Spin-dependent tunneling junctions with hard magnetic layer pinning

We have fabricated ferromagnet-insulator-ferromagnet tunnel junctions with Co and NiFe electrodes, where the Co electrodes are pinned with a hard magnetic ${\mathrm{Co}}_{\mathrm{81}}{\mathrm{Pt}}_{\mathrm{19}}$ alloy layer. This approach gives a coercivity of about 300 Oe for the Co layer, while that of the NiFe is about 80 Oe, so we obtain antiparallel magnetization over a wide field range. The ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ tunneling barrier layers were formed by in situ plasma oxidation of elemental Al layers with thicknesses from 10 to 25 Å. For the junctions, we find room temperature magnetoresistance ratios as high as 13% and nonlinear current–voltage curves that are well fit by the Simmons tunneling theory. Depth profiling x-ray photoelectron spectroscopy of oxidized Al barrier layers on Co underlayers reveals a stoichiometry of nearly ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$