  Measurements of the $^{1}$H-NMR spectrum of a small ($\sim$ 4 $\mu$g) single
crystal of the organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$ are reported
with an applied magnetic field $\bf{B}$$_{0}$ = 9 T parallel to the a-axis in
the $ac$-plane over a temperature $(T)$ range 2.0 $-$ 180 K. They provide the
distribution of the static local magnetic field at the proton sites in the
paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along
with the changes that occur at the PM$-$AFI phase transition. The spectra have
six main peaks that are significantly broadened and shifted at low $T$. The
origin of these features is attributed to the large dipolar field from the 3d
Fe$^{3+}$ ion moments (spin $S_{\rm{d}}$ = 5/2). Their amplitude and
$T-$dependence are modeled using a modified Brillouin function that includes a
mean field approximation for the total exchange interaction ($J_{0}$) between
one Fe$^{3+}$ ion and its two nearest neighbors. A good fit is obtained using
$J_{0}$ = $-$ 1.7 K. At temperatures below the PM$-$AFI transition temperature
$T_{MI}$ = 3.5 K, an extra peak appears on the high frequency side of the
spectrum and the details of the spectrum become smeared. Also, the rms
linewidth and the frequency shift of the spectral distribution are
discontinuous, consistent with the transition being first-order. These
measurements verify that the dominant local magnetic field contribution is from
the Fe$^{3+}$ ions and indicate that there is a significant change in the
static local magnetic field distribution at the proton sites on traversing the
PM to AFI phase transition.
