  We present a model compound for the $S$=1/2 ferromagnetic Heisenberg chain
composed of the verdazyl-based complex
$[$Zn(hfac)$_2]$$[$4-Cl-$o$-Py-V-(4-F)$_2]$. $Ab$ $initio$ MO calculations
indicate a predominant ferromagnetic interaction forming an $S$=1/2
ferromagnetic chain. The magnetic susceptibility and specific heat indicate a
phase transition to an AF order owing to the finite interchain couplings. We
explain the magnetic susceptibility and magnetization curve above the phase
transition temperature based on the $S$=1/2 ferromagnetic Heisenberg chain. The
magnetization curve in the ordered phase is described by a conventional AF
two-sublattice model. Furthermore, the obtained magnetic specific heat
reproduces the almost temperature-independent behavior of the $S$=1/2
ferromagnetic Heisenberg chain. In the low-temperature region, the magnetic
specific heat exhibits $\sqrt{T}$ dependence, which is attributed to the energy
dispersion in the ferromagnetic chain.
