  Ground state properties of the S=1/2 frustrated Heisenberg chain with period
3 exchange modulation are investigated using the numerical diagonalization and
DMRG method. It is known that this model has a magnetization plateau at 1/3 of
the saturation magnetization M_s. On the other hand, the ground state is
ferrimagnetic even in the absence of frustration if one of the nearest
neighbour bond is ferromagnetic and others are antiferromagnetic. In the
present work, we show that this ferrimagnetic state continues to the region in
which all bonds are antiferromagnetic if frustration is strong. This state
further continues to the above mentioned 1/3-plateau state. In between, we also
find the noncollinear ferrimagnetic phase in which the spontaneous
magnetization is finite but less than M_s/3. The intuitive interpretation for
the phase diagram is given and the physical properties of these phases are
discussed.
