Elisabetta Agostinelli

Spin-glass-like freezing and enhanced magnetization in ultra-small CoFe2O4 nanoparticles

The magnetic properties of ultra-small (3 nm) CoFe(2)O(4) nanoparticles have been investigated by DC magnetization measurements as a function of temperature and magnetic field. The main features of the magnetic behaviour are blocking of non-interacting particle moments (zero-field-cooled magnetization T(max) approximately 40 K), a rapid increase of saturation magnetization (up to values higher than for the bulk material) at low T and an increase in anisotropy below 30 K due to the appearance of exchange bias. The low temperature behaviour is determined by a random freezing of surface spins. Localized spin-canting and cation distribution between the two sublattices of the spinel structure account quantitatively for the observed increase in saturation magnetization.

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.

Superparamagnetic blocking and superspin-glass freezing in ultra small δ-(Fe0.67Mn0.33)OOH particles

The magnetic properties of ultra-small (~2 nm) δ-(Fe(0.67)Mn(0.33))OOH nanoparticles prepared by a microemulsion technique have been investigated by magnetization and ac susceptibility measurements at variable frequency. The results provide evidence of two different magnetic regimes whose onset is identified by two maxima in the zero-field-cooled susceptibility: a large one, centered at ~150 K (T(mh)), and a narrow one at ~30 K (T(ml)). The two temperatures exhibit a different frequency dependence: T(mh) follows a Vogel-Fulcher law τ = τ(0)exp[(E(a)/k(B))/(T-T(0))], indicating a blocking of weakly interacting nanoparticle moments, whereas T(ml) follows a power law τ = τ(0)(T(g)/T(mν)-T(g))(α), suggesting a collective freezing of nanoparticle moments (superspin-glass state). This picture is coherent with the field dependence of T(ml) and T(mh) and with the temperature dependence of the coercivity, strongly increasing below 30 K.

Origin of magnetic anisotropy in ZnO/CoFe2O4 and CoO/CoFe2O4 core/shell nanoparticle systems

ZnO-core/CoFe2O4-shell nanoparticles of 7.4 nm average size have been synthesized and their magnetic properties have been compared to those of CoO-core/CoFe2O4-shell nanoparticles with similar morphology. The coercive field values are much lower than those for CoO/CoFe2O4 nanoparticles (e.g., at 5 K: Hc = 7.8 kOe for ZnO/CoFe2O4; Hc = 27.8 kOe for CoO/CoFe2O4). The nature of the coercive field values is explained by a phenomenological model for the free energy of a non-magnetic core, or an antiferromagnetic core, encapsulated by a hard ferrimagnetic shell.

Static and Dynamic Magnetic Properties of Melt-Spun Granular Cu100-xCox Alloys

The magnetic properties of granular Cu-Co alloys produced by melt-spinning have been investigated by DC and AC susceptibility measurements. The results indicate the occurrence of blocking processes of interacting superparamagnetic particles. The effects of annealing by Joule heating at different current densities are discussed in relation to the giant magnetoresistance properties of the system.

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 Co–Ag nanocrystalline films deposited by Pulsed Laser Deposition were irradiated by excimer laser at different values of energy fluence. The microstructural modifications induced by such treatment were investigated (by atomic force, scanning and transmission electron microscopies) and correlated with the magnetic and magnetotransport properties. The results indicate that laser irradiation favors the silver grain growth and induces a segregation of a fraction of Co, driven by the interface energy, to sites energetically more favorable like Ag grain boundaries (as small grains with diameters less than 5 nm, to be compared to ≈10 nm in the as-deposited sample) and the film/substrate interface. Such microstructural variation leads to changes in the magnetic behavior and to an increase of the magnetoresistance below 10 kOe.

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

Granular Co-Cu alloy ( Co10Cu90) produced by melt-spinning and subsequently annealed in vacuum by DC Joule heating at different current densities have been investigated by DC and AC susceptibility measurements. High values of the heating current density lend to promote grain growth and to favore the increase of intergrain interactions strength, determining a variety of magnetic propeties from a collective fi freezing in random directions of moments of nanosized Co particles to an inhomogeneous and pl progressive blocking of moments of particles with increasing size. Meanwhile, significant changes of the giant magnetoresistance exhibited by the system were also observed. The correlation between microstructure, magnetic properties and GMR is discussed.

Magnetic and transport properties of Co-Ag nanocrystalline particles

Abstract An investigation of static magnetic properties of Co x Ag 1− x ( x =0.25, 0.40) granular films grown by the pulsed laser deposition technique is reported for samples heat treated at different temperatures. Different terms have been found to contribute to the observed temperature dependence of the magnetization: isolated clusters, superparamagnetic particles and larger blocked particles. The increase of the deposition temperature up to 250°C for x =0.40 leads to an increase of the fraction of superparamagnetic particles. An annealing treatment at 250°C for x =0.25 determines the growth of particles size.