A device for sensing a magnetic field, comprising a closed superconducting loop configured to collect a magnetic field to be sensed, hereinafter external magnetic field, the closed superconducting loop having a path width (wp) and being provided with a constriction having a width (wc) narrower than the path width, the constriction generating a non-uniform magnetic field, hereinafter internal magnetic field, in response to the external magnetic field, a vibrating mechanical oscillator coupled to, or formed by the constriction and responsive to the internal magnetic field, and a detector configured to detect deflection or vibration of the mechanical oscillator and providing a signal indicative of the deflection or vibration.