Exchange biasing and interface structure in MnNi/Fe(Mo) bilayers

The role of magnetic, structural, and chemical roughness on the origin of e xchange biasing in polycrystalline Mn52Ni48/Fe-92(Mo-8) bilayers has been i nvestigated by transmission x-ray magnetic circular dichroism (XMCD), vibra ting sample magnetometry (VSM), and transmission electron microscopy (TEM). Three bilayer samples of MnNi(22 nm)/Fe(Mo) (6 nm) were grown by molecular beam epitaxy under ultrahigh vacuum conditions with the MnNi layer at temp eratures of 200 degrees C, 250 degrees C, and 300 degrees C. The exchange b ias, H-e was observed to be the largest for the 250 degrees C growth sample . The angular dependence of H-e can be well modeled in terms of a cosine se ries with odd terms confirming the unidirectional nature of the anisotropy energy. However, the coefficients are different for the three samples indic ating different microscopic magnetic interactions at the interface. XMCD me asurements showed no magnetic moment for Mn and Ni but showed systematic va riations of the Fe moment, i.e., a decrease in Fe moment with increase in H -e was observed. We have interpreted this decrease in Fe moment in terms of antiferromagnetic (AF) ordering of Fe at the interface with the extent of the AF ordering being related to the magnitude of the exchange. Thus, for s amples grown at 250 degrees C, it is found that at least about 4 MLs of Fe appear to be AF. Preliminary energy-filtered imaging of cross-section sampl es shows that the Fe layer is chemically rough suggesting that the decrease in moment may arise from the intermixing of Fe with the MnNi layer. (C) 20 00 American Institute of Physics. [S0021-8979(00)68908-9].