  We present a detailed study of the field-dependent specific heat of the
bimetallic ferromagnetically coupled chain compound MnNi(NO$_2$)$_4$(en)$_2$,
en = ethylenediamine. For this material, which in zero field orders
antiferromagnetically below $T_N=2.45$ K, small fields suppress magnetic order.
Instead, in such fields a double-peak like structure in the temperature
dependence of the specific heat is observed. We attribute this behavior to the
existence of an acoustic and an optical mode in the spin wave dispersion as
result of the existence of two different spins per unit cell. We compare our
experimental data to numerical results for the specific heat obtained by exact
diagonalization and Quantum-Monte-Carlo simulations for the alternating spin
chain model, using parameters that have been derived from the high-temperature
behavior of the magnetic susceptibility. The interchain coupling is included in
the numerical treatment at the mean-field level. We observe remarkable
agreement between experiment and theory, including the ordering transition,
using previously determined parameters. Furthermore, the observed strong effect
of an applied magnetic field on the ordered state of MnNi(NO$_2$)$_4$(en)$_2$
promises interesting magnetocaloric properties.
