  The magnetic atom diffusion at the interface between CoMn alloy and MnGa
layer with annealing is studied using x-ray magnetic circular dichroism (XMCD)
analysis. We found that the spins in bcc CoMn are coupled parallel to those in
perpendicularly magnetized MnGa layer under the as-grown conditions, while the
post-annealing modulates the interfacial magnetic coupling to antiferromagnetic
in Co. The element-specific hysteresis curves at each absorption edge revealed
the large coercive fields in Mn and Co through the exchange coupling with MnGa.
After the annealing process, the changes of XMCD spectral line shapes are
related to the interfacial reactions promoting the formation of Mn and Co2MnGa
layers, which is deduced from the analysis of transmission electron microscopy.
The interfacial diffusion of Mn atoms modulates the magnetic exchange coupling
between Mn and Co sites and reverses the direction of perpendicular
magnetization.
