A partially unclad fiber Bragg grating (FBG) coated with Fe3O4 nanoparticles as a magnetic field sensor is experimentally demonstrated. A series of six FBGs reflecting different wavelengths at an efficient length of 30 mm are fixed on the top and outside of a cylindrical glass chamber. The chamber is equipped with a solenoid that acts as a magnetic field generator and is filled up with air, saline solution, and crude oil separately to detect the changes in the magnetic field. The magnetic-hysteresis loops confirm the super-paramagnetic properties of the synthesized Fe3O4 nanoparticles with size smaller than 20 nm. The sensor response time of �21 s confirms the high reliability and repeatability of the sensing scheme. The change in the magnetic field strength depended from FBG-solenoid distance, propagating at different media and function generator frequency leads to shift in the reflected wavelength of each single FBG's accordingly. The magnetic field strength outside the solenoid obeys the inverse-cubic law, and the decrease in the wavelength shift with an increase in the FBG-solenoid distance is in excellent agreement with the Biot-Savart law. The shift is caused by the interference of different propagating modes that are reflected from the core-cladding and cladding-magnetite layer interfaces; these modes have different phases because of the changes in the refractive index of the magnetite layer resulting from the change in the magnetic field. Our precise fabrication of the FBG probe with a maximum sensitivity of �11 000 pm mT-1 and the proposed design of experiment may be appropriate for detecting small changes in magnetic fields in the oil industry. © 2020 IOP Publishing Ltd.