Co-existence of biquadratic and unidirectional anisotropy in IrMn/Co/FeOx/Co films

Samples of Ta/Cu 3 nm/IrMn 8 nm/Co_(FM1) 1 nm/FeO_x 1.6 nm/Co_(FM2) 2.5 nm/Ta were prepared using sputtering deposition. The exchange biasing direction of the films were set by postannealing in vacuum at 200 °C for 15 min at a presence of 1 kOe. Hysteresis loops measured parallel (0°) and perpendicular (90°) to the annealing field show the existence of unidirectional anisotropy and uniaxial anisotropy, respectively. The presence of the uniaxial anisotropy at 90° induced a local energy minumum at 90°. The effects of local energy minimum at 90° are more pronounced in the R-H loops measured at 20° where the coexistence of the two anisotropies results to an extra MR minimum near the zero field. With increasing positive field, the magnetization of the free layer is switched to the appllied field direction, accompanying the increase in MR. Further increase of the positive fields lead to the re-appearance of the MR minimum due to the alignment of the magnetization of FM2 and free layer.