  Achieving magnetometers with ultrahigh sensitivity at room temperature is an
outstanding problem in physical sciences and engineering. Recently developed
non-Hermitian cavity spintronics offers new possibilities. In this work we
predict an exceptional magnetic sensitivity of cavity magnon polaritons with
the peculiar parity-time (PT) symmetry. Based on the input-output formalism, we
demonstrate a "Z"-shape spectrum including two sideband modes and a dark-state
branch with an ultranarrow linewidth in the exact PT phase. The spectrum
evolves to a step function when the polariton touches the third-order
exceptional point, accompanied by an ultrahigh sensitivity with respect to the
detuning. The estimated magnetic sensitivity can approach 10^{-15}T Hz^{-1/2}
in the strong coupling region, which is two orders of magnitude higher than
that of the state-of-the-art magnetoelectric sensor. We derive the condition
for the noiseless sensing performance. Purcell-like effect is observed when the
PT symmetry is broken. A possible experimental scheme to realize our proposal
is also discussed.
