  A single crystal S=3/2 perfect kagome lattice antiferromagnet
$\mathrm{KCr_3(OH)_6(SO_4)_2}$ (Cr-jarosite) has been studied by X-band and
high-frequency electron spin resonance (ESR). The g-values perpendicular to the
kagome plane (c-axis) and in the plane are determined to be $g_c=1.9704 \pm
0.0002$ and $g_\xi=1.9720 \pm 0.0003$, respectively, by high-frequency ESR
observed at 265 K. Antiferromagnetic resonances (AFMR) with the
antiferromagnetic gap of 120 GHz are observed at 1.9 K, which is below
$T_N$=4.5 K. The analysis of AFMR modes by the conventional molecular field
theory shows $d_p=0.27$ K and $d_z=0.07$ K, where $d_p$ and $d_z$ are in-plane
and out-of-plane components of DM d vector, respectively. From these results
and the estimated exchange interaction J=6.15 K by Okuta et al., the ground
state of Cr-jarosite is discussed in connection with the Monte Carlo
simulations result with classical Heisenberg spins on the kagome lattice by
Elhajal et al. Finally, the angular dependence of linewidth and the lineshape
observed at 296 K by X-band ESR show typical behavior of a two-dimensional
Heisenberg antiferromagnet, suggesting a good two-dimensionality of
Cr-jarosite.
