Asymmetrical Elementary Excitations Responsible for the Ferrimagnetism in an Antiferromagnetic Spin-1/2 Zigzag Ladder

The finite-temperature magnetic properties of an antiferromagnetic (AF) bond alternating S=1/2 zigzag spin chain with asymmetrical AF next-nearest-neighbor (NNN) exchange interactions in an external magnetic field are investigated by means of the many-body Greens function theory within random phase approximation. The results show that when the NNN exchange interactions are asymmetrical, the spin system exhibit a clear ferrimagnetic ordering at finite temperatures. It is shown that the ferrimagnetic behavior is attributed to asymmetrical elementary excitations, resulting from the competition between the spin frustrations and magnetic excitations reduced by the asymmetrical NNN interactions. The mechanism of this ferrimagnetism is much different from a common one which originates from mixed spins with different spin values through antiparallel spin alignments.