In-situ measurement of magnetic field gradient in a magnetic shield by a spin-exchange relaxation-free magnetometer*
Fang Jian-Chenga), Wang Taoa)†, Zhang Hongb), Li Yanga), Cai Hong-Weia)
       
The second harmonic signal of the lock-in amplifier, which is proportional to the power of the probe beam as predicted by Eq. ( 10 ).