Formation of quantum magnetization plateaux in mixed-spin Ising chains with single-ion anisotropy

We investigate the physical processes which give rise to a multi-plateau ground-state magnetization curve in ferrimagnetic Ising chains with alternating spins (S,s) and different single-ion anisotropies on each sublattice of the system under an applied magnetic field, by using an elaboration of the molecular-field theory. Our analysis is started with the system (S,12) for which we use the transfer-matrix technique for comparison. In this system, we find a double-plateau structure (initial and saturation) in the magnetization curve for all values of S>12, independent of anisotropies. Then we study two more elaborate systems, comparing the results with density-matrix renormalization group calculations, and finally generalize our argument to the general case. We find that for a specified range of the anisotropy parameters, the system exhibits 2s+1 plateaux, including the two classical and all those allowed for general quantum spin chains. This follows a similar rule as that known for spin-S (S≥1) Ising chains with single-ion anisotropy, for which 2S+1 plateaux appear in the ground-state magnetization curve, surviving even at low temperatures.