Tuning of interfacial perpendicular magnetic anisotropy and domain structures in magnetic thin film multilayers

We investigate the magnetic domain structures and the perpendicular magnetic anisotropy (PMA) arising in CoFeB films interfaced with selected heavy metal (HM) layers with large spin Hall angles in HM/CoFeB/MgO (HM = W, Pt, Pd, WxTa1-x) stacks as a function of CoFeB thickness and composition for both as-deposited and annealed materials stacks. The coercivity and the anisotropy fields of annealed material stacks are higher than for the as-deposited stacks due to crystallisation of the ferromagnetic layer. Generally a critical thickness of MgO > 1 nm provides adequate oxide formation at the top interface as a requirement for the generation of PMA. We demonstrate that in stacks with Pt as the HM, the PMA depends crucially on the alloy composition of the ferromagnet requiring a Co rich alloy to achieve PMA, i.e. it is important to have a large number of Pt-Co hybrid bonds at the interface compared to Pt-Fe hybrid bonds. In Pd/CoFeB/MgO layers, we observe for appropriate PMA a special stripe domain configuration that also includes circular skyrmion-like spin structures. Three different means of selective tuning of the PMA are discussed apart from the conventional FM thickness variation: Changing the number of repetitions of a multilayer stack is shown to change the PMA and strong PMA is found for certain compositions of WxTa1-x that also result in very low coercive fields (<1 mT) of the CoFeB. The PMA has also been shown to be tuned via annealing, very strong anisotropy fields of up to 1 T and low pinning of domain structures were measured in W/CoFeB/MgO, making these multilayers stacks a promising candidate for MRAM development.