Monodisperse magnetite Fe 3 O 4 nanoparticles of controlled size within 6 and 20 nm in diameter were synthesized by thermal decomposition of an iron organic precursor in an organic medium. Particles were coated with oleic acid. For all samples studied, saturation magnetization M s reaches the expected value for bulk magnetite, in contrast to results in small particle systems for which M s is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is also similar to that of bulk magnetite. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder. However, although the saturated state may be similar, the approach to saturation is different and, in particular, the high-field differential susceptibility is one order of magnitude larger than in bulk materials. The relevance of these results in biomedical applications is discussed.

Some of the main experimental observations related to the occurrence of exchange bias in magnetic systems are reviewed, focusing the attention on the peculiar phenomenology associated to nanoparticles with core/shell structure as compared to thin film bilayers. The main open questions posed by the experimental observations are presented and contrasted to existing theories and models for exchange bias formulated up to date. We also present results of simulations based on a simple model of a core/shell nanoparticle in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and at the interfacial regions, offering new insight on the microscopic origin of the experimental phenomenology. Adetailed study of the magnetic order of the interfacial spins shows compelling evidence that most of the experimentally observed effects can be qualitatively accounted within the context of this model and allows also to quantify the magnitude of the loop shifts in striking agreement with the macroscopic observed values.

Some of the main experimental observations related to the occurrence of exchange bias in magnetic systems are reviewed, focusing the attention on the peculiar phenomenology associated to nanoparticles with core/shell structure as compared to thin film bilayers. The main open questions posed by the experimental observations are presented and contrasted to existing theories and models for exchange bias formulated up to date. We also present results of simulations based on a simple model of a core/shell nanoparticle in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and at the interfacial regions, offering new insight on the microscopic origin of the experimental phenomenology. A detailed study of the magnetic order of the interfacial spins shows compelling evidence that most of the experimentally observed effects can be qualitatively accounted within the context of this model and allows also to quantify the magnitude of the loop shifts in striking agreement with the macroscopic observed values.

This chapter is aimed at studying the anomalous magnetic properties (glassy behaviour) observed at low temperatures in nanoparticles of ferrimagnetic oxides. This topic is discussed both from numerical results and experimental data. Ferrimagnetic fine particles show most of the features of glassy systems due to the random distribution of anisotropy axis, interparticle interactions and surface effects. Experiments have shown that the hysteresis loops display high closure fields with high values of the differential susceptibility. Low magnetisation as compared to bulk, shifted loops after field-cooling, highfield irreversibilities between zero-field and field cooling processes and ageing phenomena in the time-dependence of the magnetisation, are also observed. This phenomenology indicates the existence of some kind of freezing phenomenon arising from a complex hierarchy of the energy levels, whose origin is currently under discussion. Two models have been proposed to account for it: i) the existence of a spin-glass state at the surface of the particle which is coupled to the particle core through an exchange field; and ii) the collective behaviour induced by interparticle interactions. In real systems, both contributions simultaneously occur, being difficult to distinguish their effects. In contrast, numerical simulations allow us to build a model just containing the essential ingredients to study solely one of two phenomena. Glassy behaviour in ferrimagnetic nanoparticlesThe assemblies of fine magnetic particles with large packing fractions and/or nanometric sizes show most of the features which are characteristic of glassy systems (for a recent review see Ref. [12]). This glassy behaviour results from a complex interplay between surface and finite-size effects, interparticle interactions and the random distribution of anisotropy axis throughout the system. In many cases, these contributions are mixed and in competition, Sample characterisationThe phenomenology of the glassy state in strong interacting fine particles is illustrated through the study of the magnetic properties of nanocrystalline BaFe 10.4 Co 0.8 Ti 0.8 O 19 [11]. M -type barium ferrites have been studied for a long time because of their technological applications [33,34,35,36], such as microwave devices, permanent magnets, and high-density magnetic and magneto-optic recording media, as well as their large pure research interest [37,38]. The compounds obtained by cationic substitution of the pure BaFe 12 O 19 ferrite display a large variety of magnetic properties and structures, which go from collinear ferrimagnetism to spin-glass-like behaviour [37,39,38], depending on the degree of frustration introduced by cationic substitution. In particular, the BaFe 10.4 Co 0.8 Ti 0.8 O 19 compound seems to be ideal for perpendicular magnetic recording [40], since the Co 2+ -Ti 4+ doping scheme reduces sharply the high values of the coercive field of the pure compound, which precludes their technological applications. For this composition the magnetic structur...

We describe the presence of a high frequency of spontaneous chromosome aberrations in lymphocytes from six untreated patients with Hodgkin's disease. The characteristics of the chromosome abnormalities observed suggest the existence of a certain degree of chromosome instability in these cases, that could be a predisposing factor for the development of malignancies.

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The effect of interactions on the magnetic relaxation of nanocrystalline hexagonal barium hexaferrite BaFe 10.4 Co 0.8 Ti 0.8 O 19 is discussed. We had previously shown that according to the T ln(t/ 0 ) scaling, an enhancement of the lowest-energy barriers was detected when demagnetizing interactions were dominant. Also, the Henkel plots obtained for particles of about 10 nm of mean diameter showed that the overall interactions were demagnetizing. In the present work, we have modified the interactions by milling the particles with a nanosized SiO 2 powder. Dipolar interactions are modified by breaking the particle aggregates. The observed overall interactions resulted to be also demagnetizing for the milled sample. The time dependence of the magnetization was analyzed according to two different procedures: the fluctuation field and activation volume analysis and the T ln(t/ 0 ) scaling. Activation volumes were found to increase with demagnetizing interactions and the leading demagnetizing mechanism appeared to shift from an individual particle mode for the unmilled sample to a collective one for the milled sample. The second approach showed larger relaxation rates at short times for the milled sample. The effective energy barrier distribution obtained from the scaling suggested that demagnetizing interactions increased in the milled sample, which led to an enhancement of the amount of the lowest-energy barriers. © 1997 American Institute of Physics. ͓S0021-8979͑97͒57608-0͔M -type barium hexaferrite has been widely studied because of its technological applicability. In the last years many efforts have been devoted to the development of synthesis procedures that leads to controlled-size particles, and to the knowledge of its basic properties. The study of the magnetic properties of barium hexaferrite nanoparticles showed that interparticle interactions can affect their magnetic behaviour. The selected system has previously shown demagnetizing interactions.1 In the present work, we have modified the interactions by milling the particles with a nanosized SiO 2 powder. Dipolar interactions are modified by breaking the particle aggregates. According to the results from Ido et al., 2 the resultant interaction will depend on the way the particles were grouped. Barium hexaferrite particles tend to pile up, leading to stacks, where interactions are magnetizing, although particle clusters are also observed, where interactions are demagnetizing. The effect of interactions on the magnetic relaxation has been analyzed in terms of fluctuation field and activation volume. To complete the analysis and the interpretation of the results, the T ln͑t/ 0 ͒ scaling of the time dependence of the thermoremanence ͑which provides the energy barrier distribution͒ have been done. Nanocrystalline BaFe 10.4 Co 0.8 Ti 0.8 O 19 particles, with a mean particle diameter of about 10 nm, were prepared by using the glass crystallization method.3 Two different samples were prepared compacting the nanocrystalline powder by cold pressing ͑in order to a...

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