D. Bisero

Fragmentation of cobalt layers in Co/Cu multilayers monitored by magnetic and magnetoresistive measurements

We have monitored the structural evolution of Co(tCo)/Cu(4×tCo) multilayers when tCo ranges from 12 to 2 A. The investigation has been performed by studying their magnetization and giant magnetoresistance, since these properties are complementary in providing information about the structure of the magnetic species into the samples. In particular, in the intermediate range of thickness, we observed no correspondence between magnetic and magnetoresistive behavior. Finally, at sufficiently low thickness, the samples exhibit noninteracting superparamagnetic features. This kind of evolution has been ascribed to the progressive fragmentation of Co layers.

Role of particle size distribution on the temperature dependence of coercive field in sputtered Co/Cu granular films

The temperature dependence of coercive field and of the ratio between the remanent and saturation magnetization of granular Co/Cu films grown by sequential sputtering has been studied with magneto-optic Kerr effect measurements in the temperature range 8.5–60 K. The observed temperature dependence of coercive field does not fit any of the plausible Tx laws commonly used to describe systems of single-domain ferromagnetic particles. We worked out a generalized model, which also includes temperature effects related to nonuniform single-domain particle size distribution. The model predictions account well for the observed temperature behavior of both coercive field and ratio between the remanent and saturation magnetization.

Magnetostatic and exchange coupling in the magnetization reversal of trilayer nanodots

We present an experimental investigation of the magnetization reversal process in Ni80Fe20(10 nm)/Cu/Co(10 nm) sub-micrometric circular discs for two different thicknesses of the Cu spacer (1 and 10 nm). Magnetic hysteresis loops were measured by the longitudinal magneto-optical Kerr effect and by resonant scattering of polarized soft x-ray. The results for the 10 nm thick Cu interlayer show a complex magnetization reversal process determined by the interplay between the interlayer dipolar interaction and the different reversal nucleation fields in the two layers. It is worth noting that, during the reversal process, the magnetization of the two layers remains in a nearly single domain state due to the dipolar coupling. These findings are confirmed by three-dimensional micromagnetic simulations. In contrast, when the Cu spacer is 1 nm thick both measurements and simulations show that the reversal is accomplished via the formation of a vortex state in both discs due to the presence of a ferromagnetic exchange coupling that competes with the dipolar interaction.

Transport properties and magnetic disorder/order transition in FexAg100−x films

Abstract We have studied the magnetic disorder/order transition in Fe x Ag 100− x films, with x varying from 10 to 30, focusing our attention on the interplay between transport and magnetic properties. The samples have been deposited by DC magnetron co-sputtering and analyzed by magneto-optic Kerr effect and magnetoresistance measurements, with external magnetic field applied both in and out of the film plane. Magnetization and magnetoresistive results indicate that for low Fe content ( x x ⩽30) the effect of local magnetic ordering emerges and the films can no longer be considered as granular. The presence of magnetic coherence on different length scales in this regime is discussed and related to coalescence of magnetic particles and clusters formation, with increasing Fe concentration above 20%. This value appears as a critical iron content around which the magnetic disorder/order transition occurs.

GMR as a function of temperature in FeAg granular samples: the effect of magnetic interactions

We have deposited various FeAg granular systems having different iron content and we have investigated their giant magnetoresistive properties as a function of temperature. In a previous work we observed that, at room temperature, with increasing Fe content samples behaviour changes from superparamagnetic to ferromagnetic-like, and 20% is the concentration that separates these two regimes. In correspondence with this value, the dependence of giant magnetoresistance intensity as a function of iron concentration shows a maximum. With decreasing temperature, we observed that the position of this maximum shifts progressively towards lower iron concentrations. This behaviour has been discussed in terms of the presence of magnetic interparticle interactions. An analysis of the dependence of giant magnetoresistance as a function of applied field at the various temperatures is also presented.

Structural and compositional stability of Co oxide grown on (001) bct Co

Abstract Room temperature (RT) adsorption of oxygen on the (001) surface of metastable, cubic Co film epitaxially grown on a Fe substrate results in the formation of about 7 monolayers (ML) crystalline CoO film, in the typical rocksalt structure 45° rotated with respect to the square unit mesh of the underlying Co film. We investigated the stability of this oxide layer upon annealing in the RT–620 K temperature range, by means of primary-beam diffraction modulated electron emission (PDMEE), and X-ray photoelectron spectroscopy (XPS). For temperatures up to 570 K, film thickness and local order are preserved, in spite of an increasing number of Co and O atoms bonded in sites of reduced coordination. For larger temperatures, thickness also reduces, but the oxide film is still characterized by a high degree of CoO-like local order.

Magnetization Configurations in NiFe Slotted Rings Studied by Magneto-Optical Kerr Effect and Magnetic Force Microscopy

The magnetization states and the reversal mechanisms of 30 nm thick NiFe full and slotted rings, with outer and inner diameter of 1400 nm and 600 nm, respectively, are analysed by magneto-optical Kerr effect and in-field magnetic force microscopy. The evolution of the magnetization configuration is investigated as a function of the applied field intensity. Both the measured hysteresis loops and the MFM images are satisfactorily reproduced by a combined micromagnetic and analytical approach, showing that the presence of the slot in one arm of the ring forces the system to undergo a transition into a flux closure vortex state of well defined chirality. Both the reversal mechanism and the field range where the vortex exists are influenced by the aperture angle of the slot.

Dipolar interaction in dense chains of submicrometric rectangular dots

Dense chains of rectangular dots (715×450 nm) put side by side, with interdot spacing variable in the range between about 55 and 625 nm, have been patterned by deep UV lithography, starting from a 40 nm thick permalloy film. A magneto-optical Kerr effect study, assisted by magnetic force microscopy, revealed that the magnetization inversion proceeds through the nucleation and the annihilation of two vortices, with the nucleation and annihilation fields appreciably affected by the coupling. The latter also modifies the frequency of the fundamental resonant mode of precession of the magnetization, measured by Brillouin light scattering from spin-wave. Micromagnetic simulations enabled us to account for the main features observed in the experiment.

Coercive field vs. temperature in Fe/Ag nanogranular films

We investigate the temperature dependence of the coercive field of a nanogranular co-sputtered Fe/Ag film. The sample is superparamagnetic at room temperature and displays a 3% giant magnetoresistance. As temperature decreases from 40 to 9 K, hysteresis appears in magnetization loops. The coercive field dependence on temperature turns out to be different from the power law expected for single-domain monodispersed ferromagnetic particle ensembles. The results are analyzed in terms of a model taking into account the effect of magnetic grain size distribution on coercive field. The agreement between experiment and the model is excellent. The possibility to gain information concerning width and symmetry of the particles volume distribution from the study of coercivitys scaling with temperature is discussed.

GMR effect across the transition from multilayer to granular structure

We have studied the evolution of giant magnetoresistance in magnetic multilayers obtained by alternately growing thin cobalt and copper layers having proportional thickness. Co nominal layers thickness was progressively reduced from 12 A down to 2 A. This decrease produces a fragmentation of the Co layers and the evolution of samples magnetic properties indicates a transition from multilayer to granular behavior. Magnetic and magnetoresistive data of the granular phase have been analyzed as a function of Co thickness in order to determine particle size distribution.