Some of the most relevant finite-size and surface effects in the magnetic and transport properties of magnetic fine particles and granular solids are reviewed. The stability of the particle magnetization, superparamagnetic regime and the magnetic relaxation are discussed. New phenomena appearing due to interparticle interactions, such as the collective state and non-equilibrium dynamics, are presented. Surface anisotropy and disorder, spin-wave excitations, as well as the enhancements of the coercive field and particle magnetization are also reviewed. The competition of surface and finite-size effects to settle the magnetic behaviour is addressed. Finally, two of the most relevant phenomena in the transport properties of granular solids are summarized namely, giant magnetoresistance in granular heterogeneous alloys and Coulomb gap in insulating granular solids.

The influence of a variety of parameters on the synthesis of iron oxide nanoparticles (magnetite/maghemite Fe 3 O 4 /γ-Fe 2 O 3 ) by thermal decomposition of a metal-organic iron precursor in an organic medium is reported. We study the role of both the surfactant and the reducing agent on the shape, the size distribution, and the magnetic properties. We aim at synthesizing magnetic nanoparticles with high crystal quality and good magnetic response. A narrow size distribution of pseudospherical and faceted particles (4-20 nm) with a high saturation magnetization (M s ≈ 80-85 emu/g at 5 K) is obtained when using oleic acid as a surfactant. In contrast, decanoic acid yields much larger pseudocubic particles (45 nm) with a wider size distribution and a larger saturation magnetization (M s ) 92 emu/g at 5 K), close to the expected value for bulk magnetite. Besides, the use of a variety of reducing agents monitors the magnetic behavior. In the case of 1,2-hexadecanediol, magnetic characterization suggests that the nanoparticles have uniform oxidation. However, those particles prepared without the use of any reducing agent also show uniform oxidation just with a slightly smaller value of the saturation magnetization (M s ) 76 emu/g at 5 K). In contrast, hydrazine seems to promote a nonuniform oxidation that results in the appearance of the exchange bias phenomenon and in a smaller saturation magnetization (M s ) 67 emu/g at 5 K). New ways to tune the shape, the size, and the magnetic properties are discussed.

We report on the effect of using decanoic acid as capping ligand on the synthesis of iron oxide nanoparticles by thermal decomposition of an organic iron precursor in organic medium. This procedure allowed us to control the particle size within 5 nm and about 30 nm by modifying the precursor-to-capping ligand ratio in a systematic fashion and to further expand the particle size range up to about 50 nm by adjusting the final synthesis temperature. The nanoparticles also showed high saturation magnetization of about 80-83 emu/g at low temperature, almost size-independent and close to the value for the bulk counterpart. Decanoic acid-coated nanoparticles were transferred to water by using tetramethylammonium hydroxide, which allowed further coating with silica in a tetraethyl orthosilicate solution. Consequently, these iron oxide nanoparticles are tunable in size and highly magnetic, and they could become suitable candidates for various biomedical applications such as contrast agents for magnetic resonance imaging and magnetic carriers for drug delivery.