Exchange bias is the result of an exchange interaction between an antiferromagnetic(AFM) material and a ferromagnetic(FM) material. When materials with FM-AFM interface are cooled through the Neel temperature of the AFM, an exchange bias is induced in the FM. The hysteresis loop of the FM-AFM system shifts from center generally in the opposite direction to the cooling field. The exchange bias field (Hex) between a FM and an AFM film are reported to be sensitive to atomic scale interfacial topology, AFM crystalline texture and AFM grain size.
In this study, the effects of underlayers of IrMn and magnetic layers on exchange field, the effect of interfacial long range roughness on exchange field and annealing effect were investigated. The exchange coupled multilayers were deposited by a DC magnetron sputtering method on thermally oxidized Si wafer with the following layer structure : Si/Ta(50Å)/UL(80Å)/IrMn(200Å)/FM(40Å)/Ta(50Å), where underlayer UL represents NiFe and PtNiFe, and magnetic layer FM represents NiFe, Co, $Co_{80}Fe_{20}$, $Co_{50}Fe_{50}$.
For very thin films, the micro structural characteristics of IrMn AFM layer are closely linked to the structure of UL used. NiFe is most widely used UL. In this study and we introduce new UL, PtNiFe for the improvement of IrMn texture. We controlled PtNiFe compositions to have the same structure and lattice parameter as those of IrMn layer. As the result, crystalline texture of IrMn was extremely improved and grain size of IrMn was slightly increased. Even though the crystalline texture of IrMn was greatly improved, The increase of the exchange field was in~ 15Oe range than the film coupled to NiFe UL.. Therefore the texture of IrMn might not much affect to exchange field.
To study effect of FM on exchange field, we varied FM layer shuch as NiFe, Co, $Co_{80}Fe_{20}$ and $Co_{50}Fe_{50}$. The results showed that the exchange field is higher, when FM layer has a larger saturation magnetization value.
The effect of the interfacial long range roughness was studied. We inserted Al(600Å) bottom layer under exchange coupled multilayer. Because Al has columnar structure, the Al bottom layer gave variation in interfacial micro roughness to the multilayer. When the interfacial roughness was assumed as sinusoidal function, the amplitude divided by wavelength gives 0.1 in this sample. Even though Orange peel coupling field was ∼10Oein those samples, the film with Al bottom layer showed larger exchange field than that without Al. As the Al bottom layer did not affect the crystalline texture and grain size of IrMn, increase of exchange field might be caused by other factors.
The annealing effect on exchange field was investigated. When samples were annealed, exchange field increased up to a certain annealing temperature that depend on UL material. When the conventional NiFe UL was used, maximum exchange field was observed at 300℃ annealing. When the new PtNiFe UL was used, maximum exchange field was at 400℃ annealing. This difference in annealing is related to Mn diffusion behavior into the different underlayer at each film. Some increase of the exchange field after annealing below the optimum temperature are found to be related to improvement of IrMn texture, increase of grain size and the formation of IrMn ordered phase.