Exchange coupling and Mn valency in GaN doped with Mn and co-doped with Mg Mostefa Djermouni, Ali Zaoui, Roland Hayn and Abdelkader Boukortt Eur. Phys. J. B, 93 4 (2020) 61 Published online: 01 April 2020 DOI: 10.1140/epjb/e2020-100517-2