CoFe–Cu granular alloys: From noninteracting particles to magnetic percolation

Abstract

CoFe-Cu granular films with ferromagnetic content ranging from 0.10 to 0.33 by volume were prepared by radio frequency sputtering. As-cast samples were rapidly annealed at various temperatures up to 750°C to promote the segregation of CoFe particles within the metallic matrix. Magnetic and transport properties suggested that this family of samples may be classified into three groups: ͑i͒ below about 0.20 volume content of CoFe, all samples display the typical features of a granular solid constituted by a random distribution of nanometric CoFe particles within a Cu matrix, and the maximum magnetoresistance is about 20% at low temperature ͑giant magnetoresistance͒; ͑ii͒ for as-cast samples within 0.20 and 0.30 of volume concentration, magnetoresistance and magnetization display complex bimodal behavior and large metastable effects associated with the interparticle interactions, which stabilize a domain-like microstructure well below the volume percolation threshold ͑0.55͒, as already observed in CoFe-Ag͑Cu͒ granular alloys. As a consequence of the large magnetic correlations, magnetoresistance is very low ͑1%-3%͒. Through annealing, the microstructure and therefore the transport properties evolve to those of a classical giant magnetoresistance system with large particles; and ͑iii͒ above about 0.30 of volume content ͑and still below the volume percolation threshold͒, as-cast samples display both anisotropic and giant magnetoresistance, as also observed in other granular alloys. Annealing leads to complete segregation and to the formation of large magnetic particles, which results in a transition from mixed behavior of both anisotropic and giant magnetoresistance ͑GMR͒ regimes to a giant magnetoresistance regime, with a maximum GMR of about 7%.

Remanence breakdown in granular alloys at magnetic percolation

“…Values for K within 0.30-0.40ϫ10 6 erg/cm 3 and ϭ0 for all as-deposited samples have been found, in agreement with the magnetoelastic energy calculation from the axial distortion. 21 Similar results have been observed in CoFe-Cu granular films, 26 though K values were smaller (ϳ0.2ϫ10 6 erg/cm 3 ), as a consequence of a lower degree of texture with respect to the CoFe-AgCu films.…”

“…Moreover, while remanent enhancement has been widely studied in thin films and magnetic recording media, we have shown how remanence breakdown takes place in granular systems due to the stabilization of a longrange out-of-plane domain microstructure. Similar studies in CoFe-Cu samples 26 prepared under the same conditions also showed the evolution of M r /M s from 0.5 in asdeposited samples to 0.8 in annealed ones, although no remanence breakdown was observed in as-deposited samples with high x v . No out-of-plane component of the magnetization was observed after in-plane saturation, in agreement with the smaller anisotropy values and lower crystal texture displayed by these samples, 26 than those used here.…”

“…Similar studies in CoFe-Cu samples 26 prepared under the same conditions also showed the evolution of M r /M s from 0.5 in asdeposited samples to 0.8 in annealed ones, although no remanence breakdown was observed in as-deposited samples with high x v . No out-of-plane component of the magnetization was observed after in-plane saturation, in agreement with the smaller anisotropy values and lower crystal texture displayed by these samples, 26 than those used here. It has also been shown that magnetic correlations are able to maintain a constant M r /M s ratio up to room temperature, since they are able to avoid thermal decay of remanence, even for particles no more than 3 nm in size.…”

Effect of pulse plating and additive on phase separation in Cu–Co nano-granular alloys

“…Of course, many a times, annealing at high temperatures was found to improve such phase segregation in Cu-Co alloys. As a result, tensile stress develops [38,39] leading to formation of crack within the film and ultimately to loss of structural integrity in actual practice. The phase separation tendency is found to be more evident for PC plated samples deposited from electrolyte in the presence of saccharine as shown in Fig.…”

Interactions controlled evolution of complex magnetoresistance in as-deposited Ag100−Co nanogranular films with perpendicular magnetic anisotropy

“…These films are studied extensively owing to scientific and technological challenges involved [2][3][4][5]. However, due to the complexity of their microstructure, many of the physical properties of these granular films are yet to be clearly understood.…”