Figure 1: The Giant Magnetoresistance effect is due to the large difference in electrical resistance between two magnetic states of a metallic multilayer film.
Figure 2: An illustration of the layout of the 5x5 GMR sensor array. Each white square in the "full array" drawing on the left represents one NVE NVS5B15 sensor. The arrows on this drawing indicate the orientation of the axis of sensitivity for each sensor. The right side of the figure shows how the outputs of the elements are split up in the images that follow.
Figure 3: Layout of the image acquisition apparatus.
Figure 4: Image of a threaded #10 rod placed above the GMR array. On the left, the gray-scale images show the response of the elements to the magnetic field emanating from the rod. The "corners" image shows only data from the elements with a vertical axis of sensitivity, while the "others" image shows data from the elements with a horizontal axis of sensitivity.
Figure 5: Same as Figure 4, but with the rod flipped about a vertical axis. The image is almost a mirror reversal of Figure 4.
Figure 6: Same as Figure 4, but with a 6 Oe external applied field acting on the rod and the array. The image looks similar to Figure 4 and is even a bit clearer despite the necessary subtraction of the background signal from the external field. This image suggests that GMR arrays will be able to locate ferrous objects even in magnetic soils of volcanic origin.
Figure 7: Image of two ferrous bolts placed above the array. The outline of the bolts is not directly visible, but the symmetry of the pattern is recognizable. More GMR elements and concomitant higher spatial resolution could substantially improve this image.
Special electronic version bonus figure (!) Here is a digital camera photo of the apparatus. The sensor array is in the Bud box below the white firebrick, on which the two bolts (Figure 7) are placed. The sensor array is connected via ribbon cable (for signals) and coax (power) to the gold anodized box, which contains two encapsulated DC power supplies and a breakout board. The gold electronics box is connnected by a shielded cable to an A/D card in the PC, whose monitor is displaying the sensor readout.