  Spin polarized atomic ensembles can be used for the precise measurement of
magnetic field. Conventional atomic magnetometers have demonstrated high
sensitivities, albeit at low detection bandwidth, fundamentally limited by the
Larmor precession frequency of the atoms. Here, we introduce a new type of
atomic magnetometer which can realize sensitive detection with high temporal
resolution. The magnetometer is based on monitoring the effect of the magnetic
field on the hyperfine coherences of spin-polarized hydrogen atoms produced
from the rapid photodissociation of hydrogen chloride (HCl). This scheme
extends the magnetic measurement bandwidth to an upper frequency limit set by
the hyperfine interaction. The use of HCl as the source for polarized atoms
allows for large spin densities in the range of $10^{19}$ - $10^{20} cm^{-3}$,
many orders of magnitude higher than those achieved with conventional
alkali-atomic magnetometers. A spin-projection noise-limited magnetometer
smaller than 100 {\mu}m can detect with nanosecond temporal resolution a
magnetic field as low as 1 nT. This novel magnetometer opens new possibilities
for precise measurement of ultrafast magnetic fields, with applications in
biology, surface science and chemistry.
