We study the low-temperature properties of S = 1 and 1/2 alternating spin chains with antiferromagnetic nearest-neighbor exchange couplings using analytical techniques as well as a quantum Monte Carlo method. The spin-wave approach predicts two different low-lying excitations, which are gapped and gapless, respectively. The structure of low-lying levels is also discussed by perturbation theory in the strength of the Ising anisotropy. These analytical findings are compared with the results of quantum Monte Carlo calculations and it turns out that spin-wave theory well describes the present system. We conclude that the quantum ferrimagnetic chain exhibits both ferromagnetic and antiferromagnetic aspects.

We present a new method of constructing the matrix product (MP) states and apply it to the ground state of the Heisenberg spin chain with alternating spins 1 and 1 2 and antiferromagnetic exchange interactions between nearest neighbors (the simplest example of a quantum ferrimagnet). The elementary matrix state is constructed in a way which ensures given transformational properties under rotations, which allows one to fix the total spin and its z-projection of the entire MP wavefunction. We compare the variational MP results with the numerical results obtained through a Quantum Monte Carlo method; the agreement is found to be within 0.4% for the ground state energy and 5% for the correlation functions.

We study thermodynamic properties of the one-dimensional Heisenberg ferrimagnet with antiferromagnetically exchange-coupled two kinds of spins 1 and 1/2. The specific heat and the magnetic susceptibility are calculated employing a modified spin-wave theory as well as a quantum Monte Carlo method. The specific heat is in proportion to T 1/2 at low enough temperatures but shows a Schottky-like peak at mid temperatures. The susceptibility diverges as T −2 . We reveal that at low temperatures the model is regarded as a ferromagnet, while at mid temperatures it behaves like a gapped antiferromagnet.