M. Ghidini

Magnetic Nanostructures in Modern Technology

We present a brief overview ohhe magnetization dynamics driven by a spin-polarized current in nonconfined ferromagnetic multilayers when a point-contact setup is used. The possibility to sustain persistent oscillations in these systems could be employed in the design of currentcontrolled microwave oscillators on nanometric scale. The attempt to reproduce the main experimental results by means ofboth macrospin and micromagnetic models encounters many difficulties. We have focused on the latter, summarizing the state-of-art and pointing out the stili unsolved methodological problems arising from such a modeling.

Magnetization processes in hard Co-rich Co–Pt films with perpendicular anisotropy

We present a study of the magnetic properties and magnetization processes in hard Co-Pt (Pt∼20at.%) films. Co-rich Co-Pt films, with thickness t ranging from 5nm up to 2μm, were prepared by electrodeposition on (0001)-oriented Ru underlayers. All samples displayed strong perpendicular magnetic anisotropy and high coercivity. Virgin magnetic domain structures for varying thickness were investigated by magnetic force microscopy (MFM). The observed increase of domain width with film thickness is well understood by full two-dimensional micromagnetic computations with no adjustable parameters. The easy-axis magnetization process, as observed by measuring virgin curves by magnetometry and imaging the corresponding magnetization configurations by MFM in variable field, consists of two stages separated by a well-defined critical field, marking the onset of domain wall propagation. A thorough analysis of the out-of-plane angular dependence of the switching field points out that unpinning of domain walls is the dom...

Microstructural and magnetic properties of exchange-coupled Co/Fe multilayers

Cobalt/iron multilayers with different layer thickness were electron beam evaporated in ultrahigh vacuum, keeping constant both the number of layers and the Co/Fe thickness ratio. Structural and magnetic properties are thoroughly investigated with different techniques. The multilayers have clean and sharp interfaces. All samples show single-phase magnetic behavior in the temperature range of 5–300 K, due to the strong exchange coupling between the layers. The room temperature hysteresis loops present a sharp switching at a field of ∼8 kA/m, followed by a residual hysteresis extending up to saturation, at fields of several tens of kA/m.

Proximity effects induced by a gold layer on La0.67Sr0.33MnO3 thin films

The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with a gold capping layer. With respect to uncoated manganite layers of the same thickness, Au-capped 4nm thick manganite films reveal a dramatic reduction (≃185K) of the Curie temperature TC and a lower saturation low temperature magnetization M0. A sizable TC reduction (≃60K) is observed even when an inert SrTiO3 layer is inserted between the gold film and the 4nm thick manganite layer, suggesting that this effect might have an electrostatic origin.

Direct deposition of magnetite thin films on organic semiconductors

Technological procedures able to produce high quality electrodes from magnetic oxides in vertical organic-inorganic hybrid devices is a challenging task in the field of organic spintronics. Thin films of magnetite (Fe3O4) have been successfully grown directly on top of organic semiconductor layers, tris(8-hydroxyquinoline)aluminium(III) (Alq3), by pulsed-electron ablation technique. The films show ferromagnetic behavior and good structural quality, properties detected by magneto-optical Kerr effect, superconductor quantum interference device, micro-Raman spectroscopy, and Atomic Force Microscopy. The ferromagnetic behavior persists even for 10nm thick films. Charge injection at magnetite-organic interface has been finally demonstrated by detecting electroluminescence from Alq3.

Swift heavy ions for magnetic nanostructures

Abstract A comprehensive review on the use of swift heavy ion irradiation of intermetallic rare earth-transition metal compounds is given. We show that latent track precipitation is possible in some of these metallic compounds, with diameters in the range 1–5 nm. The behaviour is well described by the thermal spike model. By choosing the elements carefully, it is possible to prepare magnetic nanostructures with truly nanometric dimensions. Due to the quasi 1-D geometry, peculiarities in the magnetic properties are observed. We summarize the most significant results obtained and we briefly discuss some new developments.

Magnetic properties of amorphous nanocolumns created by heavy ion irradiation of paramagnetic YCo2 thin films (invited)

Recent studies have demonstrated the possibility of inducing amorphous latent tracks in metallic materials by GeV heavy ion irradiation. In the present work, 0.880 GeV 238U beams have been used to induce ferromagnetic amorphous nanocolumns in nonmagnetic crystalline YCo2 films. The loss of crystallinity deduced by x‐ray analysis is in good agreement with the one determined by magnetic measurements. The Co magnetic moment in the columns is approximately equal to 1 μB, as in bulk amorphous YCo2, but the ordering temperature (150 K) is strongly reduced with respect to the bulk. Magnetization measurements reveal the nanocolumns to be single domain. A perpendicular anisotropy is observed in samples irradiated at the smallest fluencies, which is interpreted to be due to shape anisotropy. A progressive decrease of the anisotropy with increasing fluence is observed and qualitatively described in terms of dipolar interactions between columns. At low temperature, the coercive field reaches 650 Oe. A simple nonunifo...

Magnetization processes in amorphous nanoparticles obtained by heavy ions irradiation of nonmagnetic Y—Co films

Ferromagnetic amorphous nanoparticles have been obtained by heavy ions (Pb, U) irradiation of paramagnetic YCo 2 thin films. The precipitation of elongated particles formed of an amorphous ferromagnetic core of diameter ∼ 20 A with an external core of recrystallized YCo 2 is observed. At low fluence the interactions between particles are weak and a clear perpendicular anisotropy, of shape in origin, is observed. The temperature dependence of the anisotropy field is well fitted in the chain of sphere approximation, revealing the irregular shape of the tracks. The coercive field, which amounts to 1/3 of the anisotropy field at low temperature, is analyzed by considering either 1-D curling which implies continuous exchange interaction along the ion path or symmetrical fanning for a chain of decoupled spheres. When the temperature is increased, the coercivity vanishes as a result of the large thermal activation.

Reversal modes of the multilayer exchange-spring magnet

Abstract A one-dimensional micromagnetic model of the multilayer exchange-spring magnet is utilized for calculating the analytical expression of the differential susceptibility at the nucleation field in terms of structural parameters. The phase diagram in the plane of half-layer thicknesses is given for a Sm–Co/Fe multilayer, showing the critical line of χc→∞, which is the boundary for the irreversible demagnetization mode. For thin layers the optimum soft-phase volume fraction is deduced.

Structural analysis of ferromagnetic latent tracks in RCo2 thin films (R = Y,Tm,Ce) by means of X-ray diffraction

Abstract YCo2 paramagnetic crystalline thin films have been irradiated by beams of GeV U and Pb ions, resulting in the formation of ferromagnetic amorphous latent tracks. The combination of quantitative X-ray diffraction analysis in regular θ – 2θ mode and in grazing incidence geometry reveals latent tracks of around 20Ain diameter embedded in a stressed crystalline matrix (Δa/a≈ 1%). Different behaviours are observed in the parent systems TmCo2 and CeCo2: TmCo2 is irradiation insensitive while CeCo2 exhibits strong recrystallization effects.