Experimental observations of field-dependent activation of core and surface spins inNi-ferrite nanoparticles

Abstract

The magnetic behavior of Ni-ferrite(NiFe2O4)nanoparticles synthesized in a solid state reaction process has been investigated. The cooling field,HCF, dependence of magnetization in a wide range of temperatures, from 5 to 300 K, has beenexamined for low and high field regimes. It has been observed that there is a transition region,∼3–4 T, between different mechanisms that controls the magnetization. At low fields,<3 T, classical blocking–unblocking of small particles governs themagnetization whereas spin-glass like behavior prevails at high fields,>4 T, starting below a well defined freezing temperature of 50 K. TheHCF dependence of magnetic viscosity has shown that there is a significant jump in therelaxation rate of the particles around 4 T which appears as the boundary region for thetemperature-dependent magnetization as well. These observations are interpreted asindicating that below the spin freezing temperature there is a boundary field(∼4 T) where the strongly pinned surface spins are enabled to be thermally activated whilebelow this field only core spins participate in the magnetic relaxation.