Oscillatory Interlayer Exchange Coupling in Amorphous CoSiB/Pt/CoSiB Structure

Oscillatory interlayer exchange coupling (IEC) is found in amorphous CoSiB(1.5 nm)/Pt(t_Pt)/CoSiB(1.5 nm) structure as a function of the Pt layer thickness, t_Pt, which is varied from 1.4 to 3.0 nm. We prepared the samples using dc magnetron sputtering method, and measured anomalous Hall resistance and magnetization hysteresis loops. All the samples show out-of-plane easy axes with perpendicular magnetic anisotropy energies. Surprisingly, strong antiferromagnetic IEC is found when t_Pt=2.2-2.7 nm, otherwise they are simple ferromagnets like multilayer systems of Co/Pt and Fe/Pt. The Pt layer is known as near ferromagnet, because it is easily polarized with an adjacent ferromagnetic layer or by a small amount of ferromagnetic impurities. The interlayer coupling via Pt has always been observed to be ferromagnetic so far. We report the antiferromagnetic IEC in amorphous CoSiB/Pt/CoSiB trilayer structure and propose its possible physical origin based on quantum well theory and negative spin polarization of Pt layer.