Structural characterization of thin film ferromagnetic tunnel junctions

Tunnel junctions comprised of two magnetic metal layers separated by a thin insulating oxide layer have been prepared by reactive sputtering onto thermally oxidized (100) silicon wafers at room temperature. The magnetic layers (thicknesses ∼30-50 nm) consisted of thin films of Co, Fe, and/or CoFe and the oxide barriers (thicknesses in the range 2-10 nm) included CoO, MgO, HfO₂, and SiO₂. The barriers were prepared by dc reactive sputtering from pure metal sources in mixed oxygen-argon atmospheres or by rf sputtering from oxide targets. Transmission electron microscopy in the cross-sectional geometry was used in this study to characterize the tunnel junction microstructure. Barriers of CoO and MgO were invariably polycrystalline with many crystallites extending across the entire barrier thickness, whereas barriers of SiO₂ and HfO₂ appeared to be amorphous. Although grain boundary diffusion has been proposed as a possible mechanism for providing shorts or "pinholes" between the magnetic electrodes, it was significant that both HfO₂ (amorphous) and MgO (polycrystalline) showed high magnetoresistive response at low temperature.