A magnetic field sensor

Magnetic field sensor, comprising at least one magnetic field concentrator (30; 30, 30'), configured to amplify a magnetic field (B0) to be measured, hereinafter external magnetic field, and produce a non- uniform magnetic field (B), hereinafter internal magnetic field, in a space close to the magnetic field concentrator, and an oscillating mechanical resonator (10) carrying at least one permanent magnet (15) arranged in space, the mechanical resonator having a resonance frequency that changes as a function of the external magnetic field, wherein the internal magnetic field has non-zero second derivative along a direction of oscillation of the mechanical resonator (10).