In this work, the detection and characterization of sub-surface crack’s depths using GMR, TMR and uniform magnetic field distribution with multi-frequency excitation were studied. Experimental tests were performed in two stacked aluminum plates with their thickness equal to 3 mm and 4 mm. The multi-frequency signal was used to exploit the penetration of the eddy currents into the two stacked aluminum plates in different layers. A planar coil especially designed to produce a uniform magnetic field distribution around the magnetic field sensors was used to exploit the invariance of the excitation field for small spatial translations. As only low testing frequencies (up to 500 Hz) were chosen for the detection of the sub-surface cracks, GMR and TMR sensors were used to obtain the measurement of the magnetic field perturbation due to the presence of cracks in the two stacked aluminum plates. The obtained results and the characteristics of the two sensors are discussed in this paper.