The Unusual Double-Shifted Magnetization Curves In An Exchange-Biased Perpendicular Co/Irmn System

We observed an unusual double-shifted hysteresis loop in a perpendicular exchange-coupled Co/IrMn system at room temperature, which leads to an uneven exchange bias field in the positive and the negative field. With the thickness of the antiferromagnetic layer ranging from 4.8 to 10.4 nm, the negative exchange bias field is approximately 100 Oe larger than the positive exchange bias field. This result is related to the density of net spins in the antiferromagnetic layer that have pinned on the ferromagnetic layer. In addition, the exchange bias field of this unusual double-shifted hysteresis loop can be adjusted by the magnetic field annealing temperature.