We investigated the origin of the cooperative transition of As-Ga atoms in Be-doped low-temperature-grown GaAs layers by magnetization measurements and first principle calculations. For first principle calculations, a large supercell was used to reproduce the average distance of As-Ga atoms and Be atoms in experimental samples. With one As-Ga atom and Be atom in each supercell and the periodic boundary condition, the calculated total energy of the As-Ga at the substitutional site is lower than that at the interstitial site by 0.87 eV. The magnetization measurements have shown that the transition occurs in a sample with a GaAs substrate similar to that in lift-off samples, indicating that a uniform strain induced by the substrate does not affect the occurrence of the transition. These results suggest that the complex strain field induced by the coexistence of As-Ga defects at substitution and interstitial site plays a major role in the cooperative transition. (c) 2013 Elsevier B.V. All rights reserved.