Effect of interparticle interactions in (Fe0.26Ni0.74)50B50 magnetic nanoparticles

The magnetic properties of amorphous (Fe0.26Ni0.74)50B50 nanoparticles (mean diameter: 2 nm), synthesized by chemical route, have been investigated by magnetization measurements. The results obtained on a powder sample and on homogeneous diluted dispersion of particles in a polymer matrix, prepared from the same batch, are compared. The dispersed sample shows the characteristic behaviour of an assembly of independent magnetic nanoparticles, with narrow size distribution, whose moments block progressively with decreasing temperature: the zero field cooled magnetization, MZFC, shows a narrow maximum (Tmax=55 K), whereas the field cooled one (MFC) increases continuously with decreasing temperature; in the high-temperature superparamagnetic regime, the magnetization curves are satisfactorily fitted to a superposition of Langevin functions, due to the volume distribution. In the powder sample, because of interparticle interactions, Tmax increases (Tmax∼100 K) and MFC shows a plateau below ∼30 K. Moreover, significant changes are observed at low temperature in the shape of the hysteresis cycles and in the temperature dependence of the remanent magnetization and coercive field, both showing a maximum at ∼15 K. The observed features suggest a change of regime at low temperature to a disordered collective magnetic state of particle moments