  We successfully synthesized a verdazyl-based charge-transfer salt
$[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$, which has an $S_{\rm{V}}$=1/2 on the
radical $o$-MePy-V-($p$-Br)$_2$ and an $S_{\rm{Fe}}$=5/2 on the FeCl$_4$ anion.
$Ab$ $initio$ molecular orbital calculations indicate the formation of an
$S_{\rm{V}}$=1/2 honeycomb lattice composed of three types of exchange
interaction with two types of inequivalent site. Further, the $S_{\rm{V}}$=1/2
at one site is sandwiched by $S_{\rm{Fe}}$=5/2 spins through antiferromagnetic
(AF) interactions. The magnetic properties indicate that the dominant AF
interactions between the $S_{\rm{V}}$ = 1/2 spins form a gapped singlet state,
and the remaining $S_{\rm{Fe}}$ = 5/2 spins cause an AF order. The
magnetization curve exhibits a linear increase up to approximately 7 T, and an
unconventional 5/6 magnetization plateau appears between 7 T and 40 T. We
discuss the differences between the effective interactions associated with the
magnetic properties of the present compound and ($o$-MePy-V)FeCl$_4$. We
explain the low-field linear magnetization curve through a mean-field
approximation of an $S_{\rm{Fe}}$ = 5/2 spin model. At higher field regions,
the 5/6 magnetization plateau and subsequent nonlinear increase are reproduced
by the $S_{\rm{V}}$ = 1/2 AF dimer, in which a particular internal field is
applied to one of the spin sites. The ESR resonance signals in the
low-temperature and low-field regime are explained by conventional
two-sublattice AF resonance modes with easy-axis anisotropy. These results
demonstrate that exchange interactions between $S_{\rm{V}}$ = 1/2 and
$S_{\rm{Fe}}$ = 5/2 spins in $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$ realize
unconventional magnetic properties with low-field classical behavior and
field-induced quantum behavior.
