Using density functional theory with generalized gradient approximation for exchange and correlation, we show that otherwise antiferromagnetically coupled chromium atoms in very small chromium clusters couple ferromagnetically when doped with a nitrogen atom, thus leading to giant magnetic moments. For example, the magnetic moment of Cr2N is found to be 9mu(B) while that of Cr-2 is 0mu(B). Strong bonding between Cr and N atoms brings about this magnetic transition. The Cr atoms nearest neighbor to N couple ferromagnetically with each other and antiferromagnetically with nitrogen. The significance of these results in understanding the ferromagnetic order in Cr-doped GaN is discussed. (C) 2003 American Institute of Physics.