Interparticle Magnetic Interactions and the Field Dependence of the Superparamagnetic Blocking Temperature in a Powder System of Ultrasmall Nickel Ferrite Particles

The dependence of the superparamagnetic blocking temperature on an external magnetic field T_B(H) has been studied and analyzed for a nickel ferrite nanoparticle powder system in order to establish the influence of magnetic interparticle interactions on superparamagnetic blocking. The features of this system are: (i) a small particle size (~4–5 nm on average); (ii) a pronounced “core/shell” structure of particles, in which the magnetic moment of a particle is formed by a ferrimagnetically ordered core, while the spins of the surface layer, about 1 nm thick, do not contribute to this magnetic moment. The random anisotropy model, which describes the influence of magnetic interparticle interactions on the T_B value in the external field, is used to reproduce the experimental dependence T_B(H) obtained by static magnetometry. The analysis has demonstrated strong magnetic interactions in the studied system, which are manifested in a sharp decrease in the T_B value in the weak-field region, and has made it possible to quantitatively estimate the intensity and energy of magnetic interparticle interactions, as well as to determine the magnetic anisotropy constant of individual particles (without the influence of magnetic interparticle interactions). The role of the subsystem of surface spins, which, according to the imaginary part of the magnetic susceptibility, exhibits signatures of collective behavior, is discussed as a possible source of magnetic interparticle interactions.