A magnetic field sensor comprising a transducer element (1), whereby: (I) the transducer element (1) is a Spin Tunnel Junction, comprising a first (1a) and second (1b) magnetic layer which are sandwiched about an interposed electrical insulator layer (1c); (II) the sensor comprises a yoke (3) having two arms (3a, 3b); (III) the first magnetic layer (1a) is in direct contact with a first portion of a first arm (3a) of the yoke (3).