Magnetic interactions in silica coated nanoporous assemblies of CoFe₂O₄nanoparticles with cubic magnetic anisotropy

Abstract

Magnetic interactions in silica coated spherical nanoporous assemblies of CoFe(2)O(4) nanoparticles have been investigated by low temperature field dependent remanent magnetization (M(DCD) and M(IRM)) and magnetization relaxation measurements. The synthesis procedure leads to the formation of spherical aggregates of about 50-60 nm in diameter composed of hexagonal shaped nanocrystals with shared edges. The negative deviation from the non-interacting case in the Henkel plot indicates the predominance of dipole-dipole interactions favouring the demagnetized state, although the presence of exchange interactions in the porous system cannot be excluded. The activation volume, derived from time dependent magnetization measurements, turns out to be comparable with the particle physical volume, thus indicating, in agreement with static and dynamic irreversible magnetization measurements, that the magnetization reversal actually involves individual crystals.

Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles

“…44 ‫ܪ‬ is related to the activation volume (ܸ ் ) where the magnetization reversal initiates through ܸ ் = ݇ ‫ܯ(/ܶ‬ ௌ ‫ܪ‬ ) . 50,52 ܸ ் can be defined as the smallest volume of the material that reverses coherently in an event 53 .…”

The remanence ratio in CoFe2O4 nanoparticles with approximate single-domain sizes

“…Below a certain temperature, the canted surface spins freeze into a spin-glass-like state and the hysteresis loops obtained after a field cooling (FC) shift, as a consequence of the unidirectional anisotropy resulting from the coupling between the disordered surface layer and core spins [6, 13, 14]. Furthermore, the interaction between the surface spins of different particles enhances the effective anisotropy [7, 15], making the surface anisotropy constant many orders higher than that of the bulk material [16]. …”

Magnetic behavior of nanocrystalline CoFe2O4

“…In recent years, research interest in magnetic ferrite nanostructures has greatly increased due to their importance in understanding the fundamentals for the magnetic properties and their extensive application in high-density magnetic storage, ferrofluid technology, hyperthermia treatment, magnetic resonance imaging (MRI), drug delivery, antitumor and protein separation [5][6][7]. Among these ferrite materials, cobalt ferrite (CoFe 2 O 4 ) nanoparticles have received a lot of attention due to their remarkable properties, such as moderate saturation magnetization, relatively large magnetic anisotropy, incredible mechanical hardness and chemical stability [8][9][10].…”