Synthetic antiferromagnetic soft underlayers for perpendicular recording media

Synthetic antiferromagnetically coupled films have been studied for soft underlayers for perpendicular media. Initially, FeCo bilayers were prepared by dc magnetron sputtering with a structure of glass-Ru(2.5 nm)-FeCo(t_F)-Ru(t_Ru)-FeCo(t_F)-Ru (10 nm), where t_F was varied from 5 to 200 nm and t_Ru from 0.2 to 1.4 nm. The antiferromagnetic coupling showed a maximum at t_Ru=0.6-1.0 nm as expected, depending on t_F. However, the surface coupling energy J_AF decreased very rapidly with t_F from 2.8 erg/cm² at t_F=5 nm to 0.6erg/cm² at t_F=10 nm. A structure of glass-Ru(2.5 nm)-FeCo(50 nm)-Ru(1.0 nm)-FeCo(55 nm)-Ru (10 nm) showed nearly zero remanence and very well separated hysteresis segments at ±30 Oe along the FeCo easy axis and a single narrow hysteresis loop with coercivity of 3.5 Oe along the hard axis. These characteristics were thermally stable up to at least 200°C, which is superior to IrMn exchange-biased systems. Further optimization was achieved with a trilayer structure of glass-Ru(2.5 nm)-FeCo(25 nm)-Ru(1.0nm)-FeCo(45nm)-Ru(1.0 nm)-FeCo(25 nm)-Ru (10 nm).