Device for measuring an amount of superparamagnetic material and use of such a device

A device for measuring an amount of superparamagnetic material, the device comprising:

• a first branch comprising a first coil and a second coil, which are mounted in series in a region of a first midpoint, and a second branch, mounted in parallel with the first branch and comprising a third coil and a fourth coil, which are mounted in series in the region of a second midpoint, the first coil and the third coil forming a pair of measuring coils, the second coil and the fourth coil forming a pair of compensating coils, the coils of a pair being identical to each other;
• at least one direct current generator of a current having a first frequency, referred to as a low-frequency generator, and a current having a second frequency, which is greater than the first frequency, referred to as a high-frequency generator, the generators being coupled to the first and second branches to inject into each of the coils a current having a DC component, a component having a first frequency, and a component having a second frequency, such that magnetic fields generated by the coils of the same pair are identical;
• and
• a detector of a component of an electric voltage present between the midpoint of each branch, the component being set at a mixing frequency, the mixing frequency being a linear combination of the first and the second frequency.