Development of a pinned wall sensor using cobalt-rich, near-zero magnetostrictive amorphous alloys
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I.
INTRODUCTION
SENSORS comprising magnetic materials as the principle functional elements to measure magnetic and electric field, rotation, displacement, stress torque, and medical electronics have been utilized for many years.[1–4] The unique mechanical and magnetic properties of amorphous metallic glass ribbons or wires have been of great interest to this field since they satisfy most of the particular sensor requirements, such as mechanical robustness, temperature stability, high sensitivity and resolution, quick response, and microlization in size.[5–8] Probably one of the most commercialized applications of amorphous magnetic materials is one as a sensor for electronic article surveillance (EAS), i.e., antitheft security system. Electronic article surveillance systems are currently employed in supermarkets, department stores, libraries, and video rental and other merchandise outlet stores, as well as to monitor everything such as workplace valuables from computers and diskettes to sensitive documents, and they are even used as a deterrent against infant abduction from hospitals. Such a security system consists of an elongated ferromagnetic marker (a tag, strip, or label), which is attached to the merchandise or assets to be protected. These magnetic markers produce the nonlinear hysteresis loops with nonlinear permeability, which are able to generate highorder harmonics (n: 10 to 20) of the fundamental frequency (Figure 1), and these can be detected as sharp voltage pulses by pickup coils. When they are excited in field-generating coils arrayed around a passageway, the system is C.K. KIM, formerly Research Scientist with MIT, Cambridge, MA, is now Professor at Hanyang University, Seoul, Korea. R.C. O’HANDLEY, Senior Research Scientist, Department of Materials Science and Engineering, is with the Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted January 24, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
alerted. Such tags are called harmonic tags. The usage of this characteristic is not only limited to the antitheft system but also applied to pulse-generating sensor elements for rotary encoders, noncontact switchers, and various electric and magnetic sensors. Since commonly available ferromagnetic materials do not generate such high-order harmonics, the population of ‘‘false alarm’’ signals is low. Although other high-quality ferromagnetic materials such as square permalloy[9] or ‘‘wasp-waisted’’ constricted Perminvar loop[10] can serve for such purposes as well, they are less practical due to reduced reliability in service compared to the metallic glasses owing to their lower mechanical and magnetic stability, which causes the degradation of their permeability during manufacture, dispensing, and handling. It is known that magnetic properties of cobalt-rich amorphous alloys are much less sensitive to processing conditions than are those of iron-rich amorphous alloys.[11] Part of this may be due to the lower magnetostriction of the former, which means that quenched-in stresses
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