V. Franco

Remanence breakdown in granular alloys at magnetic percolation

Microstructural effects on the magnetic behavior of rf-sputtered CoFe–AgCu granular alloys are examined through the study of the in-plane remanence-to-saturation magnetization ratio, Mr/Ms, as a function of temperature, ferromagnetic volume content, xv, and annealing temperature. At low ferromagnetic contents (xv⩽0.25), the MrMs ratio in as-deposited samples tends towards 0.5 at low temperature, as expected from the uniaxial perpendicular anisotropy displayed by all samples, which is magnetoelastic in nature and arises from the axial distortion of the CoFe face-centered-cubic cells. In as-deposited samples with xv>0.25 (well below the volume percolation threshold, xp∼0.5–0.55), a collective magnetic behavior develops due to magnetic correlations among particles. Consequently, a domain structure perpendicular to the film plane appears, which results in a remanence breakdown: Mr/Ms is about 0.2. Besides, magnetic correlations prevents the thermal decay of Mr/Ms, which is almost constant between 5 and 300 K,...

The effect of the microstructure on the magnetic interactions in CoFe–AgCu granular films: From demagnetizing to magnetizing interactions

The sign and strength of the dominant magnetic interactions in Co34Fe8Ag54Cu4 granular films were modified by changing the microstructure through annealing. Magnetic force micrographs showed that in the as-cast sample the magnetic moments of neighboring grains tended to be arranged parallel along a direction out of the film plane, forming elongated magnetic clusters that were themselves aligned antiparallel, with dominant demagnetizing interactions. This is a direct evidence that an uncompensated antiferromagneticlike microstructure is stabilized below the volume percolation threshold. However, in the sample annealed at 750 °C the particle growth led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. Thus, in this letter we present direct correlation of interactions effects with magnetic measurements and show that ΔM plots correlate with changes in the magnetic microstructure in these systems.

The nature of magnetic interactions in CoFe-Ag(Cu) granular thin films

The effect of dipolar and exchange interactions on the magnetic behaviour of sputtered CoFe-Ag(Cu) granular alloys is examined through the study of remanence curves and M plots as a function of the ferromagnetic volume content, xv , and annealing temperature. As-deposited samples, with either a random distribution of weakly interacting fine magnetic particles (3 nm in size) at low xv or with strong dipolar and exchange interactions (leading to a long range out-of-plane stripe-like domain structure) at xv 0.25, display negative M values. In the former, this is attributed to dipolar interactions being dominant in a random distribution of well separated particles. In the latter, M <0 is attributed to the flux closure between antiparallel neighbouring domains, which is strongly demagnetizing. Annealing causes phase segregation and particle growth, while particle clusterization occurs. These facts lead to an increase of the direct exchange through the surface of neighbouring grains in the same particle clusters, at the expense of dipolar interactions, which decrease as the clusters become more separated. Consequently, at high xv , annealed samples display positive M values.

From demagnetizing to magnetizing interactions in CoFe–AgCu granular films

CoFe–AgCu granular films of compositions ranging from 0.17–0.44 ferromagnetic atomic concentration were prepared by rf sputtering. The microstructure and the transport and magnetic properties suggested that this family of samples can be classified into two groups with a crossover concentration at about 32 at. %. The experimental results for samples Co34Fe8Ag54Cu4 and Co18Fe8Ag70Cu4, which are representative of both different behaviors, are discussed. For the as-prepared sample with higher CoFe content, an uncompensated out-of-plane antiferromagneticlike microstructure with dominant demagnetizing interactions was observed. The particle growth through the annealing led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. The thermal dependence of the remanence-to-saturation ratio of the as-prepared and annealed samples indicated the existence of a high degree of magnetic correlations leading to a very low magnetoresistivity: In none of the cases was a Stoner–Wohlfarth behavio...

Magnetic microstructures from magnetic force microscopy and Monte Carlo simulation in CoFe-Ag-Cu granular films

CoFe-Ag-Cu granular films, prepared by rf sputtering, displayed magnetic domain microstructures for ferromagnetic concentrations above about 32% at, and below the percolation threshold. All samples have a fcc structure with an (111) texture perpendicular to the film plane. Magnetic force microscopy (MFM) showed a variety of magnetic domain microstructures, extremely sensitive to the magnetic history of the sample, which arise from the balance of the ferromagnetic exchange, the dipolar interactions and perpendicular magnetocrystalline anisotropy, MFM images indicate that in virgin samples, magnetic bubble domains with an out-of-plane component of the magnetization are surrounded by a quasicontinuous background of opposite magnetization domains. The application of a magnetic field in different geometries drastically modifies the microstructure of the system in the remanent state: i) for an in-plane field, the MFM images show that most of the magnetic moments are aligned along the film plane, ii) for an out-of-plane field, the MFM signal increases about one order of magnitude, and out-of-plane striped domains with alternating up and down magnetization are stabilized. Numerical simulations show that a variety of metastable domain structures (similar to those observed experimentally) can be reached, depending on magnetic history, in systems with competing perpendicular anisotropy, exchange and dipolar interactions.

CoFe-based granular alloys: the role of the metallic matrix

Abstract RF-sputtered CoFe-NM granular alloys (NM=Ag, Cu) with CoFe volume content, x v , ranging from 0.10 to 0.45 have been studied. These two series of samples show similar features depending on the synthesis conditions and post-deposition annealing treatments, revealing the strong dependence of magnetotransport properties on microstructure. Three different regimes have been observed as x v is increased: the classical giant magnetoresistance (GMR) regime at low ferromagnetic contents; at intermediate x v , a domain structure appears, and GMR and anisotropic magnetoresistance (AMR) together with domain wall scattering are observed; and a third regime at x v close but below the volume percolation threshold, where the two latter contributions still coexist, while the GMR contribution has been suppressed by strong magnetic correlations. The role of the metallic matrix is crucial to determine the crossover ferromagnetic contents between these three regimes, which depend on the relative immiscibility of CoFe either in the Ag or Cu matrices and the diffusivity of Ag and Cu. Moreover, the metallic matrix settles the degree of CoFe segregation, sample crystallisation and texture, which are responsible for the magnetotransport properties.

Texture, strain and alloying in sputtered granular magnetic films

Abstract An extensive characterization of the microstructure of a large variety of CoFe–Ag(Cu) and CoFeCu granular alloys prepared by r.f. sputtering is presented. The resulting microstructure is described in terms of the ferromagnetic concentration, from the low concentration limit ( x ∼0.10 by volume) to values near the volume percolation threshold ( x th =0.50–0.55). The influence of the annealing procedure on the sample texture, particle size, crystal structure and strains, substrate–film stress and CoFe dilution in the matrix, among others, is also studied. This work is relevant to the complex magnetic and magnetotransport properties of granular alloys and their subtle and crucial dependence on the microstructure.

Training behaviour and magnetic domains in CoFeAgCu granular films

Abstract CoFeAgCu thin films display magnetic domain microstructures below the percolation threshold. It was recently shown that for x ∼ 30% a variety of magnetic domain structures appears, which are extremely sensitive to the annealing and magnetic history. This magnetic microstructure was studied by magnetic force microscopy and Monte Carlo simulation and it is correlated to the magnetic and transport properties.

Magnetic Force Microscopy: A Powerful Tool to Image Domain Structures in Granular Thin Films

Magnetic force microscopy was used to study the long-range domain structures appearing in rf-sputtered giant magnetoresistive CoFe-NM granular thin films (NM = Ag, AgCu). Even all samples were granular and the ferromagnetic concentration was far below the volume percolation threshold, out-of-plane domain-like configurations much larger than the particle size were observed due to the magnetic correlations among the CoFe particles. A large variety of remanent microstructures were also imaged as a function of the thermomagnetic history.

The microstructure of CoFeAgCu granular films: Origin of the perpendicular anisotropy

Abstract CoFeAgCu thin films display magnetic domain microstructures below the percolation threshold. Those microstructures are stabilised due to competition between perpendicular uniaxial anisotropy, dipolar and exchange interactions. This paper shows a perpendicular uniaxial anisotropy associated with rombohedral deformation and texture, whereas exchange interactions, which are responsible of the magnetic percolation, are due to 2% of CoFe atoms diluted in the matrix.