Exchange bias in [Pt∕Co]n∕IrMn multilayers with perpendicular magnetic anisotropy

Abstract

An unusual peak in ferromagnetic thickness dependence of exchange bias properties is observed for [Pt(15)∕Co(tCo)]4–IrMn(50) (in angstrom) multilayer prepared by magnetron sputtering. The exchange coupling field (Hex) shows the maximum value of 291Oe for tCo=3Å. Such atypical behavior is ascribed to a reduction of the perpendicular effective magnetic anisotropy and the concomitant reduction of MR∕MS (remanence to saturation magnetization ratio) for either very thin or exceedingly thick tCo. The hysteresis loops become asymmetric at intermediate angle with a shift not only along the field axis but also along the magnetization axis. Hex and coercivity (Hc) show a 1∕cosθ and 1∕∣cosθ∣ dependence on the angle (θ) between the applied measuring field (Ha) and the perpendicular-plane cooling field, respectively. This 1∕cosθ dependence can be accounted for by considering the strong out-of-plane magnetic anisotropy introduced during the field cooling.