Magnetic reversal and temperature dependence of coupling strength in two antiferromagnetically coupledCo∕Ptmultilayers

Abstract

Field-dependent domain imaging has been performed to understand the magnetic reversal mechanism in two antiferromagnetically coupled $\mathrm{Co}∕\mathrm{Pt}$ multilayers with out-of-plane anisotropy across a $11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ NiO spacer. Observation of only closed domain walls in the demagnetized state suggests that the two $\mathrm{Co}∕\mathrm{Pt}$ 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 $\mathrm{Co}∕\mathrm{Pt}$ 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 $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the coupling strength increases with the increase of temperature, being consistent with the previous observation, while above $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, both the coupling strength and the saturation magnetization have been observed to drop monotonically with the increase of temperature up to $526\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The antiferromagnetic coupling above room temperature may be still related to the antiferromagnetic ordering in the $11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ NiO spacer.