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

The bilinear (J(1)) and biquadratic (J(2)) exchange coupling constants were measured in sputtered trilayers of (100) Fe(40 Angstrom)/Cr(s)/Fe(40 Angst rom) for several Cr spacer layer thicknesses in the range s = 8-35 Angstrom and as a function of temperature T, using magneto-optical Kerr effect magn etometry, Brillouin light scattering, and ferromagnetic resonance. In the s amples in the range s = 8-13 Angstrom, corresponding to the first antiferro magnetic peak of J(1), J(2) follows J(1) with a room temperature ratio J(2) /J(1) congruent to 0.1, while in the range 25-35 Angstrom, corresponding to the second antiferromagnetic peak, J2 also follows J(1) but with a much la rger ratio J(2)/J(1) congruent to 1. This result, as well as the temperatur e dependence of J(2) in all samples but the one with s = 15 Angstrom, canno t be explained by any of the intrinsic or extrinsic mechanisms that have be en proposed for the origin of the biquadratic exchange coupling in Fe/Cr/Fe . (C) 1999 American Institute of Physics. [S0021-8979(99)68008-2].