Manganese (Mn) grows in a meta-stable expanded (c/a > 1) face-centered-tetragonal (fct) phase on thin fct-Co(001) template films. For small Mn thicknesses a close to a layer-by-layer growth is observed. Antiferromagnetism (AFM) of fct-Mn is evidenced by the observation of shifted magnetization loops antiparallel to the cooling field direction (negative bias) and enhanced coercive fields for Co/Mn bilayers. At low temperatures magnetic Co/Mn interface exchange interactions are detected for nominal Mn thicknesses as low as 1.3 monolayer (ML), indicating the onset of AFM in 2 ML thick Mn islands. However, between 2 ML and 5 ML Mn a positive bias of the magnetization loops is observed in a certain temperature range below the blocking temperature (TB) of the films. This peculiar behavior is explained by the dependence of TB on both the local Mn ML thickness and the Mn island size, leading to a unidirectional coercivity enhancement in a temperature range where both blocked and unblocked regions are coexisting in the Mn layers.
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