  A magnetometric technique is demonstrated that may be suitable for precision
measurements of fields ranging from the sub-microgauss level to above the Earth
field. It is based on resonant nonlinear magneto-optical rotation caused by
atoms contained in a vapor cell with anti-relaxation wall coating. Linearly
polarized, frequency-modulated laser light is used for optical pumping and
probing. If the time-dependent optical rotation is measured at the first
harmonic of the modulation frequency, ultra-narrow (~a few Hz) resonances are
observed at near-zero magnetic fields, and at fields where the Larmor frequency
coincides with half the light modulation frequency. Upon optimization, the
sensitivity of the technique is expected to exceed 10^{-11} G/sqrt{Hz}.
