M. M. Pereira de Azevedo

Tunnel magnetoresistance and magnetic ordering in ion-beam sputtered Co80Fe20/Al2O3 discontinuous multilayers

Discontinuous multilayered Co80Fe20(t)/Al2O3(30 A) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18 A. At t 13 A was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in o...

Ferromagnetic resonance in granular thin films

A theoretical analysis has been done of the effects of granule size, shape, orientation, and concentration and the temperature effects on the ferromagnetic resonance (FMR) field Hr in granular thin films. The granular CoxAg1−x thin films with 0.2 fp. For the Co–Ag system, persistence of a considerable SPM fraction is revealed by the superconducting quantum interference device data up to the highest f, and the effect at f=fp consists in a discontinuous jump of the Hr(f) slope. Otherwise, the FMR data for granular Fe–SiO2 films reveal a discontinuous jump in H...

Influence of co-evaporation technique on the structural and magnetic properties of CoCu granular films

Abstract Structural, magnetic and transport properties of granular Co x Cu 1 − x ( x = 0.11–0.45) films prepared by e-beam co-evaporation, were studied. Scanning tunneling microscopy showed the surface crystallites having an elongated shape. Magnetization and FMR data indicated a large in-plane uniaxial anisotropy in all the samples. A correlation between the orientation of the surface crystallites and the anisotropy easy axis has been found. Magnetoresistance in the fields up to 0.5 T is strongly different for transverse and longitudinal geometries.

Magnetization and magneto resistance in Fe-ion-implanted Cu and Ag thin films

Abstract Diluted granular films of CuFe and AgFe (iron content ⩽ 2%) were produced using 57 Fe ion implantation on Cu(Ag) films previously grown by laser ablation. Conversion electron Mossbauer spectroscopy shows that the implanted Fe forms either very small clusters (up to a few atoms) or large iron α-phase particles. These structural characteristics directly reflect on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters) superimposed by a significant paramagnetic contribution at low temperatures due to the small clusters. We observe deviations from strict superparamagnetic behaviour due to non-negligible local anisotropy effects at low temperatures and low fields. The Kondo effect is particularly enhanced in the CuFe films which have higher concentration of isolated Fe atoms and small sizes clusters. The magnetoresistivity Δ ϱ / ϱ of our films is dominated (for 0 ⩽ μ 0 H ⩽ 15 T) by a linear term in H , attributed to GMR-like effect from spin-dependent scattering when an electron passes between adjacent large and small clusters. At low fields we observe instead Δ ϱ / ϱ ∝ H 2 , due to the usual GMR effect between large clusters, during the alignment of their easy axes. The relevant physical differences (structural, magnetic and magnetoresistive) observed in our ion-implanted diluted Fe films, with respect to the concentrated granular films, are critically analysed.

Giant magnetoresistance behavior of granular Fe and Co implanted Ag thin films

The magnetoresistive behavior of granular thin films prepared by Fe and Co implantation in Ag thin films is reported. Ag thin films (∼2000 A) were deposited by evaporation or laser ablation onto Si and SiO2 substrates and implanted with Fe or Co at fluences up to 1017 at. %/cm2. The magnetoresistive response obtained after implantation was found to increase with the implanted fluence. An increase of the magnetoresistive response by a factor of 3–4 can be achieved after annealing the films in a conventional furnace at 620 K under vacuum. The best value of magnetoresistance obtained so far is 9% at 10 K and 1.5% at room temperature for a film implanted with Co at a fluence of 8×1016 at. %/cm2 and annealed at 620 K.

Magnetoresistance and phase analysis in splat-cooled FeCu granular alloys

Abstract Granular FeCu systems with Fe concentration in the range of 17–25% have been prepared by splat-cooling and show, upon annealing above 550 K, a significantly higher magnetoresistivity than similar alloys prepared by melt-spinning. Room temperature Mossbauer spectroscopy is used and X-ray diffractometry are used to determine the evolution of the structure and phase distribution upon annealing, and the corresponding data are correlated with the magnetoresistive behaviour. The appropriate condition to optimize the magnetoresistive amplitude are also reported.

Influence of conduction electrons and dipolar interactions on the susceptibility of granular materials

We present an analysis of the effect of intergranular interactions on magnetic susceptibility of granular magnetic systems. The RKKY-type interaction between magnetic granules, via the conduction electrons, is found non-oscillatory antiferromagnetic, unlike in multilayer systems. The demagnetizing effect of dipolar interaction is shown to produce deviations from simple superparamagnetic behavior predicted by the Langevin model.

Helimagnetism and field-induced phases in random Gd64Sc36 single crystals

We report magnetization, neutron diffraction and elastic constant measurements of the single-crystal rare earth alloy Gd64Sc36 as a function of magnetic field. For an a-axis field the material exhibits transitions from helix to fan to basal plane ferromagnet. At low temperatures the looked-in magnetic structure is very sensitive to the application of small fields (~5 mT).

GMR in high fluence ion implanted granular thin films

Abstract We report on the magnetic and magnetoresistive (MR) properties of granular materials obtained by implantation of high fluences (from 1 × 10 16 to 1.8 × 10 17 ions/cm 2 ) of Fe and Co ions into Cu and Ag thin films. The local microstructure vs. implantation doses and posterior thermal treatment is discussed to obtain MR values of practical relevance. MR measurements up to very high fields (32 T) are presented, supporting recent theoretical work on the role of short-range magnetic correlations in MR for granular materials.

Interlayer coupling across an alloy spacer: Co/Cu75Au25 multilayers

Oscillatory interlayer exchange coupling and giant magnetoresistance (MR) of magnetic multilayers have been observed in many multilayered magnetic systems. One of the most studied systems is Co-Cu [1,2], which displays giant MR values (> 50%) at room temperature and exhibits significant antiferromagnetic coupling (J ∼ 0.15 erg/cm2) leading to saturation fields of the order of a few kOe. Co-Au multilayers [3] also show oscillatory coupling but with MR values considerably smaller (∼10%). In both systems the observed periods of the oscillatory coupling are of the order of 10 A. The origin of the magnetic coupling in the case of noble - metal spacer layers is considered to be a RKKY-type interaction due to the conduction electrons of the nonmagnetic spacer [4].