Exchange coupling and Mn valency in GaN doped with Mn and co-doped with MgMostefa Djermouni, Ali Zaoui, Roland Hayn and Abdelkader BoukorttEur. Phys. J. B, 93 4 (2020) 61DOI: https://doi.org/10.1140/epjb/e2020-100517-2