HK1024327B - Magnetostrictive element for use in a magnetomechanical surveiliance system - Google Patents

Description

Magnetostrictive element for magnetic force monitoring system

This application is a continuation-in-part of a co-pending prior application serial No. 08/508,580 filed on 28.7.1995. Co-pending prior application 08/508,580 continues as part of several prior applications, namely application serial No. 08/269,651 filed 6/30 in 1994 and granted us patent No.5,469,140 at 12/21 in 1995, co-pending application serial No. 08/392,070 filed 2/22 in 1995, and co-pending application serial No. 08/420,757 filed 4/12 in 1995, all of which have the same inventors and assignee as the present application.

Technical Field

The present invention relates to magnetic (magnetocaloric) article surveillance systems, and more particularly to amorphous metal alloy magnetostrictive elements for use in such systems.

Background

U.S. patent No.4,510,489 to Anderson et al discloses a magnetomechanical Electronic Article Surveillance (EAS) system wherein a tag integrated with a magnetostrictive activation element is secured to an article to prevent theft. The active elements are formed of soft magnetic material and the tag further includes control elements that are biased or magnetized to a predetermined degree to provide a bias field that mechanically resonates the active elements at a predetermined frequency. The marker is detected by an interrogation signal generating means generating an alternating magnetic field at a predetermined resonance frequency and the signal caused by the mechanical resonance is detected by a receiving means.

According to one embodiment disclosed in Anderson et al, the interrogation signal is switched on and off, or "pulsed", and a "ringing" signal is generated by the activation element upon detection of each interrogation pulse signal.

Typically, the magnetomechanical marker is deactivated by a degaussing process on the control element. Thereby removing the bias field from the active element and thereby causing a substantial shift in the resonant frequency of the active element.

The Anderson et al patent discloses a number of materials that can be used as the activation element and describes techniques for processing these materials. The disclosed technique involves heat treatment (annealing) of an amorphous material in a saturated magnetic field. The disclosures of the Anderson et al patents are incorporated herein by reference.

U.S. patent No.5,252,144 to Martis further discloses materials suitable for use in the activation element of a magnetomechanical EAS marker, and an annealing process (no applied magnetic field) for the material.

The above-referenced copending application' 651 discloses a process in which a pre-cut strip of bulk amorphous metal alloy is annealed in the presence of a saturating transverse magnetic field. The resulting annealed strip is suitable for use as a material for a magnetomechanical marker activation element and has improved ringing performance which improves the behavior of a pulsed magnetomechanical EAS system. Furthermore, the resulting hysteresis loop characteristics of the active elements may eliminate or reduce false alarms due to exposure to harmonic EAS systems. The process disclosed in the' 651 application allows for the production of an activation element that is relatively flat in the length direction, thereby allowing for the production of very thin indicia that may be incorporated into such activation elements. The disclosure of the aforementioned application serial No. 08/269,651 is incorporated herein by reference.

The above-referenced co-pending application '757 discloses one use of the technique of the' 651 application in which a continuous process is used in which a transverse magnetic field anneal is performed in a furnace by passing a continuous ribbon of amorphous metal alloy through the furnace in a reel-to-reel fashion. The continuous ribbon is then cut into separate ribbons after annealing is complete. This continuous annealing process overcomes the inconvenience of feeding the precut strip into the out-feed furnace.

The techniques disclosed in co-pending applications '651 and' 757 represent an improvement over the prior art. However, there remains a need for further improvements in the context of these two co-pending applications to provide an activation element for an EAS marker having a resonant frequency that is relatively insensitive to fluctuations in the bias field.

According to the present invention there is provided a magnetostrictive element for a magnetic article surveillance marker, wherein: the component is formed by first annealing an amorphous metal alloy in a saturation magnetic field and then a second annealing in the absence of the saturation magnetic field, the alloy comprising iron and about 5-45 atomic percent cobalt.

Also in accordance with the present invention, there is provided a marker for use in a magnetic article surveillance system comprising a magnetostrictive strip formed by first annealing an amorphous metal alloy in the presence of a saturation magnetic field and then second annealing the amorphous metal alloy in the absence of the saturation magnetic field, the alloy comprising iron and cobalt in an atomic percent of between about 5 and 45%.

Also, according to the present invention, there is provided a magnetic article surveillance system comprising: generating means for generating an electromagnetic field alternating at a selected frequency in an interrogation zone; a marker comprising a magnetostrictive strip formed by first annealing an amorphous metal alloy in a saturation magnetic field and then a second annealing in the absence of the saturation magnetic field, and a biasing element that mechanically resonates the magnetostrictive strip when exposed to an alternating magnetic field, the alloy comprising iron and about 5-45 atomic percent cobalt; and a detection device for detecting said mechanical resonance of said magnetostrictive strip.

EAS marker activation elements having a flat spool profile and which are not prone to false alarms in harmonic EAS systems are provided according to the invention. EAS marker activation elements provided in accordance with the present invention have increased insensitivity to fluctuations in an applied bias magnetic field.

According to a first aspect of the present invention there is provided a method of manufacturing a tag for an electronic article surveillance system, comprising the steps of: the method comprises the steps of annealing the amorphous metal alloy for a first time under the condition of applying a transverse magnetic field intersecting with the length direction of the strip, so that after the first annealing is finished, when the strip is exposed to a magnetic field alternating with a resonance frequency, the strip generates mechanical resonance, and the resonance frequency changes along with the change of the bias magnetic field.

According to another aspect of the present invention there is provided a method of manufacturing a marker for a magnetomechanical article surveillance system, comprising the steps of: the strip of magnetostrictive material is annealed a first time in a saturated magnetic field and then annealed a second time in the absence of the saturated magnetic field.

According to another aspect of the present invention, there is provided a method of manufacturing a magnetostrictive element for a magnetic article surveillance system, comprising the steps of: providing a continuous ribbon of amorphous metal; passing the continuous ribbon of amorphous alloy through an annealing zone in which the amorphous metal alloy is heated and a saturation magnetic field is applied to first anneal the continuous ribbon and then a second anneal in the absence of the saturation magnetic field; and cutting the annealed strip into separate strips having a predetermined length after the conveying and the second annealing.

According to another aspect of the present invention, there is provided a method of manufacturing a magnetostrictive element for a magnetic article surveillance system, comprising the steps of: providing a continuous ribbon of amorphous metal; the continuous ribbon of amorphous alloy is conveyed through an annealing zone in which the amorphous metal alloy is heated and a saturation magnetic field is applied to first anneal the continuous ribbon, then the annealed ribbon is cut into separate ribbons of predetermined lengths, and the separate ribbons are annealed a second time without the saturation magnetic field.

According to another aspect of the present invention, there is provided an apparatus for annealing a continuous ribbon of amorphous alloy, comprising: a furnace; a magnetic field element that forms a magnetic field such that there is substantially a magnetic field throughout the first zone a and a second zone B in the furnace is substantially free of a magnetic field; and a transport mechanism that transports the continuous strip through the first and second zones of the furnace.

According to the above aspect of the invention, the conveyor conveys the continuous strip through the first zone towards the second zone. Still further according to this aspect of the invention, the apparatus comprises: a supply reel located at one side of the furnace, from which the continuous strip is unwound to be fed into the furnace; a take-up spool located in the furnace on the opposite side of the supply spool on which the continuous strip is taken up after passing through the furnace.

Further, the transport mechanism may include a capstan and a pinch roller, both located between the furnace and the take-up spool, with the continuous strip disposed between the capstan and the pinch roller and driven by the capstan in a direction from the supply spool to the take-up spool. Further, the direction of the magnetic field element forming the magnetic field intersects the direction of passage through the furnace, and the magnetic field formed within the furnace is at least 800 Oe. Also, the continuous strip (strip) may be in the form of a continuous strip (ribbon), and the apparatus may further comprise a fixture located in the furnace through which the strip is drawn to impart the desired cross-sectional shape. In addition, the fixing device may also have a flat guide clamping surface to give the belt a substantially flat cross-sectional shape.

According to another aspect of the present invention, there is also provided an apparatus for annealing a continuous ribbon of amorphous alloy, comprising: an element forming a first heating zone; a magnetic field element forming a magnetic field substantially throughout the first heating region; an element forming a second heating region substantially free of magnetic field thereon; and a conveying mechanism that conveys the continuous strip sequentially through the first zone and the second zone. The element forming the first heating zone may be a first furnace and the element forming the second heating zone may be a second furnace different from the first furnace. Alternatively, a furnace comprising elements forming the first heating zone and elements forming the second heating zone may be used.

According to yet another aspect of the present invention, there is provided a magnetostrictive element for a magnetomechanical electronic article surveillance marker, the element formed by a first annealing of an amorphous metal alloy in a saturated magnetic field, followed by a second annealing in the absence of the saturated magnetic field. The temperature is below 450 ℃ and the time is less than or equal to 5 minutes.

According to yet another aspect of the present invention there is provided a magnetostrictive element for use in a magnetomechanical electronic article surveillance marker formed by first annealing an amorphous metal alloy in the presence of a saturation magnetic field, second annealing the amorphous metal alloy in the absence of the saturation magnetic field, and then cutting the twice annealed continuous ribbon into discrete ribbons.

According to yet another aspect of the present invention, there is provided a tag for use in a magnetomechanical electronic article surveillance system, the element comprising a discrete amorphous magnetostrictive strip formed by a first annealing of an amorphous metal alloy under a saturation magnetic field, followed by a second annealing in the absence of the saturation magnetic field.

According to yet another aspect of the present invention there is provided a marker for use in a magnetic article surveillance system, the element comprising a discrete amorphous magnetostrictive strip formed by first annealing an amorphous metal alloy under a saturation magnetic field, then second annealing the amorphous metal alloy in the absence of the saturation magnetic field, and cutting the twice annealed continuous strip.

According to yet another aspect of the present invention, there is provided a magnetomechanical electronic article surveillance system comprising: a generating circuit for generating an electromagnetic field alternating at a selected frequency in an interrogation zone, the generating circuit comprising a generating coil; a tag affixed to an article to be passed through an interrogation zone, the tag comprising a magnetostrictive element formed by a first anneal of an amorphous metal alloy in a saturated magnetic field and a second anneal in the absence of the saturated magnetic field, the tag further comprising a biasing element adjacent the magnetostrictive element that causes the magnetostrictive element to mechanically resonate when exposed to an alternating magnetic field; and a detection device for detecting said mechanical resonance of said magnetostrictive strip.

According to yet another aspect of the present invention, there is provided a magnetomechanical electronic article surveillance system comprising: a generating circuit for generating an electromagnetic field alternating at a selected frequency in an interrogation zone, the generating circuit comprising a generating coil; a tag affixed to an article to be passed through an interrogation zone, the tag comprising a magnetostrictive element formed by first annealing an amorphous metal alloy in a saturating magnetic field, then annealing a second time in the absence of the saturating magnetic field, and cutting the twice annealed continuous ribbon into discrete strips, the tag further comprising a biasing element adjacent the magnetostrictive element that causes the magnetostrictive element to mechanically resonate when exposed to an alternating magnetic field; and a detection circuit for detecting the mechanical resonance of the magnetostrictive strip.

According to a further aspect of the present invention there is provided a marker for a magnetomechanical article surveillance system, the marker comprising an amorphous magnetostrictive element and a biasing element adjacent thereto, the magnetostrictive element having a hysteresis loop characteristic such that the magnetostrictive element does not produce large detectable harmonic frequencies in an alternating magnetic field, the magnetostrictive element further having a resonant frequency-biasing field slope of less than 700Hz/Oe at a biasing field of 5-7 Oe. Also in accordance with this aspect of the invention, the resonant frequency/bias field slope may be less than 500Hz/Oe at a bias field of 5-7 Oe.

According to another aspect of the present invention there is provided a tag for use in a magnetomechanical article surveillance system, the tag comprising a magnetostrictive element having a resonant frequency with a bias field slope of less than 700Hz/Oe at a bias field of 5-7Oe, the tag having a total thickness of less than 0.065 inches.

Still according to this aspect of the invention, the magnetostrictive element has a resonant frequency-bias field slope of less than 500Hz/Oe and the total thickness of the marker is less than 0.030 inches, and may be on the order of 0.005 inches.

Other objects, advantages and applications of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention.

Brief Description of Drawings

FIG. 1 is a side view of a treatment apparatus provided in accordance with the present invention;

FIG. 2 is a top view of the apparatus of FIG. 1;

fig. 3 is a perspective view of a winding fixture used in the processing apparatus shown in fig. 1 and 2;

FIG. 3A is a perspective view of another fixture that may be used in a processing apparatus that can impart a flat cross-sectional shape to a metal strip processed in the processing apparatus;

FIG. 4 shows the fluctuation of the resonance frequency and the magnitude of the output signal when the bias magnetic field applied to the amorphous metal alloy subjected to one annealing is changed;

FIG. 5 illustrates the fluctuation of the resonant frequency and the magnitude of the output signal when the bias magnetic field applied to the amorphous metal alloy subjected to the two-stage annealing according to the present invention is varied;

FIG. 6 illustrates the fluctuation of the resonant frequency and the magnitude of the output signal when a change in the bias field applied to an amorphous metal alloy in accordance with another embodiment of the process of the present invention is performed;

FIG. 7 is a graph showing fluctuations in resonance frequency and magnitude of an output signal when the temperature of the second annealing is changed in the case where the amorphous metal alloy is subjected to the two-step annealing;

FIG. 8 illustrates the sensitivity of the resonant frequency to changes in the bias field and the total amount of resonant frequency shift as the temperature of the second anneal is varied during the two-step anneal of an amorphous metal alloy;

FIG. 9 illustrates the fluctuation of the resonant frequency and the magnitude of the output signal as the bias field applied on an amorphous metal alloy formed in accordance with another embodiment of the process of the present invention is varied;

fig. 10 shows the M-H loop characteristics of a metal alloy strip formed according to the process of the previous embodiment of the present invention.

FIG. 11 is a schematic block diagram of an electronic article surveillance system employing a magnetomechanical marker incorporating an activation element formed in accordance with the present invention;

FIG. 12 shows the demagnetization curve for the alloy of example 5;

FIG. 13 is a plot of the slope of the resonant frequency and the resonant frequency shift versus second stage annealing temperature for the alloy of example 5;

FIG. 14 is a graph of amplitude at A1 versus second stage annealing temperature for the alloy of example 5;

FIG. 15 is a plot of resonant frequency slope and resonant frequency shift versus longitudinal magnetic field for the alloy of example 5;

FIG. 16 is a graph of the relationship between the amplitude and the longitudinal magnetic field at A1 for the alloy of example 5;

FIG. 17 is a plot of the slope of the resonant frequency and the resonant frequency shift versus second stage annealing temperature for the alloy of example 6;

FIG. 18 is a graph of amplitude at A1 versus second stage annealing temperature for the alloy of example 6;

FIG. 19 is a plot of resonant frequency slope and resonant frequency shift versus longitudinal magnetic field for the alloy of example 6;

FIG. 20 is a graph of amplitude versus field for alloy A1 of example 6;

FIG. 21 is a plot of the slope of the resonant frequency and the resonant frequency shift versus second stage annealing temperature for the alloy of example 7;

FIG. 22 is a graph of amplitude at A1 versus second stage annealing temperature for the alloy of example 7;

FIG. 23 is a plot of the slope of the resonant frequency and the resonant frequency shift versus second stage annealing temperature for the alloy of example 8;

FIG. 24 is a graph of amplitude at A1 versus second stage annealing temperature for the alloy of example 8;

FIG. 25 is a plot of resonant frequency slope and resonant frequency shift versus longitudinal magnetic field for the alloy of example 8;

FIG. 26 is a graph of amplitude versus longitudinal magnetic field at A1 for the alloy of example 8;

FIG. 27 is a plot of the slope of the resonant frequency and the resonant frequency shift versus second stage annealing temperature for the alloy of example 9;

FIG. 28 is a graph of amplitude at A1 versus second stage annealing temperature for the alloy of example 9;

FIG. 29 is a plot of resonant frequency slope and resonant frequency shift versus longitudinal magnetic field for the alloy of example 9;

FIG. 30 is a graph of amplitude versus longitudinal magnetic field at A1 for the alloy of example 9.

Detailed description of the preferred embodiments

Referring now first to fig. 1 and 2, a method and apparatus according to the present invention for forming a magnetomechanical EAS marker is described wherein a two-step annealing process is employed which produces an active element whose resonant frequency is relatively insensitive to fluctuations in the applied bias magnetic field. It should be noted that fig. 1 is a side view of the apparatus and fig. 2 is a top view of the apparatus.

The processing device is always indicated by the reference numeral 20. The processing apparatus includes: a furnace 22, a supply reel 24 and a take-up reel 26 disposed on opposite sides of the furnace 22. A continuous ribbon 28 of amorphous metal is unwound from supply reel 24, transported through furnace 22 along path P, and then wound onto take-up reel 26. The tape 28 is disposed between the reel 30 and a take-up reel 32 located between the oven 22 and the take-up reel 26. The reel 30, in combination with the pinch roller 32, pulls the leader 28 along path P through the furnace 22.

An array of permanent magnets 33 is positioned along the furnace 22 to generate a magnetic field in the furnace 22 that is oriented transverse to the long axis of the belt 28. It can be seen that the permanent magnet array 33 does not extend the full length of the furnace 22. Furthermore, the arrangement of the array 33 is such that substantially the entire first zone A in the furnace 22 has a magnetic field, while the second zone B in the furnace 22 is substantially free of the magnetic field generated by the array of magnets 33. Zone B is located upstream along the path of travel P relative to zone a.

It will be appreciated that the above-described arrangement of the magnetic array 33 relative to the furnace 22 results in the strip 28 being subjected to a two-step annealing process, i.e., the first step is to anneal the strip in the presence of a transverse magnetic field and the second step is to continue annealing the strip 28 in the absence of a transverse magnetic field.

The magnetic field generated by the magnet array 33 should be strong enough so that the magnetic field formed at region a is saturated with the material comprising the strip 28. Depending on the material used, the preferred magnetic field is preferably greater than 800Oe, and greater than 1000Oe is required to achieve a saturation field strength.

Furnace 22 may be of conventional type and preferably has the capability of maintaining zones a and B at different temperatures. The run lengths of the ribbon 28 in the second zone and the first zone are determined based on the time required for the second step and the first step annealing, respectively. The time for each step of annealing is the quotient of two parameters, namely the length of travel at each zone and the speed of conveyance of the ribbon 28 through the furnace 22. According to one preferred arrangement of the apparatus 20, the overall length of the entire stroke through the furnace 22 is about 231.1 cm. Although it is most convenient to provide zone a (transverse magnetic field annealing) and zone B (second stage annealing, no magnetic field applied) in the same furnace, it is also contemplated to provide zone a in a first furnace and zone B in a second furnace separate from and downstream of the first furnace.

To laterally curl the ribbon 28, a winding fixture 34 may be optionally provided in the oven 22. As can be seen clearly in fig. 3, the fixing means 34 have a curved surface 36 which rises from a low level in the transverse direction and lowers again. If present, fixture 34 may be placed in zone A in furnace 22 substantially midway along the length of zone A. Alternatively, the fastening device 34 may be placed in zone B, or between zone A and zone B. The belt 28 is pulled lengthwise through the fixture 34 and heat is applied to the belt 28 as it passes through the fixture 34 to conform the belt 28 to the curved surface 36, thereby imparting a lateral bend to the belt 28. The result of the processing is that the cut strip produced from the strip 28 has a curved surface corresponding to the curved surface 36 intersecting its length direction. The laterally curved strip may reduce or avoid the suction effect that would otherwise occur when the activation element is mounted on the EAS marker adjacent to a magnetic biasing element.

If a curved surface 36 is used, it is preferred that its curved surface be embossed so that it has a height of 0.0127 to 0.0254cm at its most convex point above the lateral edge of the belt 28.

As an alternative to the fixing means 34 shown in fig. 3, it is also possible to provide fixing means 34' (see fig. 3A) whose guide surface 37 is a flat surface instead of a curved surface, so that the activation element obtained has a substantially flat cross-section cut from the strip 28. Annealing the material on a flat surface, as described in the above-mentioned' 651 application, may eliminate bending of the activation element in the length direction, which may reduce the overall height of the EAS marker.

Reel motors (not shown) are provided for the supply reel 24 and the take-up reel 26, respectively. The take-up spool motor is actuated to take-up the tape 28 from the reel 30 and the take-up spool 32 with little or no slack and a certain amount of tension, and the supply spool motor is actuated to reduce the slack and tension in the tape 28 as it passes through the oven 22. The operating speed of the reel motor can be controlled by a manual operator or by providing an automatic control system.

After the two-step annealing process shown in fig. 1 and 2 is completed, the twice-annealed continuous ribbon is cut into strips by a conventional method. However, the magnetic properties obtained by the annealing process according to the present invention are more uniform than those of conventional as-cast amorphous ribbons, so that it is not necessary to measure the magnetic properties of the material and adjust the cut length of the ribbon as is often required when cutting an as-cast amorphous ribbon.

Before describing examples of applications of the novel two-step annealing process of the present invention, it should be noted that the two-step annealing according to the present invention does not necessarily need to be performed in a continuous process. In other words, either the second step anneal or the first and second step anneals together may be applied to a pre-cut separate strip rather than a continuous strip.

Specific examples of the novel process are described below.

Example 1

In a saturated transverse magnetic field, the component is Fe₃₂Co₁₈Ni₃₂Bi₁₃Si₅The continuous amorphous ribbon (atomic ratio) was annealed at 400 ℃ for 22 seconds. The tape is about 12.7mm wide and about 0.025mm thick. After the first step (transverse field) annealing, the strip was cut into 37.75mm long strips and annealed at 340 ℃ for 1 minute in a separate furnace held stationary. No saturation field was applied during the second annealing step, but due to the geomagnetic field, there was an ambient magnetic field of about 0.7Oe along the length of the strip.

Fig. 4 shows the magnetic force characteristics of the cut strip obtained by the first step (transverse magnetic field) annealing as a function of the bias magnetic field, before the second step annealing is applied. Fig. 5 shows a graph of magnetic force characteristics versus bias field for a strip obtained by a two-step process. In fig. 4 and 5:

the solid curve represents the variation of the resonance frequency with the bias magnetic field;

the short dashed curve represents the output signal amplitude as a function of the bias field at the very end of the interrogation signal pulse;

the dotted curve represents the change in amplitude of the output signal with the bias field at 1 millisecond after the end of the interrogation signal pulse;

the dashed curve shows the output signal amplitude as a function of the bias field 2 milliseconds after the end of the interrogation signal pulse.

(the amplitude of the output signal appearing at and after the end of an interrogation signal pulse is sometimes referred to as the "ringing" amplitude)

As shown in FIG. 4, for the cut strip subjected to only transverse magnetic field annealing, the slope of the resonance frequency-bias field curve (solid line) between the points 5Oe and 7Oe was about 700 Hz/Oe. This slope indicates that the resonant frequency is too sensitive to changes caused by the bias field. This degree of sensitivity can lead to unreliable behavior of the marking of the active element using a single step anneal. In particular, because the effects of the geomagnetic field vary with the orientation of the marker, changes in the orientation of the marker may result in changes in the effective applied bias field that may, in some cases, be sufficient to shift the resonant frequency away from the intended operating frequency of the magnetomechanical EAS detection device.

It should be noted that the frequency shift of the once annealed cut strip was about 2.3kHz when the bias field was reduced from 6Oe to 1Oe, while the ringing amplitude was about 310mV 1 millisecond after the interrogation signal pulse when the bias field was applied at 6 Oe. Despite the cobalt-rich material (Fe) described in the above-referenced' 651 application_39.5Co_39.5Si₂B₁₉) In contrast, the shifted output amplitude characteristic of the once annealed cut strip is satisfactory, and the resonant frequency-bias field curve slope is more appropriate, but the resonant frequency is too sensitive to bias field variations for reliable operation. However, as shown in fig. 5, in the characteristics of the cut strip after the two anneals, in the case where the frequency shift and the output amplitude characteristic are slightly decreased (acceptable decrease), the resonance frequency — the magnetic biasThe slope of the field curve is significantly reduced. Specifically, in the two-annealed strips, the slope between points 5Oe and 7Oe decreased to about 420 Hz/Oe. The frequency shift was about 2.0kHz when the bias field was decreased from 6Oe to 1Oe, and the ringing amplitude was 275mV at 1 ms with a bias field of 6 Oe.

It is believed that performing the second step of annealing in a very small ambient magnetic field helps to disperse to some extent the fairly regular magnetic domain boundaries obtained by transverse field annealing, thereby reducing the sensitivity of the material's resonant frequency to changes in the bias field. Thus, when incorporated as an activation element for a pulsed magnetomechanical EAS marker, the secondarily annealed material exhibits an acceptable degree of reliability despite inevitable variations in the effective applied bias field.

Example 2

The same procedure was used for the same material as in example 1, except that the time for the second annealing was 2 minutes instead of 1 minute. Fig. 6 shows the magnetic force characteristics of the cut strip after two anneals, wherein each of the four curves of fig. 6 represents the same characteristics as fig. 5, respectively. It should be noted that the increase in time of the second annealing step in this embodiment causes the slope of the resonant frequency-bias field curve to be more gradual, with the slope between points 5Oe and 7Oe being approximately 350 Hz/Oe. The frequency shift was reduced to 1.7kHz when the bias field was reduced from 6Oe to 1Oe, and the 1 millisecond ringing amplitude was substantially constant at 280mV for a bias field of 6 Oe.

Example 3

A continuous strip having the same composition and dimensions as described in example 1 was annealed in two steps using the continuous process equipment described above in connection with figures 1 and 2. The run of the continuous ribbon 28 was 152.4cm in zone a (transverse field annealing zone) and 78.7cm in zone B (second step annealing, no magnetic field applied). The continuous belt 28 was conveyed at a speed of 7cm per second so that the time for annealing in the first step (transverse magnetic field) was about 21 seconds. The time for the second (field-free) anneal step was approximately 11 seconds. The path of travel P is substantially parallel to the east-west direction so that there is really no longitudinal ambient magnetic field in zone B. The temperature of zone A was fixed at 380 ℃ but the temperature of zone B was varied within 320 ℃ and 400 ℃ to obtain multiple samples under different conditions. After two successive anneals the continuous strip was cut into separate strips (37.75 mm in length).

In FIG. 7, the open circles represent the values of the resonant frequencies obtained at each of the second-step annealing temperatures (bias field of 5.5Oe), and the filled squares represent the 1-millisecond ringing amplitudes obtained at the first second-step annealing temperature (bias field of 5.5 Oe). In fig. 8, the open circles indicate the resonance frequency versus bias field characteristic, while the filled squares indicate the shift in resonance frequency at different second step annealing temperatures (when the bias field is decreased from 6Oe to 1 Oe).

As shown in fig. 7, when the temperature of the second-step annealing is more than 340 deg.c, the resonant frequency of 5.5Oe decreases. When the temperature is greater than 360 ℃, the 1 millisecond ringing amplitude (also at 5.5Oe) decreases. FIG. 8 shows how the slope of the bias field at resonant frequency 1 (between points 5-7 Oe) and the total offset in frequency (from 6 to 1 Oe) vary with the second step annealing temperature. Specifically, when the second-step annealing temperature is raised from 320 ℃ to 400 ℃, the slope is reduced from about 610-650Hz/Oe to about 230 Hz/Oe. The frequency offset is first increased and then decreased when the second annealing temperature is greater than 360 ℃. The most satisfactory compromise of the slope of the bias field at the resonance frequency and the total offset of the frequency is obtained at a temperature of 380 c in the second annealing step. The performance values at this time are: 1 millisecond ringing amplitude-263 mV, resonant frequency/bias field slope-488 Hz/Oe, frequency offset-1.970 kHz.

Example 4

The same material and the same two-step annealing apparatus as in example 3 were used. The strip transport speed was reduced by about 1/2 and the following annealing parameter values were used: first (transverse magnetic field) step-43 seconds, 380 ℃; second (no magnetic field) step-22 seconds, 360 ℃. After the two-step annealed continuous ribbon was cut into separate ribbons as in the above examples, characteristic values as shown in fig. 9 were obtained. The four curves shown in fig. 9 represent the same characteristics as fig. 5 and 6, respectively, discussed above. It should be noted that the slope of the resonant frequency/bias field curve between points 5Oe and 7Oe is about 430Hz/Oe, the 1 millisecond ringing amplitude is 290mV for a bias field of 6Oe, and the frequency offset is 1.830kHz for a bias field decreasing from 6Oe to 1 Oe. The M-H hysteresis loop characteristics of the resulting two-step annealed cut strip material are shown in fig. 10.

It can be seen that the M-H loop is somewhat open near the origin, indicating that the material being processed is somewhat prone to false alarms in harmonic EAS systems, although to a lesser extent than conventional magnetomechanical markers employing as-cast (i.e., unannealed) activation elements.

In the above-described embodiments, materials of the same composition were used, but if materials of other compositions having 5 to 45 atomic% of cobalt were used, it is believed that satisfactory results could be obtained as long as the materials also contained a large proportion of nickel.

Moreover, it is preferable to try to supply no magnetic field other than the ambient magnetic field caused by the earth magnetic field in the second annealing step, but satisfactory results can be obtained if the magnetic field supplied in the length direction of the continuous ribbon or the divided ribbon in the second annealing step is less than 5 Oe. It is also believed that if the temperature of the second step anneal is greater than 450 c or the time is less than 5 minutes, satisfactory results are not obtained.

As described above, the two-step annealing process disclosed herein, and particularly the second annealing step provided in the substantial absence of an applied magnetic field after the first step of saturated transverse field annealing, allows the creation of an active element for a magnetomechanical EAS marker having a resonant frequency that is not overly sensitive to small changes in bias field. At the same time, the active element manufactured in this way has satisfactory characteristics also with respect to the total frequency shift and the ringing signal amplitude. And the activation element can be made with a flat reel profile and less prone to false alarms in harmonic EAS systems.

Fig. 11 illustrates a pulse interrogation EAS system employing a magnetic marker 100, the marker 100 incorporating an activation element made in accordance with the present invention. The system shown in fig. 11 includes a synchronization circuit 200 that controls the operation of an excitation circuit 201 and a reception circuit 202. The synchronization circuit 200 sends a synchronization gate pulse to the pump circuit 201, which activates the pump circuit 201. The power supply circuitry 201 is energized to generate and send an interrogation signal to the interrogation coil 206 for the duration of the synchronization pulse. In response to the interrogation signal, interrogation coil 206 generates an interrogation magnetic field that in turn excites marker 100 into mechanical resonance.

After the interrogation signal pulse is completed, the synchronization circuit 200 sends a gate pulse to the receiving circuit 202, which gate pulse activates the circuit 202. If a marker is present in the interrogating magnetic field during the time that the circuit 202 is excited, the marker will generate a signal in the receiving circuit 202 at the mechanical resonant frequency of the marker. This signal is detected by the receiver 202, and the receiver 202 generates a signal to the display 203 in response to the detected signal to generate an alarm or the like. In short, the receiver circuit 202 is synchronized with the excitation circuit 201 such that the receiver circuit 202 is only activated during the quiet period between pulses of the pulsed interrogation field.

Example 5Fe_32.91Ni_31.46Co_17.98B_12.67Si_4.98(wherein the subscripts are atomic percentages)

Using the above reel transfer method to convert the component into Fe_32.91Ni_31.46Co_17.98B_12.67Si_4.98The amorphous continuous ribbon (subscript atomic percent) was annealed with dimensions of about 12.7mm wide and about 25 μm thick (first stage annealing). The annealing conditions are as follows: 390℃ × 7.5 seconds, then 200℃ × 5 seconds with a magnetic field of 1200Oe applied along the width of the belt. The strip was cut into sample strips of approximately 37.75mm in length. The magnetic reaction of these samples was measured with a device equipped with transmitting and receiving coils. Fig. 12 shows the resonance frequency (Fr in kilohertz) and signal amplitude (in microvolts) versus the bias field applied along the length of the sample. The signal amplitude was measured after 0 milliseconds (a0), 1 millisecond (a1), 2 milliseconds (a2) of signal emission from the transmit coil. The following results were obtained: amplitude A1 was 403mV for a bias field of 6.5Oe, resonant frequency-biasThe magnetic field slope is 750 Hz/Oe; the bias field was shifted from 6.5Oe to 2Oe by a resonant frequency of 2.409 kHz.

Another batch of sample strips about 37.75mm long was cut from the first stage annealed ribbon of example 5 and annealed in a batch furnace, i.e., a second stage anneal was performed. However, it should be understood that the second stage anneal may also be performed using a reel transfer process, with a batch furnace being used only to facilitate testing. FIG. 13 is a graph showing the slope of the resonance frequency at 6.5Oe (in Hz/Oe) and the amount of shift of the resonance frequency from 6.5Oe to 2Oe as a function of the second-stage annealing temperature (in. degree. C.). The magnetic field was 0Oe and the annealing time was 1 minute. As can be seen in FIG. 13, as the annealing temperature increases, the slope of the wiping frequency decreases and has a minimum at 320 ℃. The resonant frequency shift also has the same tendency to decrease with increasing annealing temperature of the second stage and has a minimum at 320 c. FIG. 14 shows the signal amplitude A1 (in mV) of the sample at 6.5Oe with the transmitter coil turned off for 1 ms as a function of the second stage annealing temperature (in degrees Celsius).

The same first stage annealed material of example 5 was cut into sample strips of about 37.75mm in length. The strips were annealed at 360 ℃ for 1 minute by applying a longitudinal magnetic field of varying strength along the length of the specimen. FIG. 15 shows the slope (in Hz/Oe) of the resonance frequency (Fr) and the shift (in kHz) of the resonance frequency as a function of the applied magnetic field (in Oe). As can be seen from FIG. 15, both the resonant frequency slope and the resonant frequency shift decrease as the longitudinal magnetic field increases in the range of 0-1.2 Oe. The experimental results shown in fig. 16 show a significant change in amplitude a1 in the magnetic field range of 0-1.2Oe, i.e., the signal amplitude after 1 millisecond after the transmit coil was turned off.

Example 6Fe_40.87Co_40.61B_13.40Si_5.12(wherein the subscripts are atomic percentages)

The composition was Fe using the method described in example 5_40.87Co_40.61B_13.40Si_5.12(subscript as atomic percent), a dimension of about 10mm wide and about 25mm wideThe amorphous continuous ribbon with a thickness of μm is annealed (first stage annealing). The annealing conditions are as follows: 380 ℃ X7.5 seconds, then 200 ℃ X5 seconds. The strip was cut into sample strips of approximately 37.75mm in length. These strips were then annealed at 300-400 ℃ for 1 minute, applying a longitudinal magnetic field of 0.8Oe along the length of the sample. The magnetic reaction of these samples was measured using the apparatus as described in example 5. FIG. 17 is a graph of the slope of the resonant frequency at 6.5Oe (in Hz/Oe) and the amount of shift in the resonant cheek ratio from 6.5Oe to 2Oe as a function of the second stage annealing temperature (in degrees Celsius). As can be seen from fig. 17, as the annealing temperature increases, the resonant frequency slope and resonant frequency offset decrease and have a minimum at 380 ℃. FIG. 18 shows the signal amplitude A1 (in mV) of the sample at 6.5Oe with the transmitter coil turned off for 1 ms as a function of the second stage annealing temperature (in degrees Celsius).

The same first stage annealed material of example 6 was cut into sample strips of about 37.75mm in length. The strips were annealed at 360 ℃ for 1 minute by applying a longitudinal magnetic field of varying strength along the length of the specimen. FIG. 19 shows the variation of the resonant frequency slope (in Hz/Oe) and resonant frequency shift (in kHz) with applied magnetic field (in Oe). FIG. 20 shows the results of a test in which the signal amplitude A1 (in mV) of the sample at 6.5Oe with the transmitter coil open for 1 millisecond varied with the applied magnetic field (in Oe).

Example 7Fe_37.85Ni_30.29Co_15.16B_15.31Si_1.39(subscripts are atomic percentages)

The composition was Fe in a similar manner to that in example 5_37.85Ni_30.29Co_15.16B_15.31Si_1.39An amorphous continuous ribbon having a size of about 6mm wide and a thickness of about 25 μm was annealed. The annealing conditions are as follows: 405 ℃ X7.5 seconds, then 200 ℃ X5 seconds. The strip was cut into sample strips of approximately 37.75mm in length. These strips were then annealed at 300-400 ℃ for 1 minute, applying a longitudinal magnetic field of 0.8Oe along the length of the sample. The magnetic reaction of these samples was measured using the apparatus as described in example 5. FIG. 21 is the harmonic at 6.5OeThe slope of the oscillation frequency (in Hz/Oe) and the shift of the resonance frequency from 6.5Oe to 2Oe are related to the variation of the second stage annealing temperature (in ℃ C.). FIG. 22 shows the signal amplitude A1 (in mV) of the sample at 6.5Oe with the transmitter coil turned off for 1 ms as a function of the second stage annealing temperature (in degrees Celsius).

Example 8Fe_38.38Ni_29.06Co_16.10B_14.89Si_1.57(wherein the subscripts are atomic percentages)

The composition was Fe in a similar manner to that in example 5_38.38Ni_29.06Co_16.10B_14.89Si_1.57The amorphous continuous ribbon (subscript atomic percent) was annealed with dimensions of about 6mm wide and about 25 μm thick. The annealing conditions are as follows: 400 ℃ X7.5 seconds, then 200 ℃ X5 seconds. The strip was cut into sample strips of approximately 37.75mm in length. These strips were then annealed at 300-400 ℃ for 1 minute, applying a longitudinal magnetic field of 0.8Oe along the length of the sample. The magnetic reaction of these samples was measured using the apparatus as described in example 5. FIG. 23 is a graph showing the slope of the resonance frequency at 6.5Oe (in Hz/Oe) and the amount of shift of the resonance frequency from 6.5Oe to 2Oe as a function of the second-stage annealing temperature (in. degree. C.). FIG. 24 shows the signal amplitude A1 (in mV) of the sample at 6.5Oe with the transmitter coil turned off for 1 ms as a function of the second stage annealing temperature (in degrees Celsius).

The same first stage annealed material of example 8 was cut into sample strips of about 37.75mm in length. The strips were annealed at 360 ℃ for 1 minute by applying a longitudinal magnetic field of varying strength along the length of the specimen. FIG. 25 shows the slope (in Hz/Oe) of the resonance frequency (Fr) and the shift (in kHz) of the resonance frequency as a function of the applied magnetic field (in Oe). FIG. 26 shows the signal amplitude A1 (in mV) of the sample as a function of applied magnetic field (in Oe) at 6.5Oe with the transmit coil turned off for 1 millisecond.

Example 9Fe_42.62Ni_30.20Co_11.87B_14.14Si_1.17(wherein the followingThe notation is atomic percent)

The composition was Fe using the method described in example 5_38.38Ni_29.06Co_16.10B_14.89Si_1.57(subscript atomic percent) amorphous continuous ribbon having dimensions of about 6mm wide and about 25mm thick was annealed. The annealing conditions are as follows: 360 ℃ X7.5 seconds, then 200 ℃ X5 seconds. The strip was cut into sample strips of approximately 37.75mm in length. These strips were then annealed at 300-400 ℃ for 1 minute, applying a longitudinal magnetic field of 0.8Oe along the length of the sample. The magnetic reaction of these samples was measured using the apparatus as described in example 5. FIG. 27 is a graph showing the slope of the resonance frequency at 6.5Oe (in Hz/Oe) and the amount of shift of the resonance frequency from 6.5Oe to 2Oe as a function of the second-stage annealing temperature (in. degree. C.). FIG. 28 shows the signal amplitude A1 (in mV) of the sample at 6.5Oe with the transmitter coil turned off for 1 ms as a function of the second stage annealing temperature (in degrees Celsius).

The same first stage annealed material of example 9 was cut into sample strips of about 37.75mm in length. The strips were annealed at 360 ℃ for 1 minute by applying a longitudinal magnetic field of varying strength along the length of the specimen. FIG. 29 shows the variation of the resonant frequency slope (in Hz/Oe) and resonant frequency shift (in kHz) with applied magnetic field (in Oe). Fig. 30 shows the signal amplitude a1 (in mV) of the sample as a function of the applied magnetic field (in Oe) at 6.5Oe with the transmit coil turned off for 1 millisecond.

The magnetostrictive element of the present invention is Fe as compared with a conventional material such as Fe as a component used in the prior art₄₀Ni₃₈Mo₄B₁₈Metglas of^®MB represents a significant advance in that the magnetostrictive element of the present invention not only has a low slope of the resonant frequency, but can be made narrower, i.e., 6mm wide, whereas the prior art material is 12.7mm wide and can be made flat, thereby enhancing the commercial utility of thin line marking.

Referring to the examples, it can be seen that a magnetostrictive element made of an alloy containing iron and cobalt in an atomic percentage of about 12-41% and annealed according to a two-stage annealing process has a low slope of a resonant frequency, thereby providing a magnetostrictive element having improved stability of the resonant frequency with a change in a bias magnetic field. Based on these results, it is believed that the resonance frequency of an alloy containing iron and about 5-45 atomic% cobalt and annealed according to the two-stage annealing process has a high stability with respect to the change in bias field. It can also be seen from the examples that the performance of the magnetostrictive element made of an alloy containing iron and cobalt in the atomic percentage of about 12-18 annealed according to a two-stage annealing process is improved, i.e., the slope of the resonance frequency can be controlled below 550 Hz/Oe. Based on the above results, it is believed that the performance of an alloy containing iron and about 10-25 atomic percent cobalt annealed according to a two-stage annealing process is improved.

In the second stage annealing of the magnetostrictive element of the present invention, the temperature is preferably about 250 ℃ to 450 ℃ and the annealing time is about 0.05 to 5 minutes. The second stage anneal is conducted in the presence of a longitudinal magnetic field of 0 to about 5 Oe.

Variations and modifications may be made to the annealing apparatus and the above-described implementation without departing from the invention. The present invention is, therefore, to be considered in all respects as illustrative and not restrictive. The true spirit and scope of the present invention is set forth in the following claims.

Claims (34)

1. A magnetostrictive element for use in a magnetomechanical electronic article surveillance marker, wherein: the element is obtained by first annealing a strip of amorphous metal alloy containing iron and from about 5 to about 45 atomic percent cobalt in a saturation magnetic field such that, after the first annealing is complete, the strip has the property that, when a bias magnetic field is applied to the strip, the strip mechanically resonates at a resonant frequency in response to exposure to an alternating magnetic field at said resonant frequency, and the resonant frequency varies with variation of the bias magnetic field; after the first annealing, the strip is annealed a second time in the absence of a saturating magnetic field to reduce the rate of change of the resonant frequency with changes in the bias magnetic field.

2. A magnetostrictive element as claimed in claim 1, wherein the strip has a longitudinal axis and the saturation magnetic field is transverse to the longitudinal axis.

3. A magnetostrictive element as claimed in claim 2, wherein: the second annealing is performed in the presence of a longitudinal magnetic field.

4. A magnetostrictive element as claimed in claim 3, wherein: the longitudinal magnetic field is in the range of 0 to about 5 oersted.

5. The magnetostrictive element as claimed in claim 4, wherein: the second anneal is performed at a temperature in the range of about 250-450 c for about 0.05-5 minutes.

6. A magnetostrictive element as claimed in claim 1, wherein: the amount of cobalt is about 12-41 atomic percent.

7. A magnetostrictive element as claimed in claim 1, wherein: the cobalt content is about 10-25 atomic percent.

8. A magnetostrictive element as claimed in claim 1, wherein: the cobalt content is about 12-18 atomic percent.

9. The magnetostrictive element as claimed in claim 5, wherein: the cobalt content is about 12-41 atomic percent.

10. The magnetostrictive element as claimed in claim 5, wherein: the cobalt content is about 10-25 atomic percent.

11. The magnetostrictive element as claimed in claim 4, wherein: the cobalt content is about 12-41 atomic percent.

12. The magnetostrictive element as claimed in claim 5, wherein: the cobalt content is about 12-18 atomic percent.

13. A tag for use in a magnetomechanical electronic article surveillance system, comprising an amorphous magnetostrictive strip formed by first annealing an amorphous metal alloy strip comprising iron and about 5-45 atomic percent cobalt in a saturation magnetic field such that, after the first annealing is complete, the alloy strip has the property that, when a bias magnetic field is applied to the alloy strip, the strip mechanically resonates at a resonant frequency in response to exposure to an alternating magnetic field at said resonant frequency, and the resonant frequency changes in response to changes in the bias magnetic field; after the first annealing, the strip is annealed a second time in the absence of a saturating magnetic field to reduce the rate of change of the resonant frequency with changes in the bias magnetic field.

14. The tag of claim 13, wherein the strip has a longitudinal axis and the saturation magnetic field is transverse to the longitudinal axis.

15. The tag of claim 14, wherein: the second annealing is carried out in the presence of a longitudinal magnetic field.

16. The tag of claim 15, wherein: the longitudinal magnetic field is in the range of 0 to about 5 oersted.

17. The tag of claim 16, wherein: the second anneal is performed at a temperature in the range of about 250-450 c for about 0.05-5 minutes.

18. The tag of claim 13, wherein: the amount of cobalt is about 12-41 atomic percent.

19. The tag of claim 13, wherein: the cobalt content is about 10-25 atomic percent.

20. The tag of claim 13, wherein: the cobalt content is about 12-18 atomic percent.

21. The tag of claim 17, wherein: the cobalt content is about 12-41 atomic percent.

22. The tag of claim 17, wherein: the cobalt content is about 10-25 atomic percent.

23. The tag of claim 17, wherein: the cobalt content is about 12-18 atomic percent.

24. A magnetomechanical electronic 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 amorphous magnetostrictive strip and a biasing element that mechanically resonates the magnetostrictive strip when exposed to an alternating field, said amorphous magnetostrictive strip being formed by first annealing a strip of amorphous metal alloy comprising iron and about 5-45 atomic percent cobalt in a saturation magnetic field such that, after the first annealing is complete, the strip mechanically resonates at a resonant frequency in response to exposure to an alternating magnetic field at the resonant frequency, and the resonant frequency changes in response to changes in the biasing magnetic field; after the first annealing, annealing the strip for a second time without a saturation magnetic field to reduce a rate of change of the resonant frequency with a change of the bias magnetic field;

(c) a detection device for detecting said mechanical resonance of said magnetostrictive strip.

25. The magnetic article surveillance system of claim 24 wherein said bar has a longitudinal axis and said saturation magnetic field is transverse to said longitudinal axis.

26. The magnetic article surveillance system of claim 25, wherein: the second annealing is performed in the presence of a longitudinal magnetic field.

27. The magnetic article surveillance system of claim 26, wherein: the longitudinal magnetic field is in the range of 0 to about 5 oersted.

28. The magnetic article surveillance system of claim 27, wherein: the second anneal is performed at a temperature in the range of about 250-450 c for about 0.05-5 minutes.

29. The magnetic article surveillance system of claim 24, wherein: the amount of cobalt is about 12-41 atomic percent.

30. The magnetic article surveillance system of claim 24, wherein: the cobalt content is about 10-25 atomic percent.

31. The magnetic article surveillance system of claim 24, wherein: the cobalt content is about 12-18 atomic percent.

32. The magnetic article surveillance system of claim 28, wherein: the cobalt content is about 12-41 atomic percent.

33. The magnetic article surveillance system of claim 28, wherein: the cobalt content is about 10-25 atomic percent.

34. The magnetic article surveillance system of claim 28, wherein: the cobalt content is about 12-18 atomic percent.