  We investigate the thermodynamic properties of Heisenberg ferrimagnetic
mixed-spin chains both numerically and analytically with particular emphasis on
the combination of ferromagnetic and antiferromagnetic features. Employing a
new density-matrix renormalization-group technique as well as a quantum Monte
Carlo method, we reveal the overall thermal behavior: At very low temperatures,
the specific heat and the magnetic susceptibility times temperature behave like
$T^{1/2}$ and $T^{-1}$, respectively, whereas at intermediate temperatures,
they exhibit a Schottky-like peak and a minimum, respectively. Developing the
modified spin-wave theory, we complement the numerical findings and give a
precise estimate of the low-temperature behavior.
