Dependence of exchange coupling on the surface roughness and structure in α-Fe2O3/NiFe and NiFe/α-Fe2O3 bilayers

The exchange coupling characteristics for both Si/α-Fe2O3/NiFe and Si/NiFe/α-Fe2O3 bilayers have been investigated. These two bilayers showed completely different exchange coupling characteristics. The Si/α-Fe2O3 (50 nm)/NiFe(5 nm) bilayer had Hex=62.5 Oe, Hc=137.5 Oe, while the Si/NiFe(5 nm)/α-Fe2O3(50 nm) bilayer had Hex=4.5 Oe, Hc=9.5 Oe. The larger exchange bias field of α-Fe2O3/NiFe bilayer was mainly attributed to good crystallinity of α-Fe2O3 and the smooth interface between NiFe and α-Fe2O3. The interfacial exchange energy, Jex was also calculated for these two bilayers. The case of the rougher surface of bilayer (Si/NiFe/α-Fe2O3) exhibited smaller interfacial exchange energy. In order to verify the effect of α-Fe2O3 crystal structure on the exchange bias coupling characteristics, Ti (hcp), Zr (hcp), Ta (bcc), and Cu (fcc) were used as buffer layers for the Si/buffer layer/α-Fe2O3/NiFe structures. Ti and Zr buffer layers showed good exchange coupling performance, which was strongly related to good crystal matching with α-Fe2O3, but there was no exchange coupling improvement in the case of Ta and Cu buffer layers.