  Specific heat measurements down to 120mK have been performed on a
quasi-one-dimensional $S=1/2$ spin-gapped system (CH$_3$)$_2$NH$_2$CuCl$_3$ in
a magnetic field up to 8 T. This compound has a characteristic magnetization
curve which shows a gapless ground state and a plateau at 1/2 of the saturation
value. We have observed a spontaneous antiferromagnetic ordering and a
field-induced one below and above the 1/2 plateau field range, respectively.
The field versus temperature phase diagram is quite unusual and completely
different from those of the other quantum spin systems investigated so far. In
the plateau field range, a double-structure in the specific heat is observed,
reflecting the coexistence of ferromagnetic and antiferromagnetic excitations.
These behaviors are discussed on the basis of a recently proposed novel quantum
spin chain model consisting of weakly coupled ferromagnetic and
antiferromagnetic dimers.
