We report a detailed study of ac/dc magnetization and longitudinal/transverse transport properties of La _1.2 Sr _1.8 Mn ₂ O ₇ single crystals below T_c=118 K. We find that the resistivity minimum at 40 K is related to the reentrant spin glass phase at the same temperature, accompanied by additional anomalous Hall effects. The carrier concentration from the ordinary Hall effects remains constant during the transition and is close to the nominal doping level (0.4 holes/Mn). The spin glass behavior comes from the competition between ferromagnetic double exchange and antiferromagnetic superexchange interactions, which leads to phase separation, i.e., a mixture of ferromagnetic and antiferromagnetic clusters, representing the canted antiferromagnetic state. © 2001 American Institute of Physics.