Abstract
A new system, exhibiting unusually large interlayer biquadratic exchange coupling, has been discovered, consisting of single crystal FeCo/Mn/FeCo sandwiches epitaxially grown on GaAs substrates. Normalized m(H) curves yield a remanent moment of approximately 0.5 for all cases studied, with a number of examples having saturation fields as large as several Tesla. This implies a biquadratic coupling constant as large or larger than many known bilinear coupling constants. None of the normalized m(H) data exhibits a remanence of less than 0.5, indicating the absence of comparable contributions from bilinear coupling. Angular dependent FMR at 9 GHz implies a fourfold anisotropy of opposite sign to that measured for single FeCo layers, whereas FMR at 35 GHz agrees in sign with the single layer anisotropy. Detailed analysis of this contradiction shows that this is an apparent anisotropy reversal which emerges within the theory of FMR in the presence of large biquadratic coupling, requiring values of the coupling constant in excess of −j2=2 ergs/cm2, where the coupling term in the energy is written J2[(m1⋅m2)/m1m2]2. This large biquadratic coupling, together with the absence of bilinear coupling, appears to contradict existing theories of interlayer exchange.
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