  Controlling and narrowing x-ray frequency spectra in magnetically perturbed
two-target nuclear forward scattering is theoretically studied. We show that
different hard-x-ray spectral redistributions can be achieved by single or
multiple switching of magnetic field in nuclear targets. Our scheme can
generate x-ray spectral lines with tenfold intensity enhancement and spectral
width narrower than four times the nuclear natural linewidth. The present
results pave the way towards a brighter and flexible x-ray source for precision
spectroscopy of nuclear resonances using modern synchrotron radiation.
