A high-resolution sensor of magnetic field sensor system and materials for use in such a system are described. The sensor systems measure a magnetic field using inorganic and/or organic magneto-optically active materials, e.g. polymer material and have an interferometer based on Faraday rotation. The polymer material is preferably in the form of a film. The polymer material has an optical property that is sensitive to the magnetic field, e g the Faraday rotation effect. The present invention also provides a sensor head structure comprising the above polymer material. The sensor head may be designed for use with an optical fiber or with mirrors In particular the present invention provides a fiber Sagnac interferometer to measure the rotation of polarized plane of light. The present invention provides a fiber or mirror based Sagnac interferometer with passive phase bias applied to magnetic field sensing.