Magnetic and NMR relaxivity properties of γ-Fe 2 O 3 nanoparticles embedded into the walls of polyelectrolyte multilayer capsules and freely dispersed in a sodium borate buffer solution have been investigated. The different geometric distribution of both configurations provides the opportunity to study the relationship of water accessibility and magnetic properties of particles on the NMR relaxivity. Changes in their blocking temperature and average dipolar field were modeled as a function of packing fraction in the ensemble of free and entrapped nanoparticles. For free nanoparticles with relatively low concentration, relaxivity values increase with packing fraction according to an increase in the dipolar field and larger water accessibility. However for embedded NPs in the capsule wall, packing fractions should be limited to optimise the efficiency of this system as magnetic resonance imaging (MRI) contrast agent.

The phase distribution of high aspect ratio, Fe-filled carbon nanotubes prepared by pyrolyzing a mixture of powered ferrocene and C 60 has been determined by means of Mössbauer spectroscopy. Our results for that characterization are closely related to the observation, after field cooling processes, of a hysteresis loop shift and clearly suggest a spatial phase distribution which includes the presence of a ␥-Fe/␣-Fe interface. The temperature dependence of the hysteresis loop shift is discussed in terms of localized regions at that interface exhibiting uncompensated antiferromagnetism within reduced dimensions. DOI: 10.1103/PhysRevB.65.113405 PACS number͑s͒: 75.70.Cn, 76.80.ϩy, 81.07.De Since their first synthesis carbon nanotubes have been appreciated by their unique combination of electronic and mechanical properties 1,2 that make them a very promising candidate system to be used on the development of a broad range of new devices, which already includes field emission lamps, nanotransistors, spin polarized electron sources, flat display panels, and hydrogen storing systems. 3,4 Nevertheless, one of the most natural and attractive possibilities of these materials, closely linked to their morphology and extremely high aspect ratio, is their use as nanocontainers for secondary phases and, particularly, for magnetically ordered ones. 5 In fact, the encapsulation of magnetic phases in carbon nanotubes could constitute, due to the very large magnetic shape anisotropies acting on the encapsulated material, a feasible approach to, on the one side, the achievement of magnetic order stabilization against thermal fluctuations in systems having extremely reduced dimensions and, on the other, the exploration of the physics of the magnetic order in close-to-one-dimensional structures. From the point of view of the practical uses, carbon nanotubes filled with magnetic phases could also develop giant coercivities ͑coercivity values above those predictable on the basis of the bulk magnetocrystalline anisotropy͒ linked to their reduced transverse dimension and the consequential large influence on the magnetization reversal phenomenology of the reduced symmetry, high anisotropy interfaces. The control of the coercivity of these magnetic phase-filled nanotubes would make possible their use as very high density recording media, as magnetoresistive elements suitable to be packed to high densities, as probes for magnetic force microscopy, and, even, as recording heads.In this work, we present and discuss results about the structural and magnetic properties of Fe-filled nanotubes. We will show how the presence in the as-prepared samples of antiferromagnetic fcc ␥-Fe results, through the coupling to bcc ␣-Fe, on the occurrence of the so-called exchange biasing a phenomenology underlying the operation of the spin valves, one of the most relevant developments of the magnetoelectronics.Fe-filled carbon nanotubes were prepared by pyrolyzing a 1:1 mixture ͑by weight͒ of powered ferrocene and C 60 at atmospheric pressure under an Ar atmo...

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