A sensor element for testing a flat data carrier, in particular a banknote, has a spin resonance feature. The sensor element contains a magnetic core having an air gap, into which the flat data carrier can be inserted for testing, an element for generating a static magnetic flux in the air gap, and a resonator for exciting the spin resonance feature of the data carrier to be tested. The resonator is formed by a microstrip line resonator which is arranged in the air gap of the magnetic core and comprises a flat carrier having an upper side and a conducting structure which is arranged on the upper side of the carrier with a characteristic length.