Exchange Bias in Ni/Co Multilayers with Perpendicular Anisotropy*

We have studied exchange bias in Ta(5nm)/Au(10)/[Ni(t)/Co(0.4)]$\times $5/IrMn(8)/Ta(5) multilayers for Ni thicknesses of 0.8-1.2 nm. The samples were deposited via sputtering and no deposition field was used. The samples were annealed at 200 C in an applied field of 1500 Oe for different durations, then measured by polar Magneto-Optical Kerr Effect at room temperature. We find that the field annealing significantly alters the hysteresis loop shape, giving it more single domain character while simultaneously inducing exchange bias in the direction of the annealing field. After 34 (2 and 32) hrs annealing, the exchange bias of the samples each reach a maximum value ranging from 100 Oe for the thickest Ni to 35 Oe for the thinnest Ni. We find that the samples with the thinner Ni layers approach their exchange bias maximum values for shorter annealing times.