Biquadratic coupling in sputtered Fe/Cr/Fe still in need of a new mechanism

Abstract

The bilinear (J1) and biquadratic (J2) exchange coupling constants were measured in sputtered trilayers of (100) Fe(40 Å)/Cr(s)/Fe(40 Å) for several Cr spacer layer thicknesses in the range s=8–35 Å and as a function of temperature T, using magneto-optical Kerr effect magnetometry, Brillouin light scattering, and ferromagnetic resonance. In the samples in the range s=8–13 Å, corresponding to the first antiferromagnetic peak of J1, J2 follows J1 with a room temperature ratio J2/J1≅0.1, while in the range 25–35 Å, corresponding to the second antiferromagnetic peak, J2 also follows J1 but with a much larger ratio J2/J1≅1. This result, as well as the temperature dependence of J2 in all samples but the one with s=15 Å, cannot be explained by any of the intrinsic or extrinsic mechanisms that have been proposed for the origin of the biquadratic exchange coupling in Fe/Cr/Fe.