Nanocrystalline Nickel Ferrite, NiFe2O4: Mechanosynthesis, Nonequilibrium Cation Distribution, Canted Spin Arrangement, and Magnetic Behavior

Abstract

Nickel ferrite (NiFe2O4) nanoparticles with an average crystallite size of about 8.6 nm were prepared by mechanochemical synthesis (mechanosynthesis). In-field Mossbauer spectroscopy and high-resolution TEM studies revealed a nonuniform structure of mechanosynthesized NiFe2O4 nanoparticles consisting of an ordered core surrounded by a disordered grain boundary (surface) region. The inner core of a NiFe2O4 nanoparticle is considered to possess a fully inverse spinel structure with a Neel-type collinear spin alignment, whereas the surface shell is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. As a consequence of frustrated superexchange interactions in the surface shell, the mechanosynthesized NiFe2O4 exhibits a reduced nonsaturating magnetization, an enhanced coercivity, and a shifted hysteresis loop. The study also demonstrates that one can tailor the magnetic properties of mechanosynthesized NiFe2O4 particles by suit...