Anomalous exchange bias of the bottom NiFe layer in NiFe/FeMn/Al/NiFe

The exchange bias of the bottom NiFe layer in NiFe(3 nm)/FeMn(8 nm)/Al(t)/NiFe(t) prepared by rf magnetron sputtering has been investigated as a function of the thickness of the top NiFe and Al layers, where the NiFe and Al thicknesses were varied from 3 to 24 nm and from 0.3 to 2 nm, respectively. The exchange bias of the top NiFe layer is negligible when the Al thickness is larger than 0.3 nm. As the top NiFe thickness increases for a constant Al thickness of 1 nm, the exchange bias of the bottom layer increases from 21.2 to 228.8 Oe for a NiFe thickness of 12 nm, and then decreases with NiFe thickness. An exchange bias of 69.8 Oe for an Al thickness of 0.3 nm increases to 228 Oe on increasing the Al thickness up to 1 nm, and decreases with a further increase of Al thickness. The exchange bias of the bottom NiFe layer could be induced by the interfacial coupling between the bottom NiFe and FeMn layers, but this coupling is strongly dependent on the Al and top NiFe thicknesses, also revealing an anomalous character in the exchange bias of the bottom NiFe layer.