Enhancement of exchange bias in Mn-Ir/Co-Fe based spin valves with an ultrathin Cu underlayer and in situ Mn-Ir surface modification

Enhancement of exchange bias induced at the interface of the antiferromagnetic (AF)/ferromagnetic (F) layers was studied using the bottom "spin-valve films" (SVs) with the Mn-Ir/Co-Fe exchange coupled films. Exchange bias increased using an ultrathin Cu underlayer. Meanwhile, both exchange bias field, H_ex, and blocking temperature, T_B, increased intensively by heating specimens after depositing Mn-Ir film in a high vacuum. These two enhancement effects worked in an additive. As a result, an unidirectional anisotropy constant. J_K, of 0.39 erg/cm² (H_ex of 1.3 kOe) and T_B of ∼325°C were obtained for the bottom SVs with a total thickness of 233 Å including an AF layer of 68 Å Mn₇₄Ir₂₆ and a pinned layer of 20 Å Co₉₀Fe₁₀, where the SVs were field annealed at 320°C. A microstructural analysis using x-ray diffraction revealed that H_ex did not depend on the diffraction intensity from Mn-Ir (111) for the SVs with various underlayers, and no remarkable changes occurred in the microstructure of the SVs with the heating treatment in vacuum. Therefore, the enhancement effects might result from some changes in the microstructure and/or the morphology of the interface of AF/F layers.