Pinholes in antiferromagnetically coupled multilayers: Effects on hysteresis loops and relation to biquadratic exchange

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Abstract
We present a micromagnetic study of the influence of ferromagnetic bridges between consecutive ferromagnetic layers in antiferromagnetically coupled multilayers. The model is compared with experimental results for hysteresis loops obtained from the multilayer systems Co-Cu and FeNi-Ag. The presence of pinholes in Cu-Co multilayers is confirmed by transmission electron microscopy. We demonstrate that low densities of ferromagnetic pinholes in such multilayers are sufficient to give rise to significant deviations from the expected bilinear interlayer coupling and modify the observed interlayer oscillatory exchange coupling. The effects of pinholes can be simulated in certain cases by biquadratic exchange coupling, and we propose a magnetic phase diagram which correlates the apparent bilinear and biquadratic couplings to the pinholes density, size, and interlayer exchange strength.