Antiferromagnetic coupling driven by bond length contraction near the Ga1-xMnxN film surface.

Using first principles calculations based on gradient corrected density functional theory we show that Mn atoms, which couple ferromagnetically in bulk Ga1-xMnxN, couple antiferromagnetically on its surface. This change in magnetic behavior is brought about by a contraction of the Mn-Mn and Mn-N bond lengths, which is significantly greater on the surface than in the bulk. The present study provides new insight to the numerous conflicting experimental observations in Mn doped GaN systems.