Field induced biquadratic exchange in hard/soft ferromagnetic bilayers

The appearance of biquadratic exchange coupling between soft Fe and hard Sm Co thin layers is found. The remanent magnetization in the Fe layer reorien ts from parallel to perpendicular with respect to the SmCo easy axis after application of large enough negative field. To explain such an unexpected b ehavior in contacting ferromagnetic layers a model is proposed based on Slo nczewski's fluctuating exchange mechanism. In our samples a partial remagne tization of the hard layer creates fluctuations of the bilinear interaction s. The intralayer exchange averaging of the resulting magnetization fluctua tions in the soft layer causes the observed biquadratic coupling.