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. This invention has the following three major aspects each being an embodiment of the present invention: 1. Sensing material: a conjugate polymer such as polythiophene and/or a polymer containing superparamagnetic nanoparticles that exhibits a giant Faraday rotation. 2. Magnetic field probe that incorporates the above polymer or polymer/nanoparticle composite, e.g. in the form of a film. 3. Sensor: The sensor combines an optical fiber-based Sagnac interferometer and the above magnetic field probe.