A systematic study of Co(SiO 2 ) granular films by means of transmission electron microscopy ͑TEM͒, dc and ac initial magnetic susceptibility, and thermoremanent magnetization ͑TRM͒ is presented. The experimental results are compared with simulations of zero-field-cooled ͑ZFC͒ and field-cooled ͑FC͒ magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length ⌳, which results in a larger average ''magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ''particle clusters'' of volume ⌳ 3 , involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature ͗T B ͘ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

We have studied the magnetic behavior of dextran-coated magnetite (Fe 3 O 4 ) nanoparticles with median particle size d = 8 nm. Magnetization curves and in-field Mössbauer spectroscopy measurements showed that the magnetic moment M S of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or non-saturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy K ef f from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulk-like ferrimagnetic structure with ordered A and B sublattices, but nearly compensated magnetic moments. The dependence of the blocking temperature with frequency and applied fields, T B (H, ω), suggests that the observed non-monotonic behavior is governed by the strength of interparticle interactions.

Magnetic properties of Co-SiO 2 granular films are displayed and explained by means of a phenomenological model that takes into account the effects of magnetic interparticle coupling on the superparamagnetic relaxation behavior. The model is based on the analysis of coupled nanoparticles through a modified random anisotropy model that takes into account the concentration and size of the nanoscopic particles, as well as the field dependence of the correlation length. The proposed model leads to an accurate description of the field dependence of the blocking temperature, substantially better than the power law usually employed to describe noninteracting particles.

We have investigated the dynamic magnetic properties of dextran-coated magnetite (Fe3O4) nanoparticles in the form of (a) particles suspended in a carrier liquid and (b) concentrated powder obtained from lyophilization. The blocking temperature was found to increase from TB=42(2)to52(2)K (@μ0H=10mT) after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field Bhyp(T) reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy Ea=3.6×10−21J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K1=10kJ∕m3 was obtained by assuming the cubic phase with easy magnetic direction [111] of the bulk material above the Verwey transition, supporting the idea that the Verwey transition is absent in nanosized particles. Accordingly, no indication of magnetic transition at TV could be observed in our measurements. From the dynamical parameters of ac susceptibility χ(f,T) curves, the contribution of the dipolar interactions to the total anisotropy energy barrier could be estimated to be Ω=4.5×10−21J, larger than the single-particle value.

The compound [CoCu(opba)(DMSO) 3 ] (1) [opba = orthophenylenebis(oxamato)] has been synthesized and characterized. Its crystal structure has been analyzed by X-ray diffraction techniques at 100 and 298 K. A structural phase-transition has been detected at around 150 K. An orthorhombic crystalline system is found at both temperatures, with very similar unit-cell dimensions. At room temperature 1 crystallizes in the Pnam space group (α-1 phase), with a = 7.6712(2), b = 14.8003(3), c = 21.0028(5) Å, and Z = 4, whereas at low temperature it crystallizes in the Pna2 1 space group (β-1 phase), with a = 7.3530(2), b = 14.5928(4), c = 21.0510(7) Å, and Z = 4. Both crystalline phases consist of linearly ordered bimetallic chains with the [Cu(opba)] 2-units tied by Co II ions to form a one-dimensional system. The DMSO molecules in α-1, which are coordinated to either Cu II or Co II , are disor-