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.

A single-step synthesis of magnesium ferrite (MgFe 2 O 4 ) nanoparticles with an average crystallite size of about 8.5 nm synthesized via mechanochemical processing of binary oxide precursors at room temperature is reported. The study highlights the nature of the cation disorder and of the spin arrangement in mechanosynthesized MgFe 2 O 4 as well as its response to changes in temperature. An unusual property of the magnetization enhancement in nanoscale mechanosynthesized MgFe 2 O 4 is reported. Whereas the inner core of a MgFe 2 O 4 nanoparticle exhibits a partly inverse spinel structure with a Ne ´el type collinear spin alignment, the major features of the ionic and spin configurations in the grain boundary (surface) region are a nonequilibrium cation distribution and a canted spin arrangement. Although the spin-canting effect tends to reduce the magnetic moment, the magnetization enhancement exhibited by mechanosynthesized MgFe 2 O 4 is attributed to the nearly random distribution of magnetic cations in the surface regions of nanoparticles. On heating above 623 K, the mechanosynthesized MgFe 2 O 4 relaxes to a structural and magnetic state that is similar to the bulk one.

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A one-step synthesis of nanostructured bismuth ferrite (BiFeO3) via mechanochemical processing of a α-Fe2O3/Bi2O3 mixture at room temperature is reported. The mechanically induced phase evolution of the mixture is followed by XRD and 57Fe Mössbauer spectroscopy. It is shown that the mechanosynthesis of the rhombohedrally distorted perovskite BiFeO3 phase is completed after 12 h. Compared to the traditional synthesis route, the mechanochemical process used here represents a one-step, high-yield, low-temperature, and low-cost procedure for the synthesis of BiFeO3. High-resolution TEM and XRD studies reveal a nonuniform structure of mechanosynthesized BiFeO3 nanoparticles consisting of a crystalline core surrounded by an amorphous surface shell. The latter is found to exhibit an extraordinarily high metastability causing a rapid crystallization of nanoparticles under irradiation with electrons. In situ high-resolution TEM observations of the crystallization clearly show that the heterogeneous processes of nucleation and growth of bismuth iron oxide crystallites are spatially confined to the amorphous surface regions. This fact provides access to the elucidation of the mechanism of mechanosynthesis. It is demonstrated that the mechanosynthesized ferrite nanoparticles exhibit a partial superparamagnetism at room temperature. Quantitative information on the short-range structure and hyperfine interactions, provided by the nuclear spectroscopic technique, is complemented by an investigation of the magnetic behavior of nanostructured BiFeO3 on a macroscopic scale by means of SQUID technique. As a consequence of canted spins in the surface shell of nanoparticles, the mechanosynthesized BiFeO3 exhibits an enhanced magnetization, an enhanced coercivity, and a shifted hysteresis loop.

Zinc stannate (Zn 2 SnO 4 ) nanoparticles with an average size of about 26 nm are synthesized via singlestep mechanochemical processing of binary oxide precursors (ZnO and SnO 2 ) at ambient temperature, without the need for the subsequent calcination, thus making the synthesis route very simple and costeffective. The mechanically induced phase evolution of the 2ZnO + SnO 2 mixture is followed by XRD and by a variety of spectroscopic techniques including 119 Sn MAS NMR, Raman spectroscopy, 119 Sn M€ ossbauer spectroscopy, and XPS. High-resolution TEM studies reveal a non-uniform structure of mechanosynthesized Zn 2 SnO 4 nanoparticles consisting of a crystalline core surrounded by a structurally disordered surface shell. Due to the ability of the applied solid-state spectroscopies to probe the local environment of Sn cations, valuable complementary insight into the nature of the local structural disorder of mechanosynthesized Zn 2 SnO 4 is obtained. The findings hint at a highly nonequilibrium state of the as-prepared stannate characterized by its partly inverse spinel structure and the presence of deformed polyhedra in the surface shell of nanoparticles.