Interparticle interactions in (Fe1-xNix)(100-y)B-y magnetic nanoparticles

In order to study the effect of interparticle interactions the magnetic pro perties of amorphous (Fe1-xNix)(100-y)B-y nanoparticles (0.25 < x < 0.75 an d y approximate to 0.50) have been investigated by magnetization measuremen ts in a homogeneous dispersion of particles in a PVP polymer and in powder samples. The dispersed samples show the characteristic behaviour of an asse mbly of independent nanosized particles, characterized by a narrow size dis tribution, with a sharp maximum of zero field cooled (ZFC) magnetization wh ereas the field cooled (FC) magnetization increases continuously with decre asing temperature. The magnetization in the superparamagnetic regime is sat isfactorily fitted to a superposition of Langevin functions due to the volu me distribution. On the other hand, in the powder samples, where strong int erparticle interactions are expected, the M-ZFC shows a wider maximum with a significant increase of the temperature at which it occurs and M-FC prese nts a plateau at low temperature. The magnetization curves for the powder s amples can be fitted to a Langevin function if interparticle interactions a re taken into account by introducing a mean-field-like term in the argument .