Structural comparisons of ion beam and dc magnetron sputtered spin valves by high-resolution transmission electron microscopy

We have used high-resolution transmission electron microscopy to compare the nanostructures of ion-beam and dc magnetron sputter-deposited giant magnetoresistive (GMR) spin valves and to correlate nanostructure with magnetic properties. Very low coercivities and strong exchange bias (<8 Oe, 125 Oe) were achieved in ion-beam-deposited spin valves of the form NiFe(50)/Co(20)/Cu(≳25)/Co(20)/NiFe(50)/FeMn(150)/Ta(30 Å); these were compared with typical dc magnetron deposited structures of the same kind, both with and without a Ta seed layer, which exhibited similar and poorer exchange biasing but superior GMR ratios (to 8%.) Cross-sectional and plane-view samples were prepared of all three structures and examined by high-resolution electron microscopy. Near-perfect (111)-textured fcc metal and c-axis hcp Co columnar grains were revealed in the ion beam deposited sample, while some (10°) dispersion of this texture and random grain orientations were observed in the Ta-seeded and unseeded dc magnetron sputter-deposited samples, respectively. No amount of the α-FeMn (A12) phase was observed in any of the films. Exchange bias strengths and coercivity of the top Co/NiFe/FeMn layers thus correlate strongly with the degree of (111) texture.