A New Single-Sensor Magnetic Field Gradiometer

In this work a magnetic field gradiometer device has been developed. This device is not composed by two sensors separated a certain distance but by a single-sensor device which measures the real gradient value just at the point where it is wanted to be measured. The experimental set-up consists of a sensor head (a membrane with a fixed permanent magnet on it) vibrating at its resonance frequency, due to an alternating magnetic field gradient. Detection is performed using an optoelectronic method. Two different measurement techniques have been used: frequency measurements (the resonance frequency is measured for every external magnetic field gradient value) and amplitude measurements (the signal amplitude is measured without changing the resonance frequency obtained without an external magnetic field gradient applied). The mechanical stresses of the membrane are related with the dependence of both magnitudes (frequency and amplitude) on the external magnetic field gradient. The minimum and maximum value of the resonance frequency and signal amplitude respectively, correspond with magnetic forces equal to the magnet weight of the sensor head. This prototype shows a noise-limited sensitivity of 2 Gauss/m/root Hz at zero gradient. This device could be used also as a magnetic susceptometer.