Combination of ferromagnetic and antiferromagnetic features in Heisenberg ferrimagnets

We investigate the thermodynamic properties of Heisenberg ferrimagnetic mix ed-spin chains both numerically and analytically with particular emphasis o n the combination of ferromagnetic and antiferromagnetic features. Employin g a new density-matrix renormalization-group technique as well as a quantum Monte Carlo method, we reveal the overall thermal behaviour: at very low t emperatures, the specific heat and the magnetic susceptibility multiplied b y the temperature behave like T-1/2 and T-1, respectively, whereas at inter mediate temperatures, they exhibit a Schottky-like peak and a minimum, resp ectively. Developing the modified spin-wave theory, we complement the numer ical findings and give precise estimates for the low-temperature behaviour.