Effects of composition, thickness and temperature on the magnetic properties of amorphous CoFeB thin films

We report composition, thickness and temperature dependent magnetic properties of amorphous Co-80_ yFe(y)B(20) (x nm) films with y = 40,60 and x = 7-200 nm prepared directly on thermally oxidized Si substrate using magnetron sputtering technique. All the as-deposited films exhibit amorphous structure. For both the compositions, the shape of magnetic hysteresis (M-H) loops strongly depends on their thicknesses, i.e., the films with x <= 20 exhibit either rectangular shape or flat type loops, which change into transcritical type beyond a critical thickness (x > 50). Room temperature coercivity (Hc) and field required for saturation (Hs) increase almost linearly with increasing x between 7 and 20, but exhibit a rapid increase for the films with x up to 100 and then decrease slightly for x = 200 film. Thickness dependence of Hc and Hs varies also with composition. This is attributed to the development of effective magnetic anisotropy (K-eff) with increasing film thickness, which causes a change in magnetic domain structure from in-plane magnetization to stripe domains and degrades the magnetic properties above critical thickness. Temperature dependent M-H loops reveal that the loop shapes do not change with temperature for a given film. However, the values of Hc and Hs increase with decreasing temperature and the rate of increase depends on the composition and thickness. Similarly, the variation of Keff with temperature also displays a considerable dependency on the composition. The observed results are discussed on the basis of development of compositional and thickness dependent effective magnetic anisotropy, which changes the magnetic domain structure with increasing CoFeB film thickness. (C) 2016 Elsevier B.V. All rights reserved.