Magnetic properties of single-layer and multilayer structured Co40Fe40B20 thin films

We report systematic investigations of thickness dependent magnetic properties of amorphous Co40Fe40B20 (x nm) single-layer films and effects of number of multilayers and thickness of spacer layer on the magnetic properties of multilayer structured [Co40Fe40B20 (y nm)/Ta (z nm)]n = 1-3/Co40Fe40B20 (y nm) films prepared directly on thermally oxidized Si substrate using magnetron sputtering technique. All the as-deposited films at ambient temperature exhibit amorphous structure. For single-layer films, coercivity (HC) and field required for saturation (HS) increase gradually with increasing x from 10 to 30 nm, but exhibit a rapid increase when x ≥ 67 nm. This behaviour was attributed to the change in the magnetic domain structure from in-plane magnetization to stripe domains caused by the development of effective magnetic anisotropy instigated by stress accumulated during the deposition. High temperature thermomagnetization data show the Curie temperature of Co40Fe40B20 (20 nm) film as 512 K. On the other hand, the introduction of thin Ta spacer layers in multilayer films helps reducing HC and HS substantially, but magnitude of the reduction depends strongly on the values of n and z. The increase of n diminishes the development of effective magnetic anisotropy in CoFeB film. This is due to the reduction in the thickness of the ferromagnetic layers, which changes the magnetic domain structure, and the optimum value of z enhancing interlayer coupling between CoFeB layers. Hence, the magnetic properties of the multilayer thin films are improved. The observed results are elucidated on the basis of change in the magnetic domain structure with increasing film thickness in single-layer films, and with increasing number of multilayers and spacer layer thickness dependent interlayer coupling in multilayer structured thin films.