  Locking of a laser frequency to an atomic or molecular resonance line is a
key technique in applications of laser spectroscopy and atomic metrology.
Modulation transfer spectroscopy (MTS) provides an accurate and stable laser
locking method which has been widely used. Normally, the frequency of the MTS
signal would drift due to Zeeman shift of the atomic levels and rigorous
shielding of stray magnetic field around the vapor cell is required for the
accuracy and stability of laser locking. Here on the contrary, by applying a
transverse bias magnetic field, we report for the first time observation of a
magnetic-enhanced MTS signal on the transition of 87Rb D2-line Fg=1 to Fe=0
(close to the repump transition of Fg=1 to Fe=2), with signal to noise ratio
larger than 100:1. The error signal is immune to the external magnetic
fluctuation. Compared to the ordinary MTS scheme, it provides a robust and
accurate laser locking approach with more stable long-term performance. This
technique can be conveniently applied in areas of laser frequency
stabilization, laser manipulation of atoms and precision measurement.
