Effect of underlayer roughness, grain size, and crystal texture on exchange coupled IrMn/CoFe thin films

The exchange bias field (H-ex) between a ferromagnetic and antiferromagnetic film has been found to be sensitive to interface roughness, crystalline texture, and grain size. In order to isolate the effects of these three parameters, we deposited Si/UL/CoFe4/IrMn5/Ta5nm (top) and Si/UL/IrMn5/CoFe4/Ta5nm (bottom configuration) exchange coupled multilayers on three underlayers, namely Cu, Ru, and [Cu1/Ru1nm]n with thickness between 5 and 100 nm. With an increase in Cu (fcc) underlayer thickness, roughness and grain size increased rapidly up to 0.9 nm rms and 48 nm, respectively. For the Ru (hcp) underlayer, roughness increased gradually to 0.3 nm while the grain size increased to 30 nm. In case of Cu/Ru (hcp+fcc), the roughness observed is between that of Cu and Ru for comparable thickness. However, the grain size is much smaller compared to Ru and Cu. For both top and bottom exchange coupled films, H-ex (200-300 Oe) is observed above a critical underlayer thickness (greater than or equal to 5 nm for Ru and greater than or equal to 15 nm for Cu and Cu/Ru). In the top configuration, above the critical underlayer thickness, H-ex decreases from 295 to 180 Oe with an increase in underlayer (UL) thickness due to the rougher interface. However, H-ex did not change with roughness for the bottom case. The difference in H-ex dependence on roughness for top and bottom cases can be explained in terms of magnetostatic effects on domain formation in the antiferromagnetic layer. In both cases, above the critical UL thickness, no correlation between H-ex and the grain size and texture was found. After annealing at 225 degrees C for 1 h, high interfacial exchange energy (J(k)similar to 0.22 ergs/cm(2)) for the top configuration is obtained for films deposited on UL which have large grain size. In the bottom case, a high J(k) (similar to 0.3 ergs/cm(2)) is obtained for films with good fcc-IrMn(111) texture, corresponding to growth on Cu/Ru underlayers. (C) 2000 American Institute of Physics. [S0021-8979(00)69008-4].