  A control mechanism for stopping x-ray pulses in resonant nuclear media is
investigated theoretically. We show that narrow-band x-ray pulses can be mapped
and stored as nuclear coherence in a thin-film planar x-ray cavity with an
embedded $^{57}\mathrm{Fe}$ nuclear layer. The pulse is nearly resonant to the
14.4 keV M\"ossbauer transition in the $^{57}\mathrm{Fe}$ nuclei. The role of
the control field is played here by a hyperfine magnetic field which induces
interference effects reminding of electromagnetically induced transparency. We
show that by switching off the control magnetic field, a narrow-band x-ray
pulse can be completely stored in the cavity for approximately 100 ns.
Additional manipulation of the external magnetic field can lead to both group
velocity and phase control of the pulse in the x-ray cavity sample.
