  We have performed a muon spin rotation study on polycrystalline samples of
electron-doped layered manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (0.4<
x<1), in order to investigate the local magnetic structure and spin dynamics.
Our results provide evidence for phase separation into A-type antiferromagnetic
and charge-ordered phases for x=0.52 and spin freezing at low temperatures
(T<100K) for 0.52<x<0.75. A new phase diagram which includes this spin freezing
region is proposed.
