  The hybrid optical pumping spin exchange relaxation free (SERF) atomic
magnetometers can realize ultrahigh sensitivity measurement of magnetic field
and inertia. We have studied the $^{\text{85}}$Rb polarization of two types of
hybrid optical pumping SERF magnetometers based on
$^{\text{39}}$K-$^{\text{85}}$Rb-$^{\text{4}}$He and
$^{\text{133}}$Cs-$^{\text{85}}$Rb-$^{\text{4}}$He respectively. Then we found
that $^{\text{85}}$Rb polarization varies with the number density of buffer gas
$^{\text{4}}$He and quench gas N$_{\text{2}}$, pumping rate of pump beam and
cell temperature respectively, which will provide an experimental guide for the
design of the magnetometer. We obtain a general formula on the fundamental
sensitivity of the hybrid optical pumping SERF magnetometer due to shot-noise.
The formula describes that the fundamental sensitivity of the magnetometer
varies with the number density of buffer gas and quench gas, the pumping rate
of pump beam, external magnetic field, cell effective radius, measurement
volume, cell temperature and measurement time. We obtain a highest fundamental
sensitivity of $1.5073$ $aT/Hz^{1/2}$ ($1$ $aT=10^{-18}$ $T$) with
$^{\text{39}}$K-$^{\text{85}}$Rb-$^{\text{4}}$He magnetometer between above two
types of magnetometers when $^{\text{85}}$Rb polarization is $0.1116$. We
estimate the fundamental sensitivity limit of the hybrid optical pumping SERF
magnetometer to be superior to $1.8359\times10^{-2}$ $aT/Hz^{1/2}$, which is
higher than the shot-noise-limited sensitivity of $1$ $aT/Hz^{1/2}$ of K SERF
atomic magnetometer.
