Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers

Magnetization reversal and magnetic exchange coupling of (Co/Ni) _×N /Ru/(Co/Pt) _×12 were investigated as a function of the temperature. The number of repeats N of the soft multilayer (Co/Ni) was varied from 4 to 8 bilayers while the number of repeats of the hard bilayers (Co/Pt) was fixed to 12. Two steps hysteresis loops were observed for coupled structure with only 4 repeats of (Co/Ni) in a wide range of temperature (25–300 K). From the shift of the minor hysteresis loop, the antiferromagnetic exchange coupling H _ex was measured and then the interlayer exchange coupling J _ex was calculated. A non-monotonous dependence of J _ex with temperature was observed for N = 4 with a maximum J _ex of 0.13 erg/cm ² at 150 K. The annealing process performed on the same structure confirms the unusual behavior of interlayer exchange coupling J _ex . As the repetition number N increases to 8 bilayers the two steps hysteresis loops disappeared in the investigated temperature range, however a small kink appeared in the range of 125 and 225 K for the case of 6 bilayers. From the analysis of the coupled and uncoupled structures, it seems that the dipolar energy overcomes the antiferromagnetic coupling for thicker (Co/Ni) multilayer.