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, M-r/M-s, as a function of temperature, fer romagnetic volume content, x(v), and annealing temperature. At low ferromag netic contents (x(v)less than or equal to 0.25), the MrMs ratio in as-depos ited samples tends towards 0.5 at low temperature, as expected from the uni axial perpendicular anisotropy displayed by all samples, which is magnetoel astic in nature and arises from the axial distortion of the CoFe face-cente red-cubic cells. In as-deposited samples with x(v)> 0.25 (well below the vo lume percolation threshold, x(p)similar to 0.5-0.55), a collective magnetic behavior develops due to magnetic correlations among particles. Consequent ly, a domain structure perpendicular to the film plane appears, which resul ts in a remanence breakdown: M-r/M-s is about 0.2. Besides, magnetic correl ations prevents the thermal decay of M-r/M-s, which is almost constant betw een 5 and 300 K, even for ferromagnetic particles no more than 3 nm in size . The axial distortion disappears with annealing, the cubic symmetry is rec overed, the out-of-plane magnetic structure is lost and, therefore, the M-r /M-s ratio at low temperature tends towards 0.8 for highly annealed samples . Consequently, the magnetic properties of granular alloys depend on the in terplay between anisotropy, exchange, and dipolar interactions, which in tu rn depend crucially on the microstructure. (C) 2000 American Institute of P hysics. [S0021-8979(00)00115-8].