MAGNETORESISTIVE SENSORS WITH RAPIDLY SOLIDIFIED PERMALLOY FIBERS

The microstructure of Permalloy fibers shows columns growing perpendicular to the contact line between the melt and the extraction wheel. Strong tensile stresses are believed to develop along the columns and are incorporated in the fiber during solidification. As a result, the easy axis of magnetization of fibers is perpendicular to their length. Both transverse and longitudinal extraordinary magnetoresistive effects have a positive initial slope due to stress-related rotation of moments in the closure domains and to the coherent rotation of moments in the columns. Fiber response was also investigated as a function of the field direction and angle of incidence, fiber twisting and for strong transverse fields. The results are discussed in terms of the existing models. The longitudinal effect is analyzed in terms of the Jones-Soohoo model, whereas a new model is proposed to explain the transverse effect. Both effects are useful for sensing purposes, but we have been used only the longitudinal effect for our sensors. The structures and operating principles of single- and dual-fiber biased sensors are presented in detail. Distortions due to overbiasing are accounted for the pinning and expansion of large-area closure domains with moments tilted by the casting stress. The sensitivity-linearity range product is the same for several bias levels and a family of sensors was obtained using the same fiber. The performances of the fiber sensors are compared to those of thin-film sensors. Noise, self-heating, encasing and manufacturing are also discussed and optimistic conclusions are found for fiber sensors.