Magnetic Nanomaterials: Nonconventional Synthesis and Chemical Design
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The synthesis of nanomaterials presents a great interest not only for elucidation of basic research problems, but also for practical applications. Because of the presence of very small size nanoparticles, these materials acquire new properties that differ from those of bulk materials of the same composition. As a result of the special magnetic properties of small particles, nanomaterials find promising use in various applications: ferrofluids, information storage media, magnetic refrigeration, magnetic resonance imaging, etc.
Ferrites—magnetic materials—are composed of α-Fe2O3 and another metallic oxide MOn. The general formula of ferrites is xMOn · yFe2O3. If M is a divalent metal ion with ionic radius, r < 1 Å, a spinelic ferrite is formed: MO · Fe2O3 (n = 1). The “spinel structure” proceeds from the natural combination MgAl2O4 called “spinel.” Spinelic ferrite structure consists of a cubic close-packed oxygen arrangement, in which the cations reside on tetrahedral and octahedral interstices. The unit cell is obtained by the union of two face-centered cubic oxygen sublattices along each of three directions. This results in 64 tetrahedral sites (A) and 32 octahedral sites (B). Only 8 sites A and 16 sites B are occupied by cations in a stoichiometric spinel. The spinel compounds mainly belong to the space group Fd3m and the lattice parameter is ≈ 8.5 Å. Occupation of all the tetrahedral sites with divalent metal ions yields a normal spinel structure, while occupation of the octahedral sites with the divalent metal ions yields an inverse spinel structure. Ferrites can adopt both a normal spinel structure (M2 + = Zn2 +, Cd2 +, Mn2 +) and an inverse spinel one (M2 + = Fe2 +, Co2 +, Ni2 +, Cu2 +). Ferrites constitute a broad and important class of magnetic ceramic materials, with important technological applications which cover electronic and electrotechnical devices, heterogeneous catalysts, cement products, paints, and plastics. The magnetic properties of ferrites (magnetic saturation, magnetization, and coercivity) change drastically when the size of particles becomes very small. For this reason, the idea that synthesizing ferrites with nanometer-sized particles (nanoferrites) would help obtaining materials with superior magnetic properties is of great interest.