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

Abstract

The role of magnetic, structural, and chemical roughness on the origin of exchange biasing in polycrystalline Mn52Ni48/Fe92(Mo8) bilayers has been investigated by transmission x-ray magnetic circular dichroism (XMCD), vibrating 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 temperatures of 200 °C, 250 °C, and 300 °C. The exchange bias, He was observed to be the largest for the 250 °C growth sample. The angular dependence of He can be well modeled in terms of a cosine series with odd terms confirming the unidirectional nature of the anisotropy energy. However, the coefficients are different for the three samples indicating different microscopic magnetic interactions at the interface. XMCD measurements showed no magnetic moment for Mn and Ni but showed systematic variations of the Fe moment, i.e., a decrease in Fe moment with increase in He 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 samples grown at 250 °C, it is found that at least about 4 MLs of Fe appear to be AF. Preliminary energy-filtered imaging of cross-section samples 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.