Ultra-low hysteresis and self-biasing in GMR sandwich sensor elements

Abstract

The experiments performed demonstrate that by pinning the edges in a GMR sandwich sensor element a fivefold and greater increase in edge coercivity and a corresponding tenfold decrease in hysteresis can be achieved. The effect is accompanied by self-biasing at submicrometer dimensions. Analysis suggests that the edge pinning effect is facilitated by forming a bridge between the magnetic layers of a sandwich that establishes a path for exchange coupling. The energy associated with this exchange increases the field needed to reverse the edge magnetizations. This stabilizes the magnetizations at the interior of the element as they rotate under an applied field. Edge pinning is measured in terms of its "edge spin threshold," or EST, defined as the field at which the characteristic separation in the magnetoresistive transfer curve abruptly closes, signifying complete edge reversal. Hard edge pinned structures have ESTs greater than the material's ninety-percent saturation field, a test benchmark equal to approximately five times the anisotropy, while soft edge structures have smooth transitions that occur below the ninety-percent saturation field. EST thresholds range from 100 Oe to over 700 Oe depending on the process. For field excursions less than the EST, the material exhibits significantly lower hysteresis. Experimental values of less than 0.5 Oe hysteresis have been recorded for field sweeps of two hundred-fifty Oe (0.2%), compared to about 5.0 Oe observed in soft sandwiches over the same range. A second feature of hard edge devices is self-biasing, which has been demonstrated in devices with sub-micrometer line widths. Biasing at one-half the signal output can be achieved in devices approximately 0.7 micrometers wide and can be adjusted using sense current. These features are valuable for sensor and signal isolator applications by providing an ultra-low hysteresis, bipolar output without the need for power-consuming bias coils.