Sensor element for the inspection of a planar data carrier, in particular a banknote, having a spin-resonance feature, comprising - a magnetic core with an air gap, into which the planar data carrier is capable of being inserted for the purpose of inspection, and with at least one disassembly gap, along which the magnetic core is capable of being disassembled into two or more parts, and - a polarization device for generating a static magnetic flux in the air gap, characterized in that - the magnetic core exhibits at least one magnetic potentiometer capable of being switched between two states, wherein - in a first state the magnetic potentiometer exhibits a first magnetic reluctance, and in the first state of the magnetic potentiometer the sensor element has been set up for a measurement in respect of a spin-resonance feature of a data carrier inserted into the air gap, and - in a second state the magnetic potentiometer exhibits a second magnetic reluctance, different from the first magnetic reluctance, and in the second state of the magnetic potentiometer the sensor element has been set up for a disassembly of the magnetic core along the disassembly gap, with the result that a disassembly force for disassembling the magnetic core along the at least one disassembly gap is smaller in the second state than the disassembly force in the first state. - a magnetic core with an air gap, into which the planar data carrier is capable of being inserted for the purpose of inspection, and with at least one disassembly gap, along which the magnetic core is capable of being disassembled into two or more parts, and - a polarization device for generating a static magnetic flux in the air gap, - the magnetic core exhibits at least one magnetic potentiometer capable of being switched between two states, wherein - in a first state the magnetic potentiometer exhibits a first magnetic reluctance, and in the first state of the magnetic potentiometer the sensor element has been set up for a measurement in respect of a spin-resonance feature of a data carrier inserted into the air gap, and - in a second state the magnetic potentiometer exhibits a second magnetic reluctance, different from the first magnetic reluctance, and in the second state of the magnetic potentiometer the sensor element has been set up for a disassembly of the magnetic core along the disassembly gap, with the result that a disassembly force for disassembling the magnetic core along the at least one disassembly gap is smaller in the second state than the disassembly force in the first state. - in a first state the magnetic potentiometer exhibits a first magnetic reluctance, and in the first state of the magnetic potentiometer the sensor element has been set up for a measurement in respect of a spin-resonance feature of a data carrier inserted into the air gap, and - in a second state the magnetic potentiometer exhibits a second magnetic reluctance, different from the first magnetic reluctance, and in the second state of the magnetic potentiometer the sensor element has been set up for a disassembly of the magnetic core along the disassembly gap, with the result that a disassembly force for disassembling the magnetic core along the at least one disassembly gap is smaller in the second state than the disassembly force in the first state.