Readback in today’s HDDs is accomplished using a tunneljunction (TMR) sensor [16]. One disadvantage of the TMR sensor is its relatively high impedance. These heads are sometimes additionally designated as ‘current perpendicular to plane’ or CPP TMR heads since the bias current flows through (or the bias voltage appears across) the dimension perpendicular to the plane of film deposition and in an ‘along-track’ direction with respect to the recording process. The magnetic shields that sandwich the tunnel-junction sensor thus also serve as the electrodes or electrical contacts into the device. The size of the sensor and the cross-sectional area through which current flows are roughly inversely proportional to the areal density. Because of this, the resistance of the sensor can easily become a problem at high areal densities. High reader resistance can lead to excessive thermal resistive noise (the bias voltage is strictly limited by the risk of the tunnel barrier breakdown and cannot be increased to compensate) and to difficulties coupling the high-impedance source through the transmission lines on the suspension and into the preamp. For these reasons, there has also been an intense study of CPP GMR heads which offer much lower resistance [17].