Dependence of exchange bias field and coercivity on spacer layer thickness in FeMn/NiFe/Cu/NiFe spin valve structures

Magnetization reversal mechanisms in sputter deposited Cu(5)/NiFe(7)/CoFe(0.3)/ Cu(t)/CoFe(0.3)/NiFe(6)/Fe(20)/ Cu(2) spin valve systems on silicon substrates for different values of t in between 2.0 and 3.0 nm have been studied using vibration sample magnetometry (VSM) and magneto optic Kerr effect spectrometry (MOKE) in surface (longitudinal) and transverse geometries. The exchange bias field matches in magnitude in both the experiments and shows an oscillatory behaviour of interlayer coupling with alternative ferromagnetic and antiferromagnetic alignments, while displaying the highest H-ex value for Cu thickness of 2.2 nm in the investigated thickness range. The coercivity also follows an oscillatory trend with enhanced values for successive Cu thicknesses. However, the coercivity values obtained by MOKE are found to be larger compared to VSM, which is attributed to the presence of highly polarized CoFe protective layer in either side of the Cu spacer. The variations in exchange bias fields are discussed in terms of the confinement of quantum well states at the two interfaces of the Cu spacer layer.