  Novel ground-state spin structures in undoped and lightly-doped manganites
are here investigated based on the orbital-degenerate double-exchange model, by
using mean-field and numerical techniques. In undoped manganites, a new
antiferromagnetic (AFM) state, called the E-type phase, is found adjacent in
parameter space to the A-type AFM phase. Its structure is in agreement with
recent experimental results. This insulating E-AFM state is also competing with
a ferromagnetic metallic phase as well, suggesting that large magneto-resistant
effects could exist even in undoped Mn oxides. For doped layered manganites,
the phase diagram includes another new AFM phase of the ${\rm C_x
E_{1-x}}$-type. Experimental signatures of the new phases are discussed.
