G. Varvaro

Magnetic interactions in silica coated nanoporous assemblies of CoFe2O4 nanoparticles with cubic magnetic anisotropy.

Magnetic interactions in silica coated spherical nanoporous assemblies of CoFe(2)O(4) nanoparticles have been investigated by low temperature field dependent remanent magnetization (M(DCD) and M(IRM)) and magnetization relaxation measurements. The synthesis procedure leads to the formation of spherical aggregates of about 50-60 nm in diameter composed of hexagonal shaped nanocrystals with shared edges. The negative deviation from the non-interacting case in the Henkel plot indicates the predominance of dipole-dipole interactions favouring the demagnetized state, although the presence of exchange interactions in the porous system cannot be excluded. The activation volume, derived from time dependent magnetization measurements, turns out to be comparable with the particle physical volume, thus indicating, in agreement with static and dynamic irreversible magnetization measurements, that the magnetization reversal actually involves individual crystals.

Bioactive, nanostructured Si-substituted hydroxyapatite coatings on titanium prepared by pulsed laser deposition

AIMS The aim of this work was to deposit silicon-substituted hydroxyapatite (Si-HAp) coatings on titanium for biomedical applications, since it is known that Si-HAp is able to promote osteoblastic cells activity, resulting in the enhanced bone ingrowth. MATERIALS AND METHODS Pulsed laser deposition (PLD) method was used for coatings preparation. For depositions, Si-HAp targets (1.4 wt % of Si), made up from nanopowders synthesized by wet method, were used. RESULTS Microstructural and mechanical properties of the produced coatings, as a function of substrate temperature, were investigated by scanning electron and atomic force microscopies, X-ray diffraction, Fourier transform infrared spectroscopy, and Vickers microhardness. In the temperature range of 400-600°C, 1.4-1.5 µm thick Si-HAp films, presenting composition similar to that of the used target, were deposited. The prepared coatings were dense, crystalline, and nanostructured, characterized by nanotopography of surface and enhanced hardness. Whereas the substrate temperature of 750°C was too high and led to the HAp decomposition. Moreover, the bioactivity of coatings was evaluated by in vitro tests in an osteoblastic/osteoclastic culture medium (α-Modified Eagles Medium). CONCLUSIONS The prepared bioactive Si-HAp coatings could be considered for applications in orthopedics and dentistry to improve the osteointegration of bone implants.

Folate targeted coated SPIONs as efficient tool for MRI

The development of more sensitive diagnostic tools allowing an early-stage and highly efficient medical imaging of tumors remains a challenge. Magnetic nanoparticles seem to be the contrast agents with the highest potential, if properly constructed. Therefore, in this study, hybrid magnetic nanoarchitectures were developed using a new amphiphilic inulin-based graft copolymer (INU-LAPEG-FA) as coating material for 10-nm spinel iron oxide (magnetite, Fe3O4) superparamagnetic nanoparticles (SPION). Folic acid (FA) covalently linked to the coating copolymer in order to be exposed onto the nanoparticle surface was chosen as the targeting agent because folate receptors are upregulated in many cancer types. Physicochemical characterization and in vitro biocompatibility study was then performed on the prepared magnetic nanoparticles. The improved targeting and imaging properties of the prepared FA-SPIONs were further evaluated in nude mice using 7-Tesla magnetic resonance imaging (MRI). FA-SPIONs exhibited the ability to act as efficient contrast agents in conventional MRI, providing a potential nanoplatform not only for tumor diagnosis but also for cancer treatment, through the delivery of anticancer drug or locoregional magnetic hyperthermia.

Magnetic anisotropy and intergrain interactions in L10 CoPt(1?1?1)/Pt(1?1?1)/MgO(1?0?0) PLD granular films with tilted easy axes

The magnetic properties of tilted easy axis L10 CoPt(1 1 1)/Pt(1 1 1)/MgO(1 0 0) film, deposited by pulsed laser deposition, were investigated by magnetization angular dependence measurements and magnetic force microscopy (MFM). The room temperature anisotropy constant, evaluated measuring the in-plane variation of transverse magnetization under a rotating magnetic field, is K1 = 5 × 10 6 erg cm −3 . The domain structure, observed by MFM, consists of decoupled single domain grains, unlike the maze-like structure usually observed for similar systems. This was confirmed by the analysis of dc demagnetization and isothermal remanence magnetization curves, which provided evidence of predominant magnetostatic interactions.

Highly Textured FeCo Thin Films Deposited by Low Temperature Pulsed Laser Deposition

The effect of the deposition temperature (Tdep) on the crystallographic orientation of pulsed laser-deposited FeCo/MgO(100) thin film was determined by means of X-ray reflectivity and high resolution trasmission electron microscopy analysis and was correlated with the magnetic anisotropy properties measured by angle dependent hysteresis loops. Highly textured films with a bcc structure and very smooth surface were obtained even at room temperature, the film being [100] and [110] oriented, at Tdep=25 °C and 150 °C, respectively. The cubic symmetry is reflected in the angular dependence of remanent magnetization, showing a 4-fold character, whose in-plane distribution is consistent with the different crystallographic orientations of the films. The high structural quality, even at room temperature, is reflected in a high value of the saturation magnetization and low coercivity, matching the requirements for technological applications.

Study of Magnetic Easy Axis 3-D Arrangement in L1

We report a study of angular magnetic properties of high-anisotropy L10 CoPt (111) films having a tilted magnetic easy axis configuration without an oblique-grain microstructure. In particular, we investigated the field dependence of remanent magnetization while rotating the magnetic field inside three intersecting planes. The out-of-plane tilting of the L10 c-axis (the easy axis of the tetragonal cell) was induced by using a Pt (111) underlayer deposited onto a single-crystal MgO substrate in a conventional frontal pulsed laser deposition (PLD). The observed behavior is consistent with the presence of four easy axes with mutually orthogonal in-plane projections, symmetrically tilted at 36deg with respect to the film plane. Such a system can be used, like a common single-axis tilted medium, to record information in perpendicular mode, lowering the writing field to approximately 75% of the value along the easy direction, while still maintaining the high thermal stability typical of the L10 alloy. Moreover, the in-plane charge compensation arising from this easy axis arrangement when a perpendicular writing field is applied may favor a media noise reduction and better performance with respect to a single-axis tilted system.

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Abstract The magnetic behaviour of ensembles of magnetic entities depends on their intrinsic magnetic properties and many other factors, including short- and long-range magnetic interactions. This chapter focuses on the effect of interactions in assemblies of single-domain nanoparticles, each of them being considered as a ‘superspin’ with a magnetic moment related to the particle volume. Depending on the type and the strength of the interactions among the magnetic entities, the magnetic behaviour of an assembly of superspins evolves from ferromagnetic-like behaviour to paramagnetic-like behaviour, through spin glass-like behaviour. Due to the enhanced time and magnetization scale with respect to atomic systems, the suffix super has been used to define these magnetic states and the magnetism of nanoparticle assemblies has been often called supermagnetism. The aim of this chapter is to describe how the ability to synthesise suitable nanoparticles systems, makes interactions among magnetic entities a fundamental tool to modify magnetic properties of nanostructured-based materials. The last section of this chapter is dedicated to some examples of application, i.e. magnetic recording and permanent magnets, where the interactions among magnetic element plays a fundamental role in determining their performances.

CoPt(111)/Pt(111)/MgO(100) Tilted System for Perpendicular Recording

FePt-based exchange-coupled composites consisting of a magnetically hard L10-FePt phase exchange-coupled with a soft ferromagnetic material are promising candidates for future ultra-high density (>1 Tbit/in2) perpendicular magnetic recording media, also being of interest for other applications including spin torque oscillators and micro-electro-mechanical systems, among others. In this paper, the effect of the thickness of a soft Co layer (3 < thCo < 20 nm) on the magnetic behavior of ledge-type fcc(100)-Co/L10(001)-FePt composites deposited on an MgO (100) substrate is systematically studied by combining morpho-structural analyses and angular magnetization measurements. Starting from a film consisting of isolated L10(001)–FePt islands, the ledge-type structure was obtained by depositing a Co layer that either covered the FePt islands or filled-up the inter-island region, gradually forming a continuous layer with increasing Co thickness. A perpendicular anisotropy was maintained up to thCo ∼ 9.5 nm and a ...