Fluctuation mechanism for biquadratic exchange coupling in magnetic multilayers

We show how spatial fluctuations of chromium thickness amounting to one atomic monolayer account phenomenologically for the biquadratic magnetic coupling recently reported in epitaxial Fe/Cr/Fe trilayers. Corresponding fluctuations of the short-period oscillatory term of conventional exchange coupling induce static waves of magnetization whose energy has the observed biquadratic form. An existing calculation of conventional exchange and its recent direct measurements in highly ordered trilayers support our interpretation.