  The canting angles and fluctuation of the magnetic ion spins of spinel oxide
MnCr${}_2$O${}_4$ were studied by nuclear magnetic resonance (NMR) at low
temperatures, which has a collinear ferrimagnetic order below $T_C$ and a
ferrimagnetic spiral order below $T_s < T_C$. Contrary to previous reports,
only one spin canting angle of Cr ions was observed. The spin canting angles of
Mn and Cr ions in the ferrimagnetic spiral obtained at a liquid-He temperature
were $43\,^{\circ}$ and $110\,^{\circ}$, respectively. The nuclear spin-spin
relaxation was determined by the Suhl-Nakamura interaction at low temperatures
but the relaxation rate $T_2^{-1}$ increases rapidly as the temperature
approaches $T_s$. This indicates that the fluctuation of the spiral component
becomes faster as the temperature increases but not fast enough to leave an
averaged hyperfine field to nuclei in the time scale of nuclear spin precession
in the ferrimagnetic phase, which is on the order of $10^{-8}$ s. The spiral
volume fraction measured for various temperatures reveals that the collinear
and the spiral ferrimagnetic phases are mixed below the transition temperature
of the spiral order. The temperature hysteresis in the volume fraction implies
that this transition has first-order characteristics.
