Domain structures and training effects in granular thin films

Magnetic force microscopy (MFM) evidenced that sputtered CoFe-Ag(Cu) granul ar films displayed long-range magnetic domain structures (magnetic percolat ion) at ferromagnetic (FM) volume contents, x(nu), well below the volume pe rcolation threshold. A variety of remanent magnetic microstructures was obs erved as a function of the thermomagnetic history, which strongly condition both magnetic and transport properties. These microstructures are formed d ue to the competition between perpendicular anisotropy and dipolar and matr ix-mediated exchange interactions. All as-deposited samples displayed a hig h degree of [111] texture perpendicular to the film plane and a rhombohedra l distortion induced by film-substrate strains, which are suggested to be a t the origin of the observed perpendicular uniaxial anisotropy. Structural data indicate a 2% of CoFe is alloyed to the metallic matrix, which seems t o be enough to couple the FM particles when they are closer than 1.5 nm at x(nu) greater than or equal to 25, leading to magnetic percolation. These F M correlations among particles were well evidenced in the low-field suscept ibility measurements, which supports the hypothesis that exchange correlati ons propagate due to the CoFe spins diluted in the matrix. Annealing relaxe d the rhombohedral distortion, leading to a cubic anisotropy along the [100 ] axis of the CoFe cell. Besides, annealing caused the segregation of the C oFe spins alloyed to the matrix, and consequently, the domain structure was lost. (C) 2000 Elsevier Science B.V. All rights reserved.