We propose a study on the electric field control of uncompensated spins in the Co/BiFeO3/STO bilayer system by PNR. Multiferroic BiFeO3 (BFO) has attracted enormous interest because of the unique possibility to control the magnetisation via applying an electric field. However, the use of pure BFO films is limited because the magnetic phase of BFO is not ferromagnetic (FM), but antiferromagnetic (AFM) ordered. To overcome the disadvantage of the AFM phase in BFO, an exchange bias system with the combination of FM top layer was proposed. So far, investigations have concentrated on the reversal mechanism of FM layers and the magnetoelectric (ME) coupling in BFO. Nevertheless, the AFM nature of BFO has not been fully understood. PNR is an unique measurement tool capable of revealing the behaviour of uncompensated spins at the FM/AFM interface on the application of an electric field.