Magnetic reversal and temperature dependence of coupling strength in two antiferromagnetically coupled Co/Pt multilayers

Field-dependent domain imaging has been performed to understand the magnetic reversal mechanism in two antiferromagnetically coupled Co/Pt multilayers with out-of-plane anisotropy across a 11 angstrom NiO spacer. Observation of only closed domain walls in the demagnetized state suggests that the two Co/Pt multilayers are antiferromagnetically coupled domain by domain. Along the initial magnetization curve, the magnetic reversal of the top multilayer is mainly due to domain nucleation at fields lower than the reversal field, while around the reversal field, growth of nucleated domains dominates. Along the field-decreasing branch in minor loop of the top Co/Pt multilayer, the magnetic reversal is through simultaneous domain nucleation and growth of nucleated domains, while along the field-increasing branch, the reversal is via domain nucleation at lower fields but via growth of nucleated domains around the reversal field. The asymmetric magnetic reversal along the field decreasing and increasing branches can be attributed to the antiferromagnetic interlayer coupling across the thin NiO spacer. The temperature dependence of the antiferromagnetic interlayer coupling has also been investigated. It has been observed that below 300 K, the coupling strength increases with the increase of temperature, being consistent with the previous observation, while above 300 K, both the coupling strength and the saturation magnetization have been observed to drop monotonically with the increase of temperature up to 526 K. The antiferromagnetic coupling above room temperature may be still related to the antiferromagnetic ordering in the 11 angstrom NiO spacer.