Double-shifted magnetic hysteresis loops in perpendicular [Co/Ni] _N/FeMn exchange biased systems

Double exchange bias effect has been investigated in glass/Ta/Cu/[Co/Ni] _N/FeMn/Ta thin film structures with perpendicular magnetic anisotropy. For samples with a thin Cu under layer and a small repetition number N, the positive and negative subloops display equal exchange bias and coercive fields. With Cu thickness increasing, the negative exchange bias field decreases significantly but the positive one hardly changes. Although both bias fields decrease with the increase of N, the negative bias drops much faster than the positive bias. The observed difference is associated with the increase of perpendicular anisotropy, which weakens the exchange bias effect more in the negative loop with smaller domains. Thermal treatments verify that the doubly shifted phenomenon originates from the coexistence of antiparallel magnetic domains and it can be removed by annealing in a perpendicular field.