Ferrimagnetism induced by asymmetrical ferromagnetic next-nearest-neighbour exchange interactions in an antiferromagnetic spin-1/2 zigzag chain

The magnetic properties of an antiferromagnetic bond alternating spin-1/2 zigzag chain with asymmetrical ferromagnetic next-nearest-neighbour exchange interactions at finite temperature are investigated by using the many-body Green's function theory. It is found that the ferrimagnetic ordering does not appear in the symmetrical next-nearest-neighbour coupling case, and takes place only for the asymmetrical next-nearest-neighbour case at finite temperature rather than the ground state. Furthermore, as the asymmetry degree of the next-nearest-neighbour exchange interactions increases, the ferrimagnetism becomes more and more dominant. It is shown that the elementary excitation spectra are responsible for the observed magnetic behaviour.