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

Nickel ferrite (NiFe 2 O 4 ) nanoparticles with an average crystallite size of about 8.6 nm were prepared by mechanochemical synthesis (mechanosynthesis). In-field Mössbauer spectroscopy and high-resolution TEM studies revealed a nonuniform structure of mechanosynthesized NiFe 2 O 4 nanoparticles consisting of an ordered core surrounded by a disordered grain boundary (surface) region. The inner core of a NiFe 2 O 4 nanoparticle is considered to possess a fully inverse spinel structure with a Néel-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 NiFe 2 O 4 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 NiFe 2 O 4 particles by suitably controlling their size. The thickness of the surface shell of about 1 nm estimated from size-dependent magnetization measurements is found to be in good agreement with that obtained from high-resolution TEM and Mössbauer experiments. On heating above 673 K, the mechanosynthesized NiFe 2 O 4 relaxes to a structural and magnetic state that is similar to the bulk one.