  $^{63/65}$Cu- and $^{35/37}$Cl-NMR experiments were performed to investigate
triplet localization in the $S=1/2$ dimer compound NH$_4$CuCl$_3$, which shows
magnetization plateaus at one-quarter and three-quarters of the saturation
magnetization. In $^{63/65}$Cu-NMR experiments, signal from only the singlet Cu
site was observed, because that from the triplet Cu site was invisible due to
the strong spin fluctuation of onsite 3$d$-spins. We found that the temperature
dependence of the shift of $^{63/65}$Cu-NMR spectra at the singlet Cu site
deviated from that of macroscopic magnetization below T=6 K. This deviation is
interpreted as the triplet localization in this system. From the
$^{35/37}$Cl-NMR experiments at the 1/4-plateau phase, we found the two
different temperature dependences of Cl-shift, namely the temperature
dependence of one deviates below T=6 K from that of the macroscopic
magnetization as observed in the $^{63/65}$Cu-NMR experiments, whereas the
other corresponds well with that of the macroscopic magnetization in the entire
experimental temperature region. We interpreted these dependences as reflecting
the transferred hyperfine field at the Cl site located at a singlet site and at
a triplet site, respectively. This result also indicates that the triplets are
localized at low temperatures. $^{63/65}$Cu-NMR experiments performed at high
magnetic fields between the one-quarter and three-quarters magnetization
plateaus have revealed that the two differently oriented dimers in the unit
cell are equally occupied by triplets, the fact of which limits the theoretical
model on the periodic structure of the localized triplets.
