Origin of biquadratic exchange in magnetic multilayers (invited)

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Abstract
We postulate localized‐electron states with unpaired spin located within or at the interfaces of an otherwise nonmagnetic metallic spacer layer. Loose exchange coupling of these spins to two ferromagnets mediates a non‐Heisenberg exchange coupling between them which includes a biquadratic term. A particular version of the model assumes that each interfacial layer of magnetic atoms is weakly exchange coupled to the remainder of the ferromagnets. This model permits interpretation of the biquadratic‐coupling data of Gutierrez et al. for Fe/Al/Fe trilayers and of Fuss et al. for Fe/Au/Fe. According to this interpretation, the observed biquadratic coupling is intrinsic to the ideal multilayer structure rather than due to impurities or structural defects.