In a tunnel junction sensor a free layer structure has a cobalt iron (Co.sub.90 Fe.sub.10) layer and a half metallic iron oxide (Fe.sub.3 O.sub.4) layer and a pinned layer structure has a cobalt iron (Co.sub.50 Fe.sub.50) layer and a half metallic iron oxide (Fe.sub.3 O.sub.4) layer. Each of the iron oxide layers interfaces a barrier layer which is compatible therewith. The thicknesses of the iron oxide (Fe.sub.3 O.sub.4) layers are less than the thicknesses of the other layers of the free and pinned layer structures.