This paper presents the principle of fiber optic sensors for the detection of low-frequency magnetic fields. Magnetostrictive materials such as Tertenol-D and Metglas® are used as the sensing elements in a Mach-Zehnder interferometer configuration. Low-frequency fiber optic magnetometers are constructed and tested in both planar and cylindrical geometries. These magnetometers consist of an optical fiber bonded to a magnetostrictive metallic glass ribbon in either strip or cylindrical geometry. The characteristics of specific sensor designs are also discussed and suggestions to improve magnetic field sensor fabrication are given. From a number of experiments, the best sensitivity achieved is with sandwich geometries under the application of a dc biasing field. The performance of these sensors increases with improved mechanical design and thermal annealing of the metallic glass ribbon. Much of the reported work to date has been concerned with the response of the system in the frequency range of 0.1 to 10 kHz. The work presented here, however, deals with the response of an interferometric system using Metglas® in the lower frequency range of 10 to 50 Hz.