 K in air for different times. The magnetic properties, especially the exchange-bias (HE) fields of 40 and 100-nm core-shell structured particles synthesized under similar conditions behaved differently. However, their behavior was explained considering exchange biased interactions at (NiCo)O(AFM)-NiCo(FM), and (Co)O(AFM)-Co(FM) interfaces with two different exchange coupling constants. The 40 nm particles oxidized for 4 h and composed of (NiCo)0(AFM)-NiCo(FM), exhibited a maximum H-E field of 36 and 201 Oe at 298 and 77 K, respectively.