Recently, there has been considerable interest in the investigation of ferromagnetic spin-valve layered structures because of their large magnetoresistance. Spin-valve structures consist of two ferromagnetic layers (e.g., NiFe and Co) separated by a noble metal spacer (e.g., Cu, Au and Ag): when the angle between the magnetizations is varied, large changes in film resistivity result. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anistropy (e.g., NiFe/FeMn). Recent studies showed that the magnetoresistance varied strongly with NiFe/Cu interface structure (e.g., intermixing at the interface and interface roughness). Usually the presence of compositionally intermixed regions at the NiFe/Cu interface was shown to reduce the magnetoresistance in sputter deposited NiFe/Cu/NiFe/FeMn spin-valve structures. Also interface roughness weakens exchange coupling while it enhances topological coupling which is the magnetostatic interaction induced by the interface roughness. Generally, a weak or no interlayer coupling is desired in spin-valve structures.
Effect of degree of intermixing at the Ta/NiFe interface induced by varying applied substrate bias voltage during free layer NiFe deposition on magnetoresistance in Si/Ta/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayers was investigated. It was found that the optimum NiFe thickness which gives maximum MR increase with increasing the bias voltage. The cause of this phomenon is the decrease of magnetization of NiFe layer by intermixing of Ta and NiFe layers with the increasing bias voltage. Meanwhile, as the thickness of NiFe free layer increased beyond the optimum thickness in the bias sputtered films, MR decreased slowly. The cause of the MR decrease was interpreted as the increase of the shunting current through the free layer.
To study interfacial roughness effects, we have introduced AlN bump underlayer on the corning glass 2948. The interfacial roughness was controlled by varying the AlN deposition thickness. As the AlN deposition time increased, (111) peak intensity of the spin valve films decreased. The decrease of (111) texture in FeMn will result in weakening of exchange coupling between FeMn and NiFe. Also the bumps will increase magnetostatic coupling between the two NiFe layers. Therefore, the relative orientation of the magnetizations of the two ferromagnetic layers are not perpectly parallel or antiparallel when applied magnetic field changes and this will decrease the maximum MR.
