Combination of ultimate magnetization and ultrahigh uniaxial anisotropy in CoFe exchange-coupled multilayers

Abstract

CoFe exchanged-coupled multilayers are studied versus the thickness of the ferromagnetic (F) layer (eF) and the thickness of the antiferromagnetic (AF) layer (eAF). The F layer consists in Co90Fe10, Co50Fe50, and Co35Fe65, respectively. The AF layer is Ni50Mn50. Very high saturation magnetization (4πMs) are reported with 18.5, 23, and 24kG, respectively, combined with excellent soft rotational behaviors. The AF–F–AF multilayers cumulate top and bottom interfacial exchange coupling and show very high effective energy densities (Jex) of 1.01, 0.81, and 0.98ergcm−2, respectively. Jex increases with eAF up to a maximum for eAF⩾500Å. This leads to a strong bias field (Hex) with a classical 1∕eF dependence, which translates into a large uniaxial anisotropy (Hk) at 90° from the pinning direction. Typically, for eF=150Å, Hk is 741, 650, and 642Oe, respectively. However, this translation is rather complex and eF and 1∕eAF dependences of the ratio Hk∕Hex suggest a contribution of an intrinsic CoFe anisotropy. For all CoFe, it is shown that a ferromagnetic resonance frequency (fFMR) of 10GHz is achievable. Co35Fe65 mulilayers exhibit the highest eF×μ′ product, μ′ being very close to the theoretical ultimate permeability. Finally, the soft rotational behavior is shown unchanged over an angular range of ±30° from the pinning direction which is very unusual.