Abstract:
The static magnetic properties of a system of NiFe$_2$O$_4$ nanoparticles (average size 4.5 nm) coated with polyvinyl alcohol (PVA) were studied. The analysis of the magnetization curves allowed us to conclude that the small particle size causes a pronounced core/shell structure with structural and magnetic order in the core and, accordingly, disorder in the particle shell. The thickness of the disordered surface layer is
$\approx$ 1 nm, and the ferrimagnetic core can only be formed in particles of about 4 nm or more in size. The coating of the particles leads to their spatial separation and a decrease in the intensity of magnetic interparticle interactions, which follows from a comparison with the data for a reference sample with identical particle sizes without PVA coating. The weakening of magnetic interparticle interactions is manifested in a decrease in the superparamagnetic blocking temperature of the magnetic moments of the particles caused by the ferrimagnetically ordered core. Another important consequence of the spatial separation of particles is the collapse of the common integrated surface for a cluster of particles, which is realized in the reference sample. This is manifested in a smaller value of the coercive force of the magnetic hysteresis loop (at $T$ = 4.2 K) of the studied system of particles coated with PVA. Also, the paper discusses the processes of freezing of atomic magnetic moments located in disordered areas of particles.
Keywords:nickel ferrite, nanoparticles, magnetic interparticle interactions.
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