We present a detailed study of static and dynamic magnetic behavior of Fe 3 O 4 nanoparticles with average particle sizes ͗d͘ ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, T V , shifts down to ϳ20 K for ͗d͘ϭ50 nm and is no longer observable for smaller particles. The smallest particles (͗d͘ϭ5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at T B ϳ45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization M S at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with ͗d͘ϭ5 nm, dynamic ac susceptibility measurements show a thermally activated Arrhenius-Néel dependence of the blocking temperature with applied frequency. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy. From the calculus of the magnetic anisotropy, it is inferred that there is no structural transition from cubic to triclinic symmetry for ͗d͘ϭ5 nm, in agreement with the absence of the Verwey transition. A value K 1 ϭ4.68ϫ10 5 erg/cm 3 is obtained for the magnetocrystalline anisotropy constant of the cubic phase.

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This review is focused on describing state-of-the-art synthetic routes for the preparation of magnetic nanoparticles useful for biomedical applications. In addition to this topic, we have also described in some detail some of the possible applications of magnetic nanoparticles in the field of biomedicine with special emphasis on showing the benefits of using nanoparticles. Finally, we have addressed some relevant findings on the importance of having well-defined synthetic routes to produce materials not only with similar physical features but also with similar crystallochemical characteristics.

Structural and magnetic properties of γ-Fe2O3 have been studied in isometric nanoparticles ranging from 3 to 14 nm with a narrow particle size distribution. Cation vacancy order is observed for particles larger than 5 nm in diameter giving rise to a cubic superstructure, while for the smallest particles these vacancies are disordered. All magnetic properties measured showed a strong dependence on the average crystallite size. For the ordered samples, saturation magnetization was found to decrease linearly with decreasing crystallite size due to a surface spin canting effect. However, a stronger decrease was observed in the disordered samples, suggesting that also an internal spin canting (cation vacancy order−disorder) has to be taken into account to explain the magnetic properties of nanoparticles. The room-temperature coercive field decreases with decreasing crystallite size; however at low temperatures, the coercivity increases as the size decreases, reaching values larger than 3000 Oe. A model to explain the magnetic properties of these particles considering both surface and order−disorder effects is proposed.

The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of "2-line" ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.crystal structure | disorder | nano-sized ferrimagnets | soil formation | strain T he structural and physical properties of ferrihydrite, an exclusively nano-sized ferric oxyhydroxide, are of importance in explaining its chemical reactivity and wide variety of occurrences. In both pristine and contaminated soils and sediments, ferrihydrite acts as a natural filter of inorganic contaminants through sorption reactions, thus affecting their transport and fate in the environment. Biomineralization of ferrihydrite as the inorganic iron core in ferritin-the protein mainly involved in iron storage and homeostasis in the human body-also occurs in a vast number of organisms (1). Bloom-forming marine diatoms, for example, use ferritin for enhanced iron storage (2), which suggests that ferrihydrite may also have underlying importance in primary productivity in the world's oceans.A well-known example of a nanomineral (3), ferrihydrite has no known crystalline counterpart formed in the laboratory or found in nature. As such, the basic crystal structure (4-7) and physical properties of ferrihydrite [e.g., density, composition (7, 8), and magnetic properties (9-14)] have remained controversial. A variety of structural models ha...