Asymmetry of magneto-optical Kerr effect loops of Co nano-columns grown by oblique incident angle deposition

Abstract

Magneto-optical Kerr effect (MOKE) in the longitudinal mode was used to measure the in-plane magnetic anisotropy of about 300nm thick Co nano-column films that were grown by e-beam evaporation at different oblique incident angles (0–85°) on native SiO2 substrates at room temperature. The symmetry of MOKE loops measured at azimuthal angles that are 180° apart is obviously broken for Co nano-column films grown at high deposition angles >70°. The plot of coercivity Hc of loops versus azimuthal angle shows that Hc values for azimuthal angles between −90° and 90° are much larger than the values for azimuthal angles between 90° and 270°. The asymmetry of coercivity is due to MOKE measurement that combines both longitudinal and polar Kerr effects. This combination is caused by the oblique magnetic anisotropy associated with the tilted Co nano-columns. A method is introduced to separate the longitudinal and polar Kerr effects. The longitudinal Kerr effect is obtained by adding hysteresis loops measured at azimuthal angles 180° apart while the polar Kerr effect is obtained by subtracting these two loops. By comparing these two orthogonal magnetization components represented by longitudinal and polar Kerr effects, we show that magnetization rotation exists in the magnetic reversal process even at azimuthal angle φ=0° that is the closest in-plane direction to the easy axis. After separating these two Kerr effects, the two-fold symmetry of coervicity associated with tilted nano-columns measured by longitudinal Kerr effect is restored.