A series of MgO perpendicular magnetic tunneling junction (p-MTJ) films with soft/hard composite electrodes is prepared by RF-sputtering, where amorphous CoFeB is chosen as soft layer. The modulation of MTJs film magnetic properties on CoFeB thickness is investigated. The critical thickness for composite free layer transform from rigid magnet (RM) to exchange spring system (ES) is indentified. Besides, an unexpected in-plane exchange bias is observed which is attributed to the formation of closure domains. The evolution of domain structures on CoFeB is examined by magnetic force microscope. Coexistence of two distinguished magnetic domains of different sizes is observed. It is found that the evolution of domain morphology as varying CoFeB thickness is due to the modulation of effective anisotropy.