Spin-3/2 model of noninteracting nanoparticles with crystal field effects

Spin-3/2 model of noninteracting single domain nanoparticles (NPs) has been presented based on the pair approximation. Magnetic properties of homogeneous ferromagnetic (F) and composite NPs with crystal field effects having inner antiferromagnetic (AF) (and F) core, a F (and AF) coupling between the core and surface and outer FM surface for various diameters and temperatures have been studied. The free energy is calculated and minimized with respect to pair variables to obtain the field-cooled magnetization for spin-3/2 Ising system. Hysteresis loops have been plotted for various values of crystal fields, exchange coupling constants, and axial anisotropy of strength which couples the core, core-surface and surface regions within the NPs. The coercive field and its linear fit to the data plotted as a function of diameters of single domain NPs.