  We report here 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 $T_{c}$ = 121 K. We
find that the resistivity upturn below 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.
