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.

magnetic fine particles exhibit most of the features attributed to glassy behavior, e.g., irreversibility in the hysteresis loops and in the zero-field-cooling and field-cooling curves extends up to very high fields, and aging and magnetic training phenomena occur. However, the multivalley energy structure of the glassy state can be strongly modified by a field-cooling process at a moderate field. Slow relaxation experiments demonstrate that the intrinsic energy barriers of the individual particles dominate the behavior of the system at high cooling fields, while the energy states corresponding to collective glassy behavior play the dominant role at low cooling fields. ͓S0163-1829͑99͒05421-1͔Fine magnetic particle systems show most of the features of glassy systems due to the random distribution of anisotropy axis, interparticle interactions, and surface effects. These main features 1 include the flattening of the field cooling susceptibility, 2 an increase in the magnetic viscosity, 3 the occurrence of aging effects, 4 the critical slowing down observed by ac susceptibility, 5 and the increase in the nonlinear susceptibility as the blocking temperature is approached from above.3 These features do not seem to be associated with a true spin-glass transition. Nevertheless, some authors claim that they reveal the existence of some kind of collective state. 3,6 Although this state is mostly attributed to the frustration induced by magnetic interactions between randomly distributed particles, 6 some studies suggest the dominant role of surface spin disorder.7 One of the facts that makes the behavior of these systems complex is the coexistence of the freezing associated with frustration and the intrinsic blocking of the particles. Consequently, depending on the time window of the experimental technique, one or both phenomena are observed. For example, blocking effects usually determine the results of Mössbauer spectroscopy, since the measured blocking temperature decreases with increasing interactions, 8 while freezing phenomena determine the thermal dependence of the cusp of the real part of the ac susceptibility for concentrated samples, which moves to higher temperatures with increasing interactions. In this paper, we show that the glassy state of strong interacting particles can be destroyed by a field-cooling process at a moderate magnetic field, which precludes a true phase transition. We also demonstrate that the dynamics of these systems is strongly affected by the initial magnetic moment configuration, in such a way that the glassy state determines the dynamic behavior only in low-cooling-field experiments, while at high cooling fields the dynamics is mostly dominated by the intrinsic energy barriers of the individual particles. These conclusions result from comparing the effective distribution of energy barriers obtained from the T ln (t/ 0 ) analysis of the magnetic relaxation 10 measured after field cooling the sample at different fields. The results of some aging experiments also reinforce these conclusio...

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...

Binding of Mn2+ or Mg2+ to the high-affinity site of the purple membrane from Halobacterium salinarium has been studied by superconducting quantum interference device magnetometry or by ab initio quantum mechanical calculations, respectively. The binding of Mn2+ cation, in a low-spin state, to the high-affinity site occurs through a major octahedral local symmetry character with a minor rhombic distortion and a coordination number of six. A molecular model of this binding site in the Schiff base vicinity is proposed. In this model, a Mg2+ cation interacts with one oxygen atom of the side chain of Asp85, with both oxygen atoms of Asp212 and with three water molecules. One of these water molecules is hydrogen bonded to both the nitrogen of the protonated Schiff base and the Asp85 oxygen. It could serve as a shuttle for the Schiff base proton to move to Asp85 in the L-M transition.

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