MAGNETOSTRICTIVE ELEMENT FOR USE IN A MAGNETOMECHANICAL SURVEILLANCE SYSTEM

Abstract: A magnetostrictive element for use in a magnetomechanical article surveillance marker is formed by first annealing (22) an amorphous metal alloy (28), such alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field (33) and then second annealing the alloy (28) in the absence of the saturating magnetic field.

Description: MAGNETOSTRICTIVE ELEMENT FOR USE TN A MAGNETOMECHANICAL SURVEILLANCE SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending prior application serial no. 08/508,580, filed July 28, 1995. Copending prior application serial no. 08/508,580 is a continuation-in-part of prior application serial no. 08/269,651 filed June 30, 1994, now US Patent No. 5,469,140, issued November 21, 1995, and is also a continuation-in-part of co- pending prior application serial no. 08/392,070 filed February 22, 1995, and is further a continuation-in-part of co-pending prior application serial no. 08/420,757 filed April 12, 1995, all of which prior applications have a common inventor and a common assignee with the present application. FIELD OF THE INVENTION This invention relates to magnetomechanical article surveillance systems and, more particularly, to an amorphous metal alloy magnetostrictive element for use in such systems. BACKGROUND OF THE INVENTION U.S. Patent No. 4,510,489, issued to Anderson et al., discloses a magnetomechanical electronic article surveillance (EAS) system in which markers incorporating a magnetostrictive active element are secured to articles to be protected from theft. The active elements are formed of a soft magnetic material, and the markers also include a control element which is biased or magnetized to a pre-determined degree so as to provide a bias field which causes the active element to be mechanically resonant at a pre-determined frequency. The markers are detected by means of an interrogation signal generating device which generates an alternating magnetic field at the pre-determined resonant frequency, and the signal resulting from the mechanical resonance is detected by receiving equipment. According to one embodiment disclosed in the Anderson et al. patent, the interrogation signal is turned on and off, or "pulsed," and a "ring-down" signal generated by the active element after conclusion of each interrogation signal pulse is detected. Typically, magnetomechanical markers are deactivated by degaussing the control element, so that the bias field is removed from the active element thereby causing a substantial shift in the resonant frequency of the active element. The Anderson et al. patent discloses a number of materials that may be used for the active element, and also describes techniques used for treating the materials. The disclosed techniques include heat-treating (annealing) an amorphous material in a saturating magnetic field. The disclosure of the Anderson et al. patent is incorporated herein by reference. U.S. Patent No. 5,252,144, issued to Martis, discloses fiirther materials said to be suitable for use as active elements in magnetomechanical EAS markers, as well as annealing processes (without application of a magnetic field) to be applied to the materials. The above-referenced '651 co-pending application discloses a procedure in which batches of pre-cut strips of an amorphous metal alloy are annealed in the presence of a saturating transverse magnetic field. The resulting annealed strips are suitable for use as the active elements in magnetomechanical markers and have improved ring-down characteristics which enhance performance in pulsed magnetomechanical EAS systems. In addition, the hysteresis loop characteristic of the resulting active element is such as to eliminate or reduce false alarms that may result from exposure to harmonic EAS systems. Moreover, the procedure disclosed in the '651 application produces active elements that are relatively flat in longitudinal profile, permitting fabrication of quite thin markers incorporating such active elements. The disclosure of the aforesaid application serial no. 08/269,651 is incorporated herein by reference. The above-referenced '757 co-pending application discloses an adaptation of the techniques of the '651 application, in which a continuous process is employed to transport a continuous ribbon of amorphous metal alloy from reel to reel through an oven in which transverse-field annealing is carried out. Then, after annealing, the continuous ribbon is cut into discrete strips. This continuous annealing process avoids inconvenience in transporting pre-cut strips into and out of an oven. The techniques disclosed in the '651 and 757 co-pending applications represent advances over previously known techniques. However, it would be desirable to modify the techniques of those two co-pending applications so as to provide active elements for EAS markers having a resonant frequency that is relatively insensitive to variations in the biasing magnetic field. SUMMARY OF THE INVENTION In accordance with the present invention there is provided a magnetostrictive element for use in a magnetomechanical article surveillance marker formed by first annealing an amorphous metal alloy, such alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing the alloy in the absence of the saturating magnetic field. In addition, the present invention provides a marker for use in a magnetomechanical article surveillance system comprising an amorphous magnetostrictive strip formed by first annealing an amorphous metal alloy, such alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing the alloy in the absence of the saturating magnetic field. Still further, the present invention provides a magnetomechanical article surveillance system comprising generating means for generating an electromagnetic field alternating at a selected frequency in an interrogation zone, a marker comprising an amorphous magnetostrictive strip formed by first annealing an amorphous metal alloy, such alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing said alloy in the absence of the saturating magnetic field and a biasing element to cause the magnetostrictive strip to be mechanically resonant when exposed to the alternating field, and detecting means for detecting the mechanical resonance of the magnetostrictive strip. The present invention provides EAS marker active elements that are not prone to producing false alarms in harmonic EAS systems and have a flat profile. The markers of the present invention exhibit increased stability of the resonant frequency of such active elements relative to changes in biasing magnetic field. According to an aspect of the invention, there is provided a method of fabricating a marker for use in an electronic article surveillance system, including the steps of first annealing a strip of magnetostrictive material during application of a magnetic field directed transverse to a longitudinal axis of the strip, with the strip having a characteristic upon completion of the first annealing such that, upon application of a biasing magnetic field to the strip, the strip is mechanically resonant at a resonant frequency in response to exposure to an alternating magnetic field at the resonant frequency, with the resonant frequency being subject to variation in dependence on changes in the biasing magnetic field, and the method further including the step, performed subsequent to the first annealing step, of second annealing the strip to reduce a rate at which the resonant frequency varies in dependence on changes in the biasing magnetic field. According to another aspect of the invention, there is provided a method of fabricating a marker for use in an electronic article surveillance system, including the steps of first annealing a strip of magnetostrictive material during application of a saturating magnetic field, and, subsequent to the first annealing, second annealing the strip in the absence of the saturating magnetic field. According to still another aspect of the invention, there is provided a method of forming a magnetostrictive element for use in an magnetomechanical electronic article surveillance marker, including the steps of providing a continuous strip of amorphous metal alloy, transporting the continuous amorphous strip through an annealing region in which heat and a saturating magnetic field are applied to anneal the ribbon; further annealing the continuous alloy strip in the absence of the saturating magnetic field, and, after the steps of transporting and further annealing, cutting the annealed strip into discrete strips each having a pre-determined length. According to a further aspect of the invention, there is provided a method of forming a magnetostrictive element for use in a magnetomechanical electronic article surveillance marker, including the steps of providing a continuous strip of amorphous metal alloy; transporting the continuous amoφhous alloy strip through an annealing region in which heat and a saturating magnetic field are applied to anneal the ribbon; after the transporting step, cutting the annealed strip into discrete strips, each having a predetermined length; and further annealing the discrete strips in the absence of the saturating magnetic field. In accordance with yet another aspect of the invention, there is provided an apparatus for annealing a continuous strip of an amoφhous metal alloy, including an oven, a magnetic field element for forming a magnetic field that is present in substantially all of a first region of the oven, with the magnetic field being substantially absent from a second region of the oven, and a transport mechanism for transporting the continuous strip along a path through the first region of the oven and through the second region of the oven. Further in accordance with the latter aspect of the invention, the transport means transports the continuous strip through the first region in a direction towards the second region. Still further in accordance with this aspect of this invention, the apparatus may include a supply reel, located at one side of the oven, with the continuous strip being unwound from the supply reel for being supplied to the oven, and a take-up reel, located at an opposite side of the oven from the supply reel, with the continuous strip being wound on the take-up reel after passing through the oven. In addition, the transport mechanism may include a capstan and a pinch roller, both inteφosed between the oven and the take-up reel, with the continuous strip being engaged between the capstan and pinch roller and being driven by the capstan in a direction from the supply reel to the take-up reel. Further, the magnetic field element forms the magnetic field in a direction transverse to the path through the oven and the magnetic field is formed with a field strength of at least 800 Oe inside the oven. Still further, the continuous strip may be in the form of a continuous ribbon, and the apparatus may further include a fixture located in the oven, with the ribbon being drawn through the fixture for imparting a desired cross-sectional profile to the ribbon. The fixture may include a curl surface for imparting a curved cross-sectional profile to the ribbon. Alternatively, the fixture may include a flat guide surface for imparting a substantially flat cross-sectional profile to the ribbon. According to still another aspect of the invention, there is provided an apparatus for annealing a continuous strip of an amoφhous metal alloy, including an element for forming a first heated region, a magnetic field element for forming a magnetic field that is present in substantially all of the first heated region, an element for forming a second heated region from which the magnetic field is substantially absent, and a transport mechanism for transporting the continuous strip along a path through the first and second regions. The element for forming the first heated region may be a first oven and the element for forming the second heated region may be a second oven different from the first oven. Alternatively, a single oven may constitute both of the element for forming the first heated region and the element for forming the second heated region. According to a further aspect of the invention, there is provided a magnetostrictive element for use in a magnetomechanical electronic article surveillance marker, with the element being formed by first annealing an amoφhous metal alloy in the presence of a saturating magnetic field, and then second annealing the amoφhous metal alloy in the absence of the saturating magnetic field. The second annealing may be performed at a temperature less than about 450° C and for a period of not more than 5 minutes. According to still a further aspect of the invention, there is provided a magnetostrictive element for use in a magnetomechanical electronic article surveillance marker, formed by first annealing a continuous strip of an amoφhous metal alloy in the presence of a saturating magnetic field, then second annealing the continuous strip in the absence of the saturating magnetic field, and then cutting the twice-annealed continuous strip into discrete strips. According to yet another aspect of the invention, there is provided a marker for use in a magnetomechanical electronic article surveillance system, including a discrete amoφhous magnetostrictive strip formed by first annealing an amoφhous metal alloy in the presence of a saturating magnetic field and then second annealing the amoφhous metal alloy in the absence of the saturating magnetic field. According to still another aspect of the invention, there is provided a marker for use in a magnetomechanical electronic article surveillance system, including a discrete amoφhous magnetostrictive strip formed by first annealing a continuous strip of an amoφhous metal alloy in the presence of a saturating magnetic field, then second annealing the continuous strip in the absence of the saturating magnetic field, and then cutting the twice-annealed continuous strip into discrete strips. According to a further aspect of the invention, there is provided a magnetomechanical electromc article surveillance system, including generating circuitry for generating an electromagnetic field alternating at a selected frequency in an interrogation zone, with the generating circuitry including an interrogation coil, a marker secured to an article appointed for passage through the interrogation zone, with the marker including an amoφhous magnetostrictive element formed by first annealing an amoφhous metal alloy in the presence of a saturating magnetic field and then second annealing the amoφhous metal alloy in the absence of the saturating magnetic field, the marker also including a biasing element located adjacent to the magnetostrictive element, the biasing element being magnetically biased to cause the magnetostrictive element to be mechanically resonant when exposed to the alternating field, the system also including detecting means for detecting the mechanical resonance of the magnetostrictive element. According to yet another aspect of the invention, there is provided a magnetomechanical electronic article surveillance system, including generating circuitry for generating an electromagnetic field alternating at a selected frequency in an interrogation zone, the generating circuitry including an interrogation coil, and a marker secured to an article appointed for passage through the interrogation zone, the marker including an amoφhous magnetostrictive element formed by first annealing a continuous strip of an amoφhous metal alloy in the presence of a saturating magnetic field, then second annealing the continuous strip in the absence of the saturating magnetic field, and then cutting the twice-annealed continuous strip into discrete strips, the marker including a biasing element located adjacent to the magnetostrictive element, the biasing element being magnetically biased to cause the magnetostrictive element to be mechanically resonant when exposed to the alternating field. The system also includes detecting circuitry for detecting the mechanical resonance of the magnetostrictive element. According to still another aspect of the invention, there is provided a marker for use in a magnetomechanical electronic article surveillance system, including an amoφhous magnetostrictive element and a biasing element located adjacent to the magnetostrictive element, with the magnetostrictive element having a hysteresis loop characteristic such that the magnetostrictive element does not produce substantial detectable harmonic frequencies when in an alternating electromagnetic field, and the magnetostrictive element also has a resonant- frequency-to-bias-field slope characteristic of less than about 700 Hz/Oe in a bias field range of 5 Oe to 7 Oe. Further in accordance with the latter aspect of the invention, the resonant- frequency-to-bias-field slope characteristic of the magnetostrictive element may be less than 500 Hz/Oe in the bias field range of 5 Oe to 7 Oe. According to yet another aspect of the invention, there is provided a marker for use in a magnetomechanical electronic article surveillance system, including a magnetostrictive element having a resonant-frequency-to-bias-field slope characteristic of less than about 700 Hz/Oe in a bias field range of 5 Oe to 7 Oe, the marker having an overall thickness of less than .065 inches. Further in accordance with the latter aspect of the invention, the resonant-frequency-to- bias-field slope characteristic of the magnetostrictive element may be less than 500 Hz/Oe in the bias field range of 5 Oe to 7 Oe, and the overall thickness of the marker may be less than .030 inches, and may be about .005 inches. Other objectives, advantages, and applications of the present invention will be made apparent by the following detailed description of the preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a processing apparatus provided in accordance with the invention. Fig. 2 is a top view of the processing apparatus of Fig. 1. Fig. 3 is a perspective view of a curling fixture employed in the processing apparatus of Figs. 1 and 2. Fig. 3A is a perspective view of a fixture that may alternatively be employed in the processing apparatus so as to impart a flat cross-sectional profile to a metal ribbon processed in the processing apparatus. Fig. 4 is a graphical representation of variations in resonant frequency and output signal amplitude resulting from changes in a bias field applied to an amoφhous metal alloy that is subjected only to a single annealing step. Fig. 5 is a graphical representation of variations in resonant frequency and output signal amplitude resulting from changes in a bias field applied to an amoφhous metal alloy strip that is twice-annealed in accordance with the invention. Fig. 6 is a graphical representation of variations in resonant frequency and output signal amplitude resulting from changes in a bias field applied to an amoφhous metal alloy strip formed according to another example of the inventive process. Fig. 7 is a graphical representation of variations in resonant frequency and output signal amplitude resulting from changes in the temperature applied to the amoφhous metal alloy during the second step of a two-step annealing process. Fig. 8 is a graphical representation of variations in resonant frequency sensitivity to bias field changes, and total resonant frequency shift, resulting from changes in the temperature applied to the amoφhous metal alloy during the second step of a two-step annealing process. Fig. 9 is a graphical representation of variations in resonant frequency and output signal amplitude resulting from changes in a bias field applied to an amoφhous metal alloy strip formed according to another example of the inventive process. Fig. 10 illustrates an M-H loop characteristic of a metal alloy strip formed according to the latter example of the inventive process. Fig. 11 is a schematic block diagram of an electronic article surveillance system which uses a magnetomechanical marker incoφorating an active element formed in accordance with the invention. Fig. 12 is a graphical representation of the bias sweep curves for the alloy of Example 5. Fig. 13 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the second stage annealing temperature for the alloy of Example 5. Fig. 14 is a graphical representation of the amplitude at time Al versus the second stage annealing temperature for the alloy of Example 5. Fig. 15 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the longitudinal field for the alloy of Example 5. Fig. 16 is a graphical representation of the amplitude at time Al versus the longitudinal field for the alloy of Example 5. Fig. 17 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the second stage annealing temperature for the alloy of Example 6. Fig. 18 is a graphical representation of the amplitude at time Al versus the second stage annealing temperature for the alloy of Example 6. Fig. 19 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the longitudinal field for the alloy of Example 6. Fig. 20 is a graphical representation of the amplitude at time Al versus the longitudinal field for the alloy of Example 6. Fig. 21 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the second stage annealing temperature for the alloy of Example 7. Fig. 22 is a graphical representation of the amplitude at time Al versus the second stage annealing temperature for the alloy of Example 7. Fig. 23 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the second stage annealing temperature for the alloy of Example 8. Fig. 24 is a graphical representation of the amplitude at time Al versus the second stage annealing temperature for the alloy of Example 8. Fig. 25 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the longitudinal field for the alloy of Example 8. Fig. 26 is a graphical representation of the amplitude at time A! versus the longitudinal field for the alloy of Example 8. Fig. 27 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the second stage annealing temperature for the alloy of Example 9. Fig. 28 is a graphical representation of the amplitude at time Al versus the second stage annealing temperature for the alloy of Example 9. Fig. 29 is a graphical representation of the resonance frequency slope and resonance frequency shift versus the longitudinal field for the alloy of Example 9. Fig. 30 is a graphical representation of the amplitude at time A! versus the longitudinal field for the alloy of Example 9. DESCRIPTION OF PREFERRED EMBODIMENTS There will now be described, with initial reference to Figs. 1 and 2, a method and apparatus provided in accordance with the invention for forming the active elements of magnetomechanical EAS markers using a two-step annealing process that yields an active element having a resonant frequency that is relatively insensitive to variations in applied bias field. It is to be noted that Fig. 1 is a side view of the apparatus and Fig. 2 is a top view of the apparatus. Reference numeral 20 generally indicates the processing apparatus. The processing apparatus includes an oven 22, and supply and take-up reels 24, 26 provided at opposite sides of the oven 22. A continuous ribbon 28 of amoφhous metal is unwound from the supply reel 24 and transported along a path P through the oven 22 and then is taken up on the take-up reel 26. The ribbon 28 is engaged between a capstan 30 and a pinch roller 32 positioned between the oven 22 and the take-up reel 26. The capstan 30, in cooperation with the pinch roller 32, draws the ribbon 28 along its path P through the oven 22. Arrays 33 of permanent magnets are provided alongside the oven 22 so as to generate a magnetic field, within the oven 22, that is transverse to the longitudinal axis of the ribbon 28. It will be observed that the arrays 33 of permanent magnets do not extend along the entire length of the oven 22. Rather, the arrays 33 are provided so that the magnetic field is present in substantially all of a first zone A within the oven 22, but the magnetic field generated by the magnet arrays 33 is substantially absent from a second zone B in the oven 22. Zone B is downstream from zone A along the path of travel P. It is to be understood that the foregoing arrangement of the magnetic arrays 33 relative to the oven 22 results in the ribbon 28 being subjected to a two-step annealing process in which, during a first step, the ribbon is annealed in the presence of a transverse magnetic field, whereas in the second step the ribbon 28 is further annealed in the absence of the magnetic field. The field generated by the magnet arrays 33 should be strong enough so that the magnetic field formed in zone A is saturating for the material making up the ribbon 28. Depending on the material used, the optimum field may be in excess of 800 Oe, and a field as strong as 1 ,000 Oe may be necessary to achieve saturation. The oven 22 may be of a conventional type, and preferably has the capability of maintaining different temperatures in zone A and B. The length of the path of travel of the ribbon 28 in zone B relative to the length of the path of travel in zone A is determined according to the desired length of time during which the second annealing step is to be performed relative to the duration of the first annealing step. The duration of each annealing step is the product of two parameters: length of the path of travel through the respective zone and the speed at which the ribbon 28 is transported through the oven 22. According to a preferred arrangement of the apparatus 20, the total length of the path of travel through the oven 22 is about 231.1 cm. Although it is most convenient to provide both zone A (transverse-field annealing) and zone B, (second-stage annealing, without applied field) within a single oven, it is also contemplated that zone A could be provided in a first oven, and zone B provided in a second oven separate from, and downstream from, the first oven. A curling fixture 34 is optionally provided within the oven 22 for the puφose of imparting a transverse curl to the ribbon 28. As best seen in Fig. 3, the fixture 34 has a curl surface 36 which, proceeding in a direction transverse to the longitudinal axis of the ribbon 28, rises and then falls. The fixture 34, if present, may be placed in zone A of the oven 22, extending substantially halfway along the length of zone A. Alternatively, the fixture 34 may be placed in zone B, or may extend within both zones A and B. The ribbon 28 is drawn longitudinally through the fixture 34, and the heating applied to the ribbon 28 during its passage through the fixture 34 causes the ribbon 28 to conform itself to the curl surface 36, thereby imparting a transverse curve to the ribbon 28. The result of the treatment is that cut strips subsequently produced from the ribbon 28 have a curve transverse to the longitudinal axis of the strips, in correspondence to the curl surface 36. The transversely-curved active elements are provided to reduce or avoid a clamping effect that might otherwise occur when the active element is mounted in the EAS marker in proximity to a magnetic biasing element. The curl surface 36, if employed, is preferably contoured so as to impart to the ribbon 28 a curve which has a height at its crown that is about 0.0127 cm to 0.0254 cm above the transverse edges of the ribbon 28. As an alternative to the fixture 34 shown in Fig. 3, there can be provided a fixture 34' (shown in Fig. 3 A) with a flat guide surface 37 instead of a curved surface, so as to produce active elements that are substantially flat sections cut from the ribbon 28. As indicated in the above-referenced '651 application, annealing the material on a flat surface tends to eliminate longitudinal curling in the active element and makes it possible to reduce the overall height of the EAS marker. Reel motors (not shown) are respectively provided for the supply reel 24 and the take-up reel 26. The take-up reel motor is operated so that the ribbon 28 is taken up, upon emerging from the capstan 30 and the pinch roller 32, with little or no slack and a modest amount of tension, and the motor for the supply reel 24 is also operated so as to minimize both slack and tension in the ribbon 28 while it passes through the oven 22. The speed of operation of the reel motors may be controlled by a human operator, or an automatic control system may be provided. Upon completion of the two-step annealing process illustrated in Figs. 1 and 2, the twice-annealed continuous ribbon is cut into strips according to a conventional technique. However, the magnetic properties imparted by the annealing process in accordance with the invention are more uniform than the properties exhibited by conventional as-cast amoφhous ribbons, so that the magnetic properties of the material need not be measured, nor the cut-length of the strips adjusted, as frequently as is required when cutting as-cast amoφhous ribbon. Before turning to specific examples of the application of the inventive two-step annealing process, it should be noted that two-step annealing in accordance with the invention need not be performed with a continuous process. That is, either the second annealing step, or both the first and second annealing steps, can be applied to pre-cut discrete strips rather than to a continuous ribbon. Particular examples of the inventive process will now be described. Example 1 A continuous amoφhous ribbon having the composition Fe 32 Co I8 Ni 32 B 13 Si 5 (atomic percent) was annealed at 400°C for 22 seconds in a saturating transverse magnetic field. The ribbon had a width of about 12.7 mm and a thickness of about .025 mm. After the first (transverse-field) annealing step, the ribbon was cut into strips having a length of 37.75 millimeters and the cut strips were then further annealed at 340°C for 1 minute while being maintained in a stationary position in a separate oven. During the second annealing step the saturating magnetic field was absent, but there was an ambient field of about 0.7 Oe in the longitudinal direction of the strips, due to the earth's magnetic field. Fig. 4 illustrates magnetomechanical characteristics of the cut strips produced by the first (transverse-field) annealing step, and before application of the second annealing step, according to variations in bias field. Fig. 5 illustrates bias-field-dependent magnetomechanical characteristics of the strips produced by the entire two-step process. In both of Figs. 4 and 5: The solid-line curve illustrates changes in resonant frequency with variations in applied bias field. The dashed-line curve illustrates output signal magnitude immediately at the end of an interrogation signal pulse, according to changes in the bias field. The dotted-line curve illustrates output signal amplitude one millisecond after the end of the interrogation field pulse, according to changes in the bias field. The dot-dash-line curve illustrates output signal amplitude two milliseconds after the end of the interrogation field pulse, according to changes in the bias field. (The output signal amplitudes exhibited at and after the end of the interrogation signal pulse are sometimes referred to as "ring-down" amplitudes.) As indicated by Fig. 4, for cut-strips which are only transverse-field annealed, the slope of the resonant-frequency-to-bias-field curve (solid-line curve) exhibits a slope of about 700 Hz/Oe between the 5 Oe and 7 Oe points. This slope is indicative of an excessive sensitivity in resonant frequency to changes in the bias field. This degree of sensitivity would tend to result in unreliable performance by markers using single-step annealed active elements. Specifically, variations in the orientation of the marker result in variations in the effective applied bias field because of the fact that the effect of the earth's magnetic field varies with orientation of the marker, and these variations are sufficient in some cases to shift the resonant frequency away from the predetermined frequency at which magnetomechanical EAS detection equipment operates. It will be noted that the once-annealed cut strips provide a frequency shift of about 2.3 kHz when the bias field is decreased from 6 Oe to 1 Oe and a ring-down amplitude at 1 millisecond after the interrogation signal pulse of about 310 mV with an applied bias field of 6 Oe. Although the frequency shift and output amplitude characteristics of the once-annealed cut- strips are satisfactory, and the resonant frequency versus bias field curve slope is more favorable than that exhibited by the cobalt-rich material (Fe 39 5 Cθ 39 5 Si 2 B 19 ) described in the above- referenced '651 application, still the sensitivity of resonant frequency to bias field variations is too great for reliable operation. However, the characteristics of the twice-annealed cut-strips, as shown in Fig. 5, provide for a significantly reduced slope of the resonant-frequency-to-bias-field curve at the cost of an acceptable reduction in the frequency shift and output amplitude characteristics. In particular, in the twice-annealed strips, the slope between the 5 Oe and 7 Oe points is reduced to about 420 Hz/Oe. The frequency shift is about 2.0 kHz, upon reduction of the bias field from 6 Oe to 1 Oe, and the ring-down amplitude at 1 millisecond is 275 mV with a 6 Oe bias field. It is believed that the second annealing step, in the presence of only a minimal ambient magnetic field, serves to somewhat disperse the rather well defined magnetic domain boundaries produced by the transverse-field annealing step, thereby reducing the sensitivity of the resonant frequency of the material to changes in the bias field. As a result, the twice- annealed material, when incoφorated as an active element in pulsed magnetomechanical EAS markers, exhibits an acceptable degree of reliability, notwithstanding the inevitable variations in effective applied bias field. Example 2 The same process was applied to the same material as in Example 1 , except that the duration of the second annealing step was 2 minutes rather than 1 minute. Fig. 6 illustrates the resulting magnetomechanical characteristics of the two-step annealed cut-strips with each of the four curves in Fig. 6 illustrating, respectively, the same characteristics as in Fig. 5. It will be noted that the increased duration of the second annealing step in this example has produced a less steep slope of the resonant-frequency-to-bias-field curve, the slope being approximately 350 Hz/Oe between the 5 Oe and 7 Oe points. The frequency shift was modestly reduced to 1.7 kHz for a bias field reduction from 6 Oe to 1 Oe, and the one-millisecond ring-down amplitude with a 6 Oe bias field is essentially unchanged at 280 mV. Example 3 A continuous ribbon having the same composition and dimensions described above in connection with Example 1 was two-step annealed using the continuous-process apparatus described above in connection with Figs. 1 and 2. The path of travel of the continuous ribbon 28 in zone A (transverse-annealing zone) was 152.4 cm and the path of travel in zone B (second-step anneal; no applied field) was 78.7 cm. The continuous ribbon 28 was transported at a speed of about 7 centimeters per second, producing a duration of about 21 seconds for the first (transverse-field) annealing step and about 11 seconds for the second (field-absent) step. The path of travel P was substantially aligned in an east-west direction so that virtually no ambient longitudinal field was present in zone B. The temperature in zone A was fixed at 380°C, but the temperature in zone B was varied within a range of 320°-400°C to obtain respective lots of samples. The continuous strip was cut into discrete strips (37.75 mm in length) after the two-step continuous annealing was carried out. In Fig. 7, the shaded circles indicate resonant frequency values obtained (at a 5.5 Oe bias field) for each of the second-step annealing temperatures, and the solid squares indicate the 1- millisecond ring-down amplitudes (at a 5.5 Oe bias field) obtained with the various second-step annealing temperatures. In Fig. 8, the shaded circles indicate the resonant-frequency-to-bias- field dependency characteristic (i.e., the slope), and the solid squares indicate the resonant frequency shift (upon reduction of bias field from 6 Oe to 1 Oe), obtained at the various second- step annealing temperatures. As indicated by Fig. 7, the resonant frequency at 5.5 Oe decreases for second-step annealing temperatures above 340° and the 1 millisecond ring-down amplitude (also at a 5.5 Oe bias field) decreases for temperatures above 360°C. Fig. 8 illustrates how the resonant frequency/bias field slope (between the 5 and 7 Oe points) and the total frequency shift (from 6 to 1 Oe) varies depending on the second-step annealing temperature. In general, the slope decreases from about 610-650 Hz/Oe to about 230 Hz/Oe, as the second-step annealing temperature is increased from 320° to 400°C. The frequency shift initially increases, and then decreases when the second-step annealing temperature is greater than 360°C. A satisfactory trade-off of resonant frequency/bias field slope versus total frequency shift is obtained with a second step annealing temperature of 380°C, yielding the following characteristics: 1 millisecond ring-down—263 mV, resonant frequency/bias field slope~488 Hz/Oe, frequency shift- 1.970 kHz. Example 4 The same material and the same two-step continuous annealing apparatus as in Example 3 were used. The alloy ribbon transport speed was reduced by a factor of about two, and the following annealing parameters were employed: first (transverse field) step~43 seconds at 380°C; second (field-absent) step~22 seconds at 360°C. After cutting the two-step annealed continuous strip into discrete strips as in the previous example, characteristics as shown in Fig. 9 were obtained. The four curves shown in Fig. 9 illustrate, respectively, the same characteristics discussed in connection with Figs. 5 and 6 above. It will be noted that the slope of the resonant frequency/bias field curve is about 430 Hz/Oe between the 5 and 7 Oe points. The 1 millisecond ring-down amplitude is 290 mV at a 6 Oe bias field, and the frequency shift is 1.830 kHz when the bias field is reduced from 6 Oe to 1 Oe. An M-H loop characteristic of the resulting two-step annealed cut-strip material is shown in Fig. 10. It will be observed that the M-H loop is somewhat open near the origin, indicating that the treated material is somewhat susceptible to causing false alarms in harmonic EAS systems, although less so than conventional magnetomechanical markers which employ as-cast (i.e., non- annealed) active elements. In each of the examples given above, a material having the same composition was used. However, it is believed that satisfactory results can be obtained with other compositions, having a proportion of cobalt ranging from 5 to 45% by atomic percent, provided that the material also includes a substantial proportion of nickel. Also, although it is preferred that no field other than an ambient magnetic field provided by the earth's magnetic field be provided during the second annealing step, it is believed that satisfactory results can also be obtained by providing a magnetic field of less than 5 Oe in the longitudinal direction of the continuous strip or discrete strips during the second annealing step. It is also believed that satisfactory results will not be obtained if the second (field-absent) annealing step is carried out at a temperature of more than 450°C or with a duration of not more than 5 minutes. As noted above, the two-step annealing process disclosed herein, and particularly the provision of a second annealing step, carried out substantially without any magnetic field, after an initial saturating transverse-field annealing step, permits fabrication of active elements for magnetomechanical EAS markers having a resonant frequency that is not unduly sensitive to small variations in the bias field. At the same time, active elements produced in this manner exhibit satisfactory characteristics in terms of overall frequency shift and ring-down signal amplitude. Also, the active elements can be made to have flat profiles and have reduced susceptibility to causing false alarms in harmonic EAS systems. Fig. 11 illustrates a pulsed-interrogation EAS system which uses a magnetomechanical marker 100 that incoφorates an active element produced in accordance with the invention. The system shown in Fig. 11 includes a synchronizing circuit 200 which controls the operation of an energizing circuit 201 and a receive circuit 202. The synchronizing circuit 200 sends a synchronizing gate pulse to the energizing circuit 201, and the synchronizing gate pulse activates the energizing circuit 201. Upon being activated, the energizing circuit 201 generates and sends an interrogation signal to interrogating coil 206 for the duration of the synchronizing pulse. In response to the interrogation signal, the interrogating coil 206 generates an interrogating magnetic field, which, in turn, excites the marker 100 into mechanical resonance. Upon completion of the interrogation signal pulse, the synchronizing circuit 200 sends a gate pulse to the receiver circuit 202, and the latter gate pulse activates the circuit 202. During the period that the circuit 202 is activated, and if a marker is present in the interrogating magnetic field, such marker will generate in the receiver coil 207 a signal at the frequency of the mechanical resonance of the marker. This signal is sensed by the receiver 202, which responds to the sensed signal by generating a signal to an indicator 203 to generate an alarm or the like. In short, the receiver circuit 202 is synchronized with the energizing circuit 201 so that the receiver circuit 202 is only active during quiet periods between the pulses of the pulsed interrogation field. Example 5: Fe 32 91 Ni 31 46 Co 17 98 B, 2 67 Si 4 9g , wherein the subscripts are in atomic percent. A continuous amoφhous ribbon having a composition Fe 32 9 iNi 31 46 Co I7 g8 B, 2 67 Si 49 , wherein the subscripts are in atomic percent, and dimensions of about 12.7 millimeters wide and about 25 micrometers thick was annealed (first stage anneal) with the reel to reel method described hereinabove. The annealing conditions were 390 degrees centigrade for 7.5 seconds followed by 200 degrees centigrade for 5 seconds under a magnetic field of 1200 oersteds applied along the ribbon width direction. The ribbon was cut into sample strips having a length of about 37.75 millimeters. A device equipped with transmitting and receiving coils was used to measure the magnetomechanical response of the samples . Fig. 12 shows the resonance frequency (Fr) in kilohertz and the signal amplitudes in millivolts as a function of the bias field in oersteds applied along the sample length. The signal amplitudes were measured at 0 milliseconds (A0), 1 millisecond (Al), and 2 milliseconds (A2) after turning off the transmitting coil. The following results were obtained: amplitude Al at a bias field of 6.5 oersteds was 403 millivolts; the resonance frequency versus bias field slope was 759 hertz per oersted at a bias field of 6.5 oersteds, and the resonance frequency shift from a bias field of 6.5 oersteds to 2 oersteds was 2.409 kilohertz. Additional sample strips of about 37.75 millimeters long were cut from the first stage annealed ribbon of this Example 5 and were further annealed, i.e., a second stage anneal, in a batch furnace. However, it should be understood that the second stage anneal could also be done in a reel to reel process and that the batch furnace anneal was done only to facilitate testing. Fig. 13 shows the relationship of the resonance frequency slope in hertz per oersted at 6.5 oersteds and the resonance frequency shift in kilohertz from 6.5 oersteds to 2 oersteds on the second stage annealing temperature in degrees centigrade. The magnetic field was 0 oersteds and the annealing time was 1 minute. From Fig. 13 it can be seen that the slope of the resonance frequency decreases as the annealing temperature increases and reaches a minimum at 320 degrees centigrade. The resonance frequency shift shows a similar trend in that it decreases with an increase in the second stage annealing temperature until it reaches a minimum at 320 degrees centigrade. Fig. 14 illustrates the amplitude Al of the samples in millivolts at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the second stage annealing temperature in degrees centigrade. The same first stage annealed material of this Example 5 was cut into sample strips having a length of about 37.75 millimeters. These strips were annealed at 360 degrees centigrade for 1 minute with various longitudinal magnetic field strengths applied along the length of the samples. Fig. 15 shows the relationship of resonance frequency (Fr) slope in hertz per oersted and the resonance frequency shift in kilohertz to the applied field in oersteds. From Fig. 15, it can be seen that as the longitudinal magnetic field increases the range from 0 to 1.2 oersteds, both the resonance frequency slope and the resonance frequency shift decrease. Fig. 16 shows the test results that illustrate that the amplitude Al, i.e., 1 millisecond after the transmitting coil was turned off, exhibits insignificant change within the magnetic field range of 0 to 1.2 oersteds. Example 6: Fe 40 87 Co 40 61 B 13 40 Si 5 12 , wherein the subscripts are in atomic percent. A continuous amoφhous ribbon having a composition Fe 40 87 Cθ 40 6I B 13 40 Si 5 12 , wherein the subscripts are in atomic percent, and dimensions of about 10 millimeters wide and 25 micrometers thick was annealed with a magnetic field applied to the ribbon width according to the method described in Example 5. The first stage annealing conditions were 380 degrees centigrade for 7.5 seconds followed by 200 degrees centigrade for 5 seconds. The ribbon was then cut into sample strips about 37.75 millimeters long that were annealed at a temperature in the range from 300 degrees centigrade to 400 degrees centigrade for 1 minute under a magnetic field of 0.8 oersteds applied along the ribbon length. The same device described in Example 5 was used to measure the samples' magnetomechanical responses. Fig. 17 illustrates the dependence of the resonance frequency slope in hertz per oersted at 6.5 oersteds and the resonance frequency shift in kilohertz from 6.5 oersteds to 2 oersteds on the second stage annealing temperature in degrees centigrade. From Fig. 17 it can be seen that the slope of the resonance frequency and resonance frequency shift decrease as the second stage annealing temperature increases and reach a minimum at 380 degrees centigrade. Fig. 18 illustrates the amplitude Al of the samples at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the second stage annealing temperature. The same first stage annealed material of this Example 6 was cut into sample strips having a length of about 37.75 millimeters. These strips were annealed at 360 degree centigrade for 1 minute with a longitudinal magnetic fields of various strength applied along the sample lengths. Fig. 19 shows the relationship of resonance frequency slope in hertz per oersted at 6.5 oersteds and the resonance frequency shift in kilohertz from 6.5 oersteds to 2 oersteds to the applied field in oersteds. Fig. 20 shows the test results for the amplitude Al, i.e., 1 millisecond after the transmitting coil had been turned off, at 6.5 oersteds as a function of the applied field in oersteds over a range of 0 to 1 oersted.. Example 7: Fe 37 85 Ni 3029 Cθι 5 16 B 15 31 Siι 39 , wherein the subscripts are in atomic percent. A continuous amoφhous ribbon having a composition Fe 37 85 Ni 3029 Co I5 ,16 B 15 i31 Siι 39 , wherein the subscripts are in atomic percent, and dimensions of about 6 millimeters wide and about 25 micrometers thick was annealed with a magnetic field applied to the ribbon width according to the method described in Example 5. The annealing conditions were 405 degrees centigrade for 7.5 seconds followed by 200 centigrade for 7.5 seconds. The ribbon was then cut into sample strips of about 37.75 millimeters long. These strips were annealed at a temperature in the range from 300 degrees centigrade to 400 centigrade for 1 minute under a magnetic field of 0.8 oersteds applied along the ribbon length. The same device described in Example 5 was used to measured the samples' magnetomechanical responses. Fig. 21 illustrates the relationship of resonance frequency slope in hertz per oersted at 6.5 oersteds and resonance frequency shift in kilohertz from 6.5 oersteds to 2 oersteds to the second stage annealing temperature in degrees centigrade. Fig. 22 illustrates the amplitude Al of the samples at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the second stage annealing temperature in degrees centigrade. Example 8: Fe 3838 Ni 29 . 06 o 16 10 B I4 89 Si 1 57 , wherein the subscripts are in atomic percent. A continuous amoφhous ribbon having the composition Fe 38 38 Ni 29. o 6 Co 16 10 B 14 89 Siι 57 , wherein the subscripts are in atomic percent, and dimensions of about 6 millimeters wide and about 25 micrometers thick was annealed with a magnetic field applied to the ribbon width similar to the method described in Example 5. The annealing conditions were 400 degrees centigrade for 7.5 seconds followed by 200 degrees centigrade for 5 seconds. The ribbon was then cut into sample strips about 37.75 millimeters long and annealed at a temperature in the range from 300 degrees centigrade to 400 degrees centigrade for 1 minute under a magnetic field of 1 oersted applied along the ribbon length. The same device described in Example 5 was used to measure the samples' magnetomechanical responses. Fig. 23 illustrates the dependence of resonance frequency slope in hertz per oersted at 6.5 oersted and resonance frequency shift in kilohertz from 6.5 oersted to 2 oersted to the second stage annealing temperature in degrees centigrade. Fig. 24 illustrates the amplitude Al of the samples at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the second stage annealing temperature in degrees centigrade. The same transverse magnetic field annealed material of this Example 8 was annealed at 360 degrees centigrade for 1 minute with longitudinal magnetic fields of various strengths. Fig. 25 shows the relationship of resonance frequency slope in hertz per oersted at 6.5 oersteds and the resonance frequency shift in kilohertz from 6.5 oersted to 2 oersted to the applied field in oersteds. Fig. 26 illustrates amplitude Al of the samples at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the applied field in oersteds. Example 9: Fe 42 . 62 Ni 302 oCθι 1 87 B 14 ι Siι 17 , wherein the subscripts are in atomic percent. A continuous amoφhous ribbon having a composition Fe 42.62 Ni 302 oCo u 87 B 14 ι 4 Si j Λη , wherein the subscripts are in atomic percent and dimensions of about 6 millimeters wide and about 25 micrometers thick was annealed with a magnetic field applied to the ribbon width according to the method described in Example 5. The annealing conditions were 360 degrees centigrade for 7.5 seconds followed by 200 degrees centigrade for 5 seconds. The ribbon was then cut into sample strips of about 37.75 mm long. These strips were annealed at a temperature in the range from 300 degrees centigrade to 400 degrees centigrade for 1 minute under a magnetic field of 1 oersted applied along the ribbon length. The same device described in Example 5 was used to measured the samples' magnetomechanical responses. Fig. 27 shows the relationship of resonance frequency slope in hertz per oersted at 6.5 oersteds and resonance frequency shift in kilohertz from 6.5 oersteds to 2 oersteds to the second stage annealing temperature in degrees centigrade. Fig. 28 illustrates the amplitude Al of the samples at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the second stage annealing temperature in degrees centigrade. The same transverse magnetic field annealed material of this Example 9 was annealed at 360 degrees centigrade for 1 minute with longitudinal magnetic fields of various strengths. Fig. 29 shows the relationship of resonance frequency slope in hertz per oersted at 6.5 oersteds and the resonance frequency shift in kilohertz from 6.5 oersteds to 2 oersteds to the applied field in oersteds. Fig. 30 illustrates the amplitude Al of the samples at 1 millisecond after the transmitting coil was turned off at 6.5 oersteds as a function of the applied field in oersteds. The magnetostrictive elements of the present invention are a significant improvement over conventional materials used in the prior art such as Metglas® 2826 MB, which has a composition of Fe 40 Ni 38 Mo 4 B 18 , in that the magnetostrictive elements of the present invention not only have a low resonance frequency slope but also can be made narrower, that is, 6 millimeters wide compared to 12.7 millimeters wide with the prior art material, and can also be made flat which enhances commercial usage of the slim-line markers. With reference to the Examples, it can be seen that the magnetostrictive elements made of alloys having iron and a proportion of cobalt in the range of about 12 to about 41 atomic percent and annealed in accordance with the two stage annealing process have a lower resonance frequency slope thereby providing a magnetostrictive element that has an increased stability of the resonance frequency relative to changes in the biasing magnetic field. Based on these results it is believed that alloys having iron and a cobalt content in the range of about 5 to about 45 atomic percent and annealed in accordance with the two stage annealing process will exhibit the increased stability of the resonance frequency relative to changes in the biasing magnetic field. It can also be seen from Examples, that the magnetostrictive elements made of alloys having iron a proportion of cobalt in the range of about 12 to about 18 atomic percent and annealed in accordance with the two stage annealing process have an enhanced result in that the resonance frequency slope can be controlled below 550 hertz per oersted. Based on these results it is believed that alloys having iron and a cobalt content in the range of about 10 to 25 atomic percent and annealed in accordance with the two stage annealing process will exhibit the enhanced result. In the second stage annealing of the magnetostrictive elements of the present invention the temperature is preferably in the range of about 250 to about 450 degrees centigrade, and this second annealing is done for a period of about 0.05 to about 5 minutes. The second annealing is performed in the presence of a longitudinal magnetic field in the range from 0 to about 5 oersteds. Various changes in the foregoing annealing apparatus and modifications in the described practices may be introduced without departing from the invention. The particularly preferred embodiments of the invention are thus intended in an illustrative and not limiting sense. The true spirit and scope of the invention is set forth in the following claims.

Claims: CLAIMS What is claimed is: 1. A magnetostrictive element for use in a magnetomechanical article surveillance marker formed by first annealing an amoφhous metal alloy, said alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing said alloy in the absence of the saturating magnetic field. 2. A magnetostrictive element as recited in claim 1, wherein said saturating magnetic field is a transverse magnetic field. 3. A magnetostrictive element as recited in claim 2, wherein said second annealing is performed in the presence of a longitudinal magnetic field. 4. A magnetostrictive element as recited in claim 3, wherein said longitudinal magnetic field is in the range of 0 to about 5 oersteds. 5. A magnetostrictive element as recited in claim 4, wherein said second annealing is performed at a temperature in the range of about 250 degrees centigrade to about 450 degrees centigrade for a time period in the range of about 0.05 to 5 minutes. 6. A magnetostrictive element as recited in claim 1 , wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 7. A magnetostrictive element as recited in claim 6, wherein the proportion of cobalt is in the range of about 10 to about 25 atomic percent. 8. A magnetostrictive element as recited in claim 7, wherein the proportion of cobalt is in the range of about 12 to about 18 atomic percent. 9. A magnetostrictive element as recited in claim 5, wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 10. A magnetostrictive element as recited in claim 5, wherein the proportion of cobalt is in the range of about 10 to about 25 atomic percent. 11. A magnetostrictive element as recited in claim 4, wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 12. A magnetostrictive element as recited in claim 5, wherein the proportion of cobalt is in the range of about 12 to about 18 atomic percent. 13. A marker for use in a magnetomechanical article surveillance system comprising an amoφhous magnetostrictive strip formed by first annealing an amoφhous metal alloy, said alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing said alloy in the absence of the saturating magnetic field. 14. A marker as recited in claim 13, wherein said saturating magnetic field is a transverse magnetic field. 15. A marker as recited in claim 14, wherein said second annealing is performed in the presence of a longitudinal magnetic field. 16. A marker as recited in claim 15, wherein said longitudinal magnetic field is in the range of 0 to about 5 oersteds. 17. A marker as recited in claim 16, wherein said second annealing is performed at a temperature in the range of about 250 degrees centigrade to about 450 degrees centigrade for a time period in the range of about 0.05 to 5 minutes. 18. A marker as recited in claim 13, wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 19. A marker as recited in claim 18, wherein the proportion of cobalt is in the range of about 10 to about 25 atomic percent. 20. A marker as recited in claim 19, wherein the proportion of cobalt is in the range of about 12 to about 18 atomic percent. 21. A marker as recited in claim 17, wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 22. A marker as recited in claim 17, wherein the proportion of cobalt is in the range of about 10 to about 25 atomic percent. 23. A marker as recited in claim 17, wherein the proportion of cobalt is in the range of about 12 to about 18 atomic percent. 24. A magnetomechanical article surveillance system comprising (a) generating means for generating an electromagnetic field alternating at a selected frequency in an interrogation zone; (b) a marker comprising an amoφhous magnetostrictive strip formed by first annealing an amoφhous metal alloy, said alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing said alloy in the absence of the saturating magnetic field and a biasing element to cause said magnetostrictive strip to be mechanically resonant when exposed to said alternating field; and (c) detecting means for detecting said mechanical resonance of said magnetostrictive strip. 25. A magnetomechanical article surveillance system as recited in claim 24, wherein said saturating magnetic field is a transverse magnetic field. 26. A magnetomechanical article surveillance system as recited in claim 25, wherein said second annealing is performed in the presence of a longitudinal magnetic field. 27. A magnetomechanical article surveillance system as recited in claim 26, wherein said longitudinal magnetic field is in the range of 0 to about 5 oersteds. 28. A magnetomechanical article surveillance system as recited in claim 27, wherein said second annealing is performed at a temperature in the range of about 250 degrees centigrade to about 450 degrees centigrade for a time period in the range of about 0.05 to 5 minutes. 29. A magnetomechanical article surveillance system as recited in claim 24, wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 30. A magnetomechanical article surveillance system as recited in claim 29, wherein the proportion of cobalt is in the range of about 10 to about 25 atomic percent. 31. A magnetomechanical article surveillance system as recited in claim 30, wherein the proportion of cobalt is in the range of about 12 to about 18 atomic percent. 32. A magnetomechanical article surveillance system as recited in claim 28, wherein the proportion of cobalt is in the range of about 12 to about 41 atomic percent. 33. A magnetomechanical article surveillance system as recited in claim 28, wherein the proportion of cobalt is in the range of about 10 to about 25 atomic percent. 34. A magnetomechanical article surveillance system as recited in claim 28, wherein the proportion of cobalt is in the range of about 12 to about 18 atomic percent.

Inventors: LIU NEN-CHIN, LIU, NEN-CHIN
Applicants: SENSORMATIC ELECTRONICS CORP, SENSORMATIC ELECTRONICS CORPORATION
Application year: 1997
Main classification: G08B13
IPC codes: C21D6, C22C45, G06K19, G08B13, G09F3, H01F1, H01L41