In recent years, spin valves have received increasing attention for possible application in magnetoresistive sensors, most notably in read heads for hard disk(HDD) and Magnetic Random Access Memory(MRAM) devices. Spin valve structures consist of two ferromagnetic layers seperated by a noble metal spacer. When the angle between the magnetizations is varied, there are large changes in film resistivity. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them is constrained by exchange anisotropy. Because of exchange coupling at the AFM/FM interface, the antiferomagnetic film provides a unidirectional bias, or unidirectional anisotropy, to an adjacent ferromagnetic film. But the fundamental mechanism is not yet fully understood. It has been known that FeMn(AFM) is antiferromagnet when FeMn is grown on fcc NiFe or Cu (111). In this study, magnetoresistnace and magnetic properties of Co/Cu/Co/FeMn spin valve is investigated.
In order to study exchange-bias coupling, seed layer(Cu) thickness and FeMn deposition conditions are varied. It is found that FeMn could be γ-FeMn antiferromagnet though it is not grown on fcc Cu(111) and optimum deposition conditions of FeMn for maximum exchange-bias coupling are found. Interface roughness and deposition rate are also important factors.
The dependence of MR ratio on layer thickness is that MR ratio is decreased with respect to increase layer thickness because of shunt current effect. But if layer thickness is smaller than ↓spin mean free path, MR ratio decreases because spin scattering is not enough and interlayer coupling between Co(free) and Co(pinned) layers is increase. In this study maximum 6.2% MR ratio is obtained.
By annealing, exchange-bias coupling is much increased and MR ratio is increased from 3.5% to 4.9%. This enhancement of exchange-bias coupling is due to atom rearrangement and grain size effect other than FeMn/Co interface. For MR ratio, Co/Cu interface smoothing is the cause of MR ratio enhancement. But as annealing tempreature is much increased, MR ratio is decreased.
