S5A.18.pdf Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014 Low Magnetic Field Detection Technique using Fiber Beam Cantilever Method for a Very Hazardous Environment Somarpita Pradhan and Partha Roy Chaudhuri* Department of Physics, Indian Institute of Technology Kharagpur-721302, India *e-mail: roycp@phy.iitkgp.ernet.in Abstract: Fiber-optic low magnetic field sensor based on fiber-beam-cantilever technique using cobalt-doped nickel-ferrite nanoparticles coated optical fiber is proposed. Sensitivity is increased by incorporating etched single-mode fiber. Magnetic field as low as 2.0mT is successfully detected. OCIS codes: (060.2370) Fiber optics sensors; (280.4788) Optical sensing and sensors 1. Introduction Magnetic field measurement using different optical and non-optical methods has become an established fact. But during the last few years, attention in optical fiber based magnetic field sensors [1] has widely increased. This is because of their several advantages namely, flexibility, remotely addressable and multiplexed operation and, perhaps for the most important and unique advantage that is their capability of working in a very hazardous environment encompassing high voltage, significant electrical noise and high temperature. Consequently such sensors find essential place in many nuclear reactors, defence projects and mine industries. However, the main challenging task for designing such sensors is to choose proper magnetic probe material. Metallic glasses have been used by many researchers [2] but the metallic glass materials exhibit a saturation of the magnetostriction at low magnetic fields around 100 A/m. For decades, optical fiber coated with ferrite nanoparticles has found wide application for sensing of a small magnetic field. Ferrites are having cubic spinel structure and show high magnetostriction property. A step ahead, spinel type oxides with appropriate doping (several compositions of A 1-x B x Fe 2 O 4 where A, B are transition elements such as Ni, Co, Fe, Mn, Zn and Cu) may be more convenient for this purpose. Spinel type nickel ferrites with 2–3 atomic % of Co doping were reported to have exceptionally high magnetic properties [3]. In this work, we have prepared cobalt-doped nickel ferrite (Ni 0.97 Co 0.03 Fe 2 O 4 ) nanoparticles as probe sample and developed experimental prototype set-up for sensing magnetic field using fiber beam cantilever technique. Variation of fiber-to-fiber transmitted power actually conveyed the signature of magnetic field in the vicinity. The sensitivity of the sensor was further improved by incorporating etched optical fiber in the configuration. The sensor configuration is very simple, easy to fabricate and also the sensors can perform in a very hazardous environment. As the sensing property does not deal with any strain within the fiber, temperature cross-sensitivity is not an issue in our sensor configuration. Detailed procedure of preparation methodology, characterizations of the magnetic nanoparticles and the magnetic field sensing experimental results are discussed in the following sections. 2. Probe sample preparation and characterization Cobalt-doped nickel ferrite (Ni 0.97 Co 0.03 Fe 2 O 4 ) sample was prepared from iron nitrate [Fe(NO₃)₃.9H 2 O], nickel nitrate [Ni(NO₃) 3 .6H 2 O] and cobalt nitrate [Co(NO₃)₂.6H 2 O] precursor materials by sol-gel method [3] due to its capability of producing high purity products, low processing temperature, ease of fabrication and low cost. After preparation of the material, a series of investigations in terms of different characterization processes were performed to confirm crystalline structure, magnetic properties, nano dimension and other related properties of the sample. The crystalline structure of the probe material (Ni 0.97 Co 0.03 Fe 2 O 4 ) was investigated by X-ray diffraction (XRD) analysis using CuK α source (wavelength 1.54A 0 ) at room temperature. The reflection peaks (220), (311), (222), (400), (422), (511), (440), (533) confirm the cubic spinel structure (see Fig. 1(a)) of the prepared material as appeared in literature [4]. The lattice constant was estimated to be ~8.35Å. The M-H loop (MHL) was obtained by Superconducting Quantum Interfering Devices (SQUID) measurement and is presented in the Fig. 1(b). The measurement was performed at room temperature (300K). We evaluated coercive field (H c ) defined as a half-width of magnetic hysteresis loop at M = 0. The average value of H c was approximately 175.50 Oe. The remanent magnetization M r was observed to be 10 emu/g. The high resolution transmission electron microscopy (HRTEM) image estimated the size of the cobalt doped nickel ferrite particles on the average about 8 nm (see Fig. 1(c)).