Fiber-optic Mach-Zehnder interferometric magnetic field sensor employing metallic glass 2605SC was fabricated for the detection of low frequency magnetic field. The signal fading problem, which was due to random phase drift resulting from temperature fluctuations and other types of disturbances, was eliminated by feedback phase compensation on the reference arm. This allows the sensitivity to be maintained at a maximum by always keeping the system in quadrature. The interferometer was tested by simulating phase shift generated by PZT phase modulator. The minimum detectable phase shift was the order of $8.08×10^{-5}rad/Hz^{1/2}$. Before ac field sensing, frequency and dc bias field response of the system was investigated. At fixed ac field, maximum ac strain for dc bias field and frequency was found at 3 G and at 190Hz, respectively. It was observed at 200Hz, 1000Hz, 2000Hz that the interferometer output was propotional to ac field. The minimum detectable magnetic field in the range of $9.51×10^{-6}Gm/Hz^{1/2}$ was obtained.
