Ferrimagnetic States in S=1/2 Frustrated Heisenberg Chains with Period 3 Exchange Modulation

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.