  The hybrid optical pumping spin exchange relaxation free (HOPSERF) atomic
co-magnetometers make ultrahigh sensitivity measurement of inertia achievable.
The wall relaxation rate has a big effect on the polarization and fundamental
sensitivity for the co-magnetometer, but it is often neglected in the
experiments. However, there is almost no work about the systematic analysis of
the influence factors on the polarization and the fundamental sensitivity of
the HOPSERF co-magnetometers. Here, we systematically study the polarization
and the fundamental sensitivity of 39K-85Rb-21Ne and 133Cs-85Rb-21Ne HOPSERF
co-magnetometers with low polarization limit and the wall relaxation rate. The
21Ne number density, the power density and wavelength of pump beam will affect
the polarization greatly by affecting the pumping rate of pump beam. We obtain
a general formula on the fundamental sensitivity of the HOPSERF
co-magnetometers due to shot-noise and the fundamental sensitivity changes with
multiple systemic parameters, where the suitable number density of buffer gas
and quench gas make the fundamental sensitivity highest. The fundamental
sensitivity $7.5355\times10^{-11}$ $\rm rad/s/Hz^{1/2}$ of 133Cs-85Rb-21Ne
co-magnetometer is higher than the ultimate theoretical sensitivity
$2\times10^{-10}$ $\rm rad/s/Hz^{1/2}$ of K-21Ne co-magnetometer.
