Hysteresis loop collapse for linear response in magnetic-tunnel-junction sensors

In our Al2 O3 magnetic-tunnel junction (MTJ) samples that have a Neel coupling equal to or larger than their easy-axis coercivities, we found that the hysteresis loop totally collapses into a near-linear response upon rotating the easy axis of the sample off the applied field axis. A very high magnetic field sensitivity of 138%/mT (13.8%/Oe), which is among the highest reported so far, with near-linear response, was exhibited by Al2 O3 MTJs using this loop collapsing technique. This phenomenon can be explained by the switching astroid curve. In order to make this technique applicable in an actual sensor, we devised a two-step procedure whereby both easy-axis and hard-axis fields are used to accomplish the same loop collapsing effect as rotation. These result is one of the best combinations of saturation field and tunneling magnetoresistance ever achieved for MTJs, in addition to the linear nonhysteretic response at zero field that is so important for magnetic sensors.