Effect of annealing on magnetic exchange coupling in CoPt/Co bilayer thin films

Thin film CoPt/Co bilayers have been prepared as a model system to investig ate the relationship between microstructure and exchange coupling in two-ph ase nanocomposite permanent magnets. The bilayers were prepared by magnetro n sputter deposition of near-equiatomic CoPt with a thickness of 25 nm onto oxidized Si wafers. In the as-deposited state, CoPt had the A1 (fcc) struc ture and was magnetically soft. Before reinsertion into the sputtering cham ber for the deposition of 2.8-16.7 nm thick Co layers, the CoPt films were annealed at 700 degrees C for 120 min to produce the magnetically hard, ful ly ordered L1(0) phase. The presence of exchange coupling in the bilayers w as verified by magnetic hysteresis and recoil measurements and showed that only for Co thicknesses below 6.3 nm was this layer (in its as-deposited st ate) coupled through its full thickness to the CoPt layer. Annealing the bi layer samples at 300 and 550 degrees C for 20 min resulted in improvement o f the interlayer magnetic coupling and produced clear differences in the ma gnetic reversal coherency and the recoil curves. However, for some samples, the improved coupling resulted in a decrease in coercivity, indicating tha t there is an optimum in the coupling strength for the attainment of high c oercivity. Transmission electron microscopy studies of the bilayers in plan view showed that the increased interlayer coupling with annealing was a re sult of improved granular epitaxy of Co to CoPt. (C) 2000 American Institut e of Physics. [S0021-8979(00)52208-7].