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 Mössbauer 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 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 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 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 NiFe2O4 relaxes to a structural and magnetic state that is similar to the bulk one.

The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism

“…24 As can be seen in Fig. 9, the present experimental data M 1/3 sat and 1/D indeed show a good linear relationship.…”

“…24 In the following, we will estimate the shell thickness using the experimentally determined D and M sat values of BaFe 12 O 19 milled for various times (see Table 2). Assuming that t is independent of D and that the shell is magnetically "dead" (M shell ¼ 0), the variation of M sat with D will then be described by…”

Exchange-biased NiFe2O4/NiO nanocomposites derived from NiFe-layered double hydroxides as a single precursor

“…According to the literature, the T B value of single-phase NiFe 2 O 4 nanoparticles is much lower than room temperature when the particle size of the NiFe 2 O 4 is smaller than 10 nm. For instance, the T B value of NiFe 2 O 4 nanoparticles with a diameter of ~6.4 nm ± 1.4 nm has been reported to be about 95 K at a magnetic field of 100 Oe [41], while a value of 60 K has been reported for NiFe 2 O 4 with a particle size of 8.6 nm with the same magnetic field [42]. In contrast, in NF-3-500, at a magnetic field of 100 Oe the nanoparticles displayed a T B of 380 K (Fig.…”

Study of structure and magnetic properties of Ni–Zn ferrite nano-particles synthesized via co-precipitation and reverse micro-emulsion technique

“…The chemical methods have advantages over the physical one such as low cost, reaction taking place at low temperature, and largescale production possibility. It is widely appreciated that the cation distribution in spinel ferrites upon which many physical and chemical properties depend is a complex function of processing parameters and depends on the preparation method of the material (Sepelak et al 2007). Selection of an appropriate method is the key factor to obtain ferrites of high quality.…”