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 similar to 30-50 nm) consisted of thin f ilms 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 p repared by de reactive sputtering from pure metal sources in mixed oxy gen-argon atmospheres or by rf sputtering from oxide targets. Transmis sion electron microscopy in the cross-sectional geometry was used in t his 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 Si O2 and HfO2 appeared to be amorphous. Although grain boundary diffusio n has been proposed as a possible mechanism for providing shorts or '' pinholes'' between the magnetic electrodes, it was significant that bo th HfO, (amorphous) and MgO (polycrystalline) showed high magnetoresis tive response at low temperature. (C) 1998 American Institute of Physi cs.