The exchange coupling strength and microstructure of the multilayers, Si/Ta(22)/Cu(t)/IrMn(10)/FeCo(4nm) with t Cu =1–30nm, have been investigated using XRD and high-resolution TEM. It has been found that randomly oriented small IrMn crystallites embedded in amorphous-like matrix are observed for t Cu =1 and 5nm, with their size increasing with increasing t Cu , while columnar IrMn crystallites grow epitaxially for t Cu =10nm. It has turned out that the exchange coupling strength depends on the number and size of the IrMn crystallites directly coupled with the FeCo layer rather than the orientation of the IrMn crystallites.