Tuning Magnetic Properties of Thick CoFeB Film by Interlayer Coupling in Trilayer Structured Thin Films

This study deals with tuning magnetic properties of a thick amorphous (a-)Co20Fe60B20 (CoFeB262) film by using interlayer magnetic coupling in trilayer structured films of [CoFeB262 (100 nm)/[Cr,Ta (x nm)]/CoFeB262 (y nm)] with y=2-50, xCr=0.75, 2 and xTa=1, 4. All the films are deposited directly on thermally oxidized Si substrate at ambient temperature using magnetron sputtering. The as-deposited a-CoFeB262 (100 nm) film exhibits magnetic stripe domain and transcritical hysteresis loop due to large effective magnetic anisotropy caused by stress induced during deposition of the films. On the other hand, the shape of magnetic hysteresis (M-H) loops in trilayer films transforms from transcritical to rectangular shaped one with enhanced remanence ratio (MR/MS) of ≥ 75% and single magnetization reversal behavior. This effectively reduces coercivity (HC) and field required to saturate magnetization (HS) in trilayer films. However, the changes in the loop shape and reductions in HC and HS depend strongly on x and y. Magnetic domain images obtained using Kerr microscopy in trilayer films show a rapid switching of large-sized domains along easy-axis and weak ripple domains along hard-axis. In addition, the magnetization reversal behavior along the hard-axis strongly depends on x(Cr,Ta). M-H loops obtained at different temperatures between 30 K and 300 K reveal no change in loop shape for trilayer films with small x and y, while the disappearance of shearing and formation of additional steps at low temperatures are observed for films with large x and y. The observed results are explained on the basis of change in interlayer coupling between CoFeB262 layers with x, y and temperature. Furthermore, these results clearly confirm that the magnetic properties of thick CoFeB262 film with stripe domain can easily be tuned into in-plane magnetization by this simple trilayer structured thin films.