  Present protocols for obtaining the ultimate magnetic sensitivity of
optically pumped magnetometers (OPMs) utilizing alkali-metal ensembles rely on
uncorrelated atoms in stretched states. A new approach for calculating the spin
projection noise (SPN)-limited signal to noise ratio (SNR) and the magnetic
sensitivity of OPMs is proposed. Our model is based solely on the mean-field
density matrix dynamics and in contrast to previous models, it applies to both
low and high field regimes, it takes into account the degree of spin
polarization, the intra- and interhyperfine correlations, the decoherence
processes, the atom-light coupling and the effects of the spin dynamics on the
spin-noise spectra. Fine tuning of the probe frequency allow us to explore
different hyperfine states and ground-state correlations. Especially in the
spin-exchange-relaxation-free (SERF) regime, alongside the magnetic resonance
narrowing and the increased number density, hallmarks of SERF magnetometers, we
report on a new SERF feature; the reduction of spin-projection noise at the
spin precession frequency as a consequence of strongly-correlated hyperfine
spins that attenuate and redistribute SPN when properly probed.
