Abstract:
A coded marker in a magnetomechanical resonant electronic article identification system, includes a plurality of ductile magnetostrictive elements or strips based on an amorphous magnetic alloy ribbon with improved magnetomechanical resonance performance. The coded marker takes full advantage of the improved magnetomechanical properties, and an electronic article identification system utilizes the coded marker. The improved encodable and decodable marker/identification system is capable of identifying considerably larger number of articles than conventional systems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to ferromagnetic amorphous alloy ribbon and to a marker for use in an electronic article identification system, the marker including a plurality of rectangular strips based on an amorphous magnetostrictive material that vibrates in an alternating magnetic field mechanically at multiple resonant frequencies, whereby the magnetomechanical effect of the marker is effectively utilized for encoding and decoding purposes. The present invention is also directed to an electronic identification system utilizing such a marker.  
         [0003]     2. Background of the Invention  
         [0004]     Magnetostriction of a magnetic material is a phenomenon in which a dimensional change takes place upon application of an external magnetic field on the magnetic material. When the dimensional change is such that the material elongates upon its being magnetized, the material is termed “positive-magnetostrictive”. When a material is “negative-magnetostrictive”, the material shrinks upon its magnetization. Thus in either case, a magnetic material vibrates when it is in an alternating magnetic field. When a static magnetic field is applied along with the alternating field, the frequency of the mechanical vibration of the magnetic material varies with the applied static field through magneto-elastic coupling. This is commonly known as ΔE effect, which is described, for example, in “Physics of Magnetism” by S. Chikazumi (John Wiley &amp; Sons, New York, 1964, page 435). Here E(H) stands for Young&#39;s modulus which is a function of an applied field H, and the material&#39;s vibrational or resonance frequency f r  is related to E(H) through 
 
 f   r =(1/2/)[ E ( H )/ρ] 1/2 ,   (1) 
 
 where l is the length of the material and ρ is the mass density of the material. The magneto-elastic or magneto-mechanical effect described above is utilized in electronic article surveillance systems which were first taught in the U.S. Pat. Nos. 4,510,489 and 4,510,490 (hereinafter the &#39;489 and &#39;490 patents). Such surveillance systems are advantageous systems, in that they offer a combination of high detection sensitivity, high operating reliability and low operating costs. 
 
         [0005]     The marker in such systems is a strip, or a plurality of strips, of known length of a ferromagnetic material, packaged with a magnetically harder ferromagnet (material with a higher coercivity) that provides a static field termed as biasing field to establish peak magneto-mechanical coupling. The ferromagnetic marker material is preferably an amorphous alloy ribbon, since the efficiency of magneto-mechanical coupling in the alloys is very high. The mechanical resonance frequency, f r  is determined essentially by the length of the alloy ribbon and the biasing field strength, as the above Equation (1) indicates. When an interrogating signal tuned to the resonance frequency is encountered in an electronic identification system, the marker material responds with a large signal field which is detected by a receiver in the system.  
         [0006]     Several amorphous ferromagnetic materials were considered in the U.S. Pat. No. 4,510,490 for coded identification systems based on magnetomechanical resonance described above and included amorphous Fe—Ni—Mo—B, Fe—Co—B—Si, Fe—B—Si—C and Fe—B—Si alloys. Of the alloys, a commercially available amorphous Fe—Ni—Mo—B based METGLAS®2826MB alloy was used extensively until accidental triggering, by a magnetomechanical resonance marker, of other systems based on magnetic harmonic generation/detection. This occurs because a magnetomechanical resonance marker used at that time sometimes exhibited non-linear BH characteristics, resulting in generation of higher harmonics of the exciting field frequency. To avoid this problem, sometimes called a system “pollution problem,” a series of new marker materials have been invented, examples of which are disclosed in U.S. Pat. Nos. 5,495,231, 5,539,380, 5,628,840, 5,650,023, 6,093,261 and 6,187,112. Although the new marker materials perform, on average, better than the materials utilized in the surveillance systems of the original &#39;489 and &#39;490 patents, somewhat better magnetomechanical performance has been found in the marker materials disclosed, for example, in U.S. Pat. No. 6,299,702 (hereinafter &#39;702 patent). These new marker materials require complex heat-treatment processes to achieve desired magnetomechanical properties as disclosed, for example, in the &#39;702 patent. Clearly, a new magnetomechanical marker material is needed which does not require such complicated post-ribbon fabrication processes and it is one aim of the present invention to provide such a marker material with high magnetomechanical performance without causing “pollution problem” mentioned above. Fully utilizing the new magnetomechanical marker material of the present invention, the present invention includes a marker with encoding and decoding capability and an electronic identification system utilizing the marker. A coded surveillance system having a magnetomechanical marker was taught in U.S. Pat. No. 4,510,490, but the number of constituent marker strips was limited due to a limited space available in a marker, thus limiting the universe of encoding and decoding capability using such a marker.  
         [0007]     Clearly, a marker is needed in which the number of marker strips is increased considerably without sacrificing the performance as a coded marker in an electronic article identification system having encoding and decoding capability, hereinafter termed “coded electronic article identification system.” 
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the invention, a soft magnetic material is included in a marker of an electronic identification system based on magnetomechanical resonance.  
         [0009]     A marker material with enhanced overall magnetomechanical resonance properties is fabricated from an amorphous alloy ribbon so that a multiple of marker strips are housed in a coded marker. A soft magnetic material in a ribbon form having magnetomechanical resonance capability is cast on a rotating substrate, as taught in the U.S. Pat. No.4,142,571. When the as-cast ribbon width is wider than the predetermined width for a marker material, the said ribbon is slit to said predetermined width. The ribbon thus processed is cut into ductile, rectangular amorphous metal strips having different lengths to fabricate a magnetomechanical resonance marker using a plurality of said strips with at least one semi-hard magnet strip which provides a bias static magnetic field.  
         [0010]     A coded electronic article identification system utilizes a coded marker of the present invention. The system has an article interrogation zone in which a magnetomechanical marker of the present invention is subject to an interrogating magnetic field with varying frequencies, the signal response to the interrogating magnetic field excitation being detected by a receiver having a pair of antenna coils situated in the article interrogation zone.  
         [0011]     In accordance with an embodiment of the invention, there is provided a coded marker of a magnetomechanical resonant electronic article identification system, adapted to resonate mechanically at preselected frequencies, comprising: a plurality of ductile magnetostrictive strips cut to predetermined lengths from amorphous ferromagnetic alloy ribbons that have curvatures along a ribbon length direction and exhibit magnetomechanical resonance under alternating magnetic field excitations with a static bias field, the strips having a magnetic anisotropy direction perpendicular to a ribbon axis, wherein at least two of the strips are adapted to be magnetically biased to resonate at a single, different one of the preselected frequencies.  
         [0012]     Where selected, a radius of curvature of the marker strip curvatures is less than 100 cm.  
         [0013]     In accordance with an embodiment of the invention, encoding is carried out by cutting an amorphous magnetostrictive alloy ribbon having its magnetic anisotropy direction perpendicular to ribbon axis to a rectangular strip with a predetermined length having a length-to-width aspect ratio greater than 3.  
         [0014]     Where selected, the strips have a strip width ranging from about 3 mm to about 15 mm.  
         [0015]     In accordance with an embodiment of the invention, the strips have a slope of resonance frequency versus bias field ranging from about 4 Hz/(A/m) to about 14 Hz/(A/m).  
         [0016]     Where selected, the strips have a length greater than about 18 mm when a strip width is 6 mm.  
         [0017]     In accordance with an embodiment of the invention, the strips have a magnetomechanical resonance frequency less than about 120,000 Hz.  
         [0018]     In accordance with an embodiment of the invention, the amorphous ferromagnetic alloy ribbons have a saturation magnetostriction between about 8 ppm and about 18 ppm and a saturation induction between about 0.7 tesla and about 1.1 tesla.  
         [0019]     In accordance with an embodiment of the invention, an amorphous ferromagnetic alloy of the amorphous ferromagnetic alloy ribbons has a composition based on Fe a —Ni b —Mo c —B d  with 30≦a≦43, 35≦b≦48, 0≦c≦5, 14≦d≦20 and a+b+c+d=100, up to 3atom % of Mo being optionally replaced by Co, Cr, Mn and/or Nb and up to 1 atom % of B being optionally replaced by Si and/or C.  
         [0020]     In accordance with an embodiment of the invention, an amorphous ferromagnetic alloy of the amorphous ferromagnetic alloy ribbons has a composition of one of: Fe 40.6  Ni 40.1  Mo 3.7  B 15.1  Si 0.5 , Fe 41.5  Ni 38.9  Mo 4.1  B 15.5 , Fe 41.7  Ni 39.4  Mo 3.1  B 15.8 , Fe 40.2  Ni 39.0 Mo 3.6  B 16.6  Si 0.6,  Fe 39.8  Ni 39.2  Mo 3.1  B 17.6  C 0.3 , Fe 36.9  Ni 41.3  Mo 4.1  B 17.8 , Fe 35.6  Ni 42.6  Mo 4.0  B 17.9,  Fe 40  Ni 38  Mo 4  B 18 , or Fe 38.0  Ni 38.8  Mo 3.9  B 19.3 .  
         [0021]     Where selected, the coded marker comprises at least two marker-strips with different lengths.  
         [0022]     Where selected, coded marker comprises five marker-strips with different lengths.  
         [0023]     Where selected, the coded marker has a magnetomechanical resonance frequency between about 30,000 and about 130,000 Hz.  
         [0024]     Where selected, the coded marker has an electronic identification universe containing up to about 1800 and about 115 million separately identifiable articles for a coded marker with two and five marker strips, respectively.  
         [0025]     Where selected, the coded marker has an electronic identification universe containing more than 115 million separately identifiable articles.  
         [0026]     In accordance with an embodiment of the invention, the strips have a magnetomechanical resonance frequency less than about 120,000 Hz.  
         [0027]     In accordance with an embodiment of the invention, an electronic article identification system has a capability of decoding coded information of a coded marker. The system comprises one of: a pair of coils emitting an AC excitation field aimed at the coded marker to form an interrogation zone; a pair of signal detection coils receiving coded information from the coded marker; an electronic signal processing device with an electronic computer with a software to decode information coded on the coded marker; or an electronic device identifying the coded marker, wherein the coded marker is adapted to resonate mechanically at preselected frequencies, wherein the coded marker comprises a plurality of ductile magnetostrictive strips cut to predetermined lengths from amorphous ferromagnetic alloy ribbons that have curvatures along a ribbon length direction and exhibit magnetomechanical resonance under alternating magnetic field excitations with a static bias field, the strips having a magnetic anisotropy direction perpendicular to a ribbon axis, wherein at least two of the strips are adapted to be magnetically biased to resonate at a single, different one of the preselected frequencies.  
         [0028]     Where selected, a radius of curvature of the marker strip curvatures is between about 20 cm and about 100 cm. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiments and the accompanying drawings in which:  
         [0030]      FIG. 1A  illustrates a side view of a strip cut from an amorphous alloy ribbon in accordance with an embodiment of the present invention and having a bias magnet, and  FIG. 1B  illustrates a view of a conventional strip with a bias magnet;  
         [0031]      FIG. 2  illustrates magnetomechanical resonance characteristics of a single strip marker in accordance with an embodiment of the present invention and magnetomechanical resonance characteristics of a conventional single strip marker, showing resonance frequency as a function of bias field;  
         [0032]      FIG. 3  illustrates resonance signals of a single strip marker in accordance with an embodiment of the present invention and resonance signals of a conventional strip marker, showing resonance signal amplitudes as a function of a bias field;  
         [0033]      FIG. 4  illustrates a BH loop taken at 60 Hz on a marker strip of an embodiment of the present invention having a length of approximately 38 mm, a width of approximately 6 mm and a thickness of about 28 μm;  
         [0034]      FIG. 5A  illustrates a comparison of a physical profile of an embodiment of a magnetomechanical resonant marker in accordance with embodiments of the present invention, and  FIG. 5B  illustrates a comparison of a conventional marker, utilizing two marker-strips with different lengths in both cases;  
         [0035]      FIG. 6A  illustrates magnetomechanical resonance characteristics of a marker having two strips with different lengths of an embodiment of the present invention, and  FIG. 6B  illustrates magnetomechanical resonance characteristics of a conventional marker having two strips with different lengths;  
         [0036]      FIG. 7  illustrates a resonance signal profile near the lower resonance frequency region of  FIG. 6A ;  
         [0037]      FIG. 8  illustrates a resonance signal profile near the upper resonance frequency region of  FIG. 6A ;  
         [0038]      FIGS. 9-1  and  9 - 2  illustrate a marker of an embodiment of the present invention, in which three strips with different lengths are housed;  
         [0039]      FIG. 10  illustrates magnetomechanical resonance characteristics of a marker having three strips with different lengths of an embodiment of the present invention;  
         [0040]      FIG. 11  illustrates magnetomechanical resonance characteristics of a marker having five strips with different lengths of an embodiment of the present invention; and  
         [0041]      FIG. 12  illustrates a coded electronic article identification system in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     A marker material with enhanced overall magnetomechanical resonance properties is fabricated from an amorphous ferromagnetic alloy ribbon so that a multiple of marker strips are housed in a coded marker, wherein at least two of the strips are adapted to be magnetically biased to resonate mechanically at a single, different one of a plurality of preselected frequencies. A magnetic material in a ribbon form having magnetomechanical resonance capability is cast on a rotating substrate, as taught in the U.S. Pat. No. 4,142,571. When the as-cast ribbon width is wider than the predetermined width for a marker material, the ribbon is slit to the predetermined width. The ribbon thus processed is cut into ductile, rectangular amorphous metal strips having different lengths to fabricate a magnetomechanical resonance marker using a plurality of the strips with at least one semi-hard magnet strip which provides a bias static magnetic field.  
         [0043]     In one embodiment of the present invention, the amorphous ferromagnetic alloy utilized to form a ribbon for the marker strip has a composition based on Fe a —Ni b —Mo c —B d  with 30 ≦a≦43, 35≦b≦48, 0≦c≦5, 14≦d≦20 and a+b+c+d=100, up to 3 atom % of Mo being optionally replaced by Co, Cr, Mn and/or Nb and up to 1 atom % of B being optionally replaced by Si and/or C.  
         [0044]     In one embodiment of the present invention, the amorphous ferromagnetic alloy utilized to form a ribbon for the marker strip has a composition of one of: Fe 40.6  Ni 40.1  Mo 3.7  B 15.1  Si 0.5 , Fe 41.5  Ni 38.9  Mo 4.1  B 15.5 , Fe 41.7  Ni 39.4  Mo 3.1  B 15.8 , Fe 40.2  Ni 39.0  Mo 3.6  B 16.6  Si 0.6 , Fe 39.8  Ni 39.2  Mo 3.1  B 17.6  C 0.3 , Fe 36.9  Ni 41.3  Mo 4.1  B 17.8 , Fe 35.6  Ni 42.6  Mo 4.0  B 17.9 , Fe 40  Ni 38  Mo 4  B 18 , or Fe 38.0  Ni 38.8  Mo 3.9  B 19.3 .  
         [0045]     Thus, an amorphous alloy ribbon with a chemical composition similar to a chemical composition of a commercially available amorphous magnetostrictive METGLAS®2826MB ribbon was cast in accordance with the invention described in the U.S. Pat. No. 4,142,571. The cast amorphous alloy had a saturation induction of about 0.88 Tesla and a saturation magnetostriction of about 12 ppm. The ribbon had widths of about 100 mm and about 25 mm, and its thickness was about 28 μm. The ribbon was then slit into narrower ribbons with different widths. The slit ribbon then was cut into ductile, rectangular strips having a length ranging from about 15 mm to about 65 mm. Each strip had a slight curvature reflecting ribbon casting wheel surface curvature. During slitting, the original curvature was modified. The curvature of a slit and cut strip was determined as described in Example 1.  FIG. 1A  illustrates the physical appearance of a marker strip  10  of an embodiment of the present invention, and  FIG. 1B  illustrates the physical appearance of a conventional strip  20  produced in accordance with a complex heat-treatment method disclosed in the U.S. Pat. No. 6,299,702. As indicated, magnetic flux lines  11  are more closed in a resonance marker-bias strip configuration of an embodiment of the present invention than the magnetic flux lines  21  of a conventional strip, as is illustrated in  FIG. 1B . This enables better coupling between a marker strip  10  of an embodiment of the present invention and a bias magnet strip  12  than is achieved by a conventional strip  20  and a bias magnet  22 , which results in less magnetic flux leakage at the two ends of a resonance marker strip of an embodiment of the present invention. Each resonance marker strip of an embodiment of the present invention and of the conventional strip was examined in light of magnetomechanical resonance performance using a characterization method of Example 2.  FIG. 2  compares the resonance frequency as a function of bias field for a single strip marker  830  of an embodiment of the present invention and the resonance frequency of a conventional strip  831 .  FIG. 2  indicates that the resonance frequency change as a function of bias field is about the same for both cases. The resonance characteristics depicted in  FIG. 2  are important in designing a resonance marker with deactivation capability because deactivation is accomplished by a change in the resonance frequency by changing bias field strength. During deactivation, the slope of the resonance frequency f r  with respect to bias field H b , i.e. df r ldH b , determines the effectiveness of deactivation and therefore is an important factor for an effective resonance marker strip. For a marker in an electronic coded identification system, a larger slope of resonance frequency versus bias field is generally preferred when a higher sensitivity is desired in an identification system.  
         [0046]     A comparison of the resonance response between the two cases is illustrated in  FIG. 3 , in which V 0  is the response signal amplitude when the exciting field is turned off, and V 1  is the signal amplitude at 1 msec after the termination of the exciting field. Clearly, a higher V 1 /V 0  ratio is preferred for a better performance of a resonance marker. Both of the signal amplitudes are therefore used in industry as part of the figure of merit for a magnetomechanical resonance marker.  FIG. 3  indicates that the signal amplitudes, V 0    441  and V 1    442  become maximum at bias fields of H b0 =500 A/m and H b1 =400 A/m, respectively, for a resonance marker strip of an embodiment of the present invention, and V 0    443  and V 1    444  become maximum at bias fields of H b0 =460 A/m and H b1 =400 A/m , respectively, for a conventional resonance marker strip. In addition,  FIG. 3  indicates that the ratio of V 1 /V 0  at these maximum points is higher for a resonance marker strip of an embodiment of the present invention than for a conventional marker strip, illustrating that signal retention of a marker strip of an embodiment of the present invention is better than in a conventional marker strip, thus enhancing the effectiveness of the present coded electronic identification system.  
         [0047]     Table I summarizes a comparison of parameters critical for the performance of a marker strip as a magnetomechanical resonator between representative conventional marker strips and examples from the marker strips of an embodiment of the present invention. It is noted that the performance of the marker strips of an embodiment of the present invention is close to, or superior to, the performance of a conventional marker strips. All of the marker strips of an embodiment of the present invention in Table I are acceptable for use as markers of the embodiment of the present invention.  
         [0048]     In Table I, maximum signal voltages for V 0  and V 1  measured at bias field strengths, H b0  and H b1 , respectively, and the resonance frequency slope, df r ldH b , measured at H b1  for marker strips of an embodiment of the present invention with strip curvature h as defined in  FIG. 1A  were compared with corresponding characteristics for ten conventional marker strips, randomly selected. The length l of the strips were all about 38 mm and their widths were about 6 mm. A radius of curvature for each marker strip was calculated from h and l. The resonance frequency of each strip was about 58 kHz.  
                                                 TABLE I                           Magnetomechanical Resonance Characteristics                                        Radius of           V 0max     H b0     V 1max     H b1     df r /dH b     h   Curvature       Marker   (mV)   (A/m)   (mV)   (A/m)   [Hz/(A/m)]   (mm)   (cm)               Conventional   140˜180   440˜500   60˜102   360˜420   5.60˜11.5   —   —       Present Invention   167   490   97   400   12.0   0.18   100       No. 1       No. 2   156   470   86   410   9.50   0.18   100       No 3   159   490   84   410   12.5   0.20   90       No. 4   167   490   94   400   11.8   0.20   90       No. 5   183   458   110   390   11.8   0.23   78       No. 6   165   488   94   370   12.5   0.23   78       No. 7   178   471   106   391   12.3   0.28   65       No. 8   160   460   92   379   10.8   0.28   65       No. 9   157   461   87   351   9.10   0.36   50       No. 10   147   420   76   391   10.3   0.64   28                  
 
         [0049]     Table I contains data for a marker strip width of about  6  mm which is presently widely used. It is one aspect of the present invention to provide marker strips with widths different than about  6  mm. Marker strips with different widths were slit from the same ribbon used in Table I, and their magnetomechanical resonance characteristics were determined. The results are summarized in Table II. The resonance signal voltages, V 0 max  and V 1 max  decreased with decreasing width as expected. Decrease in the characteristic field values, H b0  and H b1  with decreasing width is due to demagnetizing effects. Thus, a bias field magnet must be selected accordingly. A marker with a smaller width is suited for a smaller article identification area, whereas a marker with a larger width is suited for a larger article identification area because resonance signals are larger from larger marker strips, as Table II indicates. Since the resonance frequency depends primarily on the strip length, as Equation (1) indicates, the strip width change does not affect the resonance frequency of the article identification system used.  
         [0050]     Table II shows the magnetomechanical resonance characteristics of marker strips of an embodiment of the present invention with strip height h, as defined in  FIG. 1A  and with different strip widths. The definitions for V 0 max , H b0 , V 1 max  and df r ldH b  were the same as in Table I. The length l of the strips were all about 38 mm. A radius of curvature for each marker strip was calculated from h and l. The resonance frequency of each strip was about 58 kHz.  
                                                                                   TABLE II                           Magnetomechanical Resonance Characteristics            Marker       H b0                     Radius of       Width   V 0max     (A/   V 1max     H b1     df r /dH b     h   Curvature       (mm)   (mV)   m)   (mV)   (A/m)   [Hz/(A/m)]   (mm)   (cm)                    4   107   310   56   330   4.69   0.61   30       5   153   300   76   300   6.05   0.41   44       9   194   500   101   440   4.84   0.81   22       14   321   590   174   511   4.86   0.84   21                  
 
         [0051]     Another aspect of the present invention is to provide a variety of available markers operated under different conditions. For this purpose, magnetomechanical resonance characteristics were varied by changing the chemical composition of the amorphous magnetic alloy ribbon from which marker strips were produced. The chemical compositions of the alloys examined are listed in Table III in which values of the saturation induction and magnetostrictions for the alloys are given. The results of the magnetomechanical resonance properties of these alloys are given in Table IV below.  
         [0052]     Table III shows examples of magnetostrictive amorphous alloys with their compositions, saturation inductions, B s , and saturation magnetostrictions, λ s , for magnetomechanical resonance markers of an embodiment of the present invention. The values of B s  were determined from DC BH loop measurements described in Example 3 and the values of λ s , were calculated by using an empirical formula λ s =k B s   2 , with k=15.5 ppm/tesla 2 , following S. Ito et al.,  Applied Physics Letters , vol. 37, p. 665 (1980).  
                                                   TABLE III                           Magnetostrictive Amorphous Alloy                    Saturation   Saturation       Alloy   Marker Chemical Composition   Induction   Magnetostriction       No.   (numbers in atom %)   B s  (tesla)   λ s  (ppm)                    A   Fe 40.6 Ni 40.1 Mo 3.7 B 15.1 Si 0.5     0.88   12       B   Fe 41.5 Ni 38.9 Mo 4.1 B 15.5     0.98   15       C   Fe 41.7 Ni 39.4 Mo 3.1 B 15.8     1.03   16       D   Fe 40.2 Ni 39.0 Mo 3.6 B 16.6 Si 0.6     0.93   13.5       E   Fe 39.8 Ni 39.2 Mo 3.1 B 17.6 C 0.3     0.94   14       F   Fe 36.9 Ni 41.3 Mo 4.1 B 17.8     0.83   10.5       G   Fe 35.6 Ni 42.6 Mo 4.0 B 17.9     0.81   10       H   Fe 39.6 Ni 38.3 Mo 4.1 B 18.0     0.88   12       I   Fe 38.0 Ni 38.8 Mo 3.9 B 19.3     0.84   11                  
 
         [0053]     Table IV shows the magnetomechanical resonance characteristics of marker strips having different chemical compositions listed in Table III of an embodiment of the present invention with strip height h as defined in  FIG. 1A . The definitions for V 0 max , H b0 , V 1 max  and df r ldH b  were the same as in Table I. The lengths l of the strips were all about 38 mm. A radius of curvature for each marker strip was calculated from h and l. The resonance frequency of each strip was about 58 kHz.  
                                                                           TABLE IV                           Magnetomechanical Resonance Characteristics of the Alloys in Table III                                    Radius of       Alloy   V 0max     H b0     V 1max     H b1     df r /dH b     Curvature       No.   (mV)   (A/m)   (mV)   (A/m)   [Hz/(A/m)]   (cm)                    A   184   370   94   330   8.10   71       B   174   490   89   348   10.4   36       C   188   471   70   368   13.0   33       D   158   580   83   580   4.85   33       E   160   320   72   300   8.80   25       F   160   341   84   329   7.06   34       G   154   420   94   389   8.51   36       H   171   472   85   351   9.73   27       I   146   352   60   250   13.4   30                  
 
         [0054]     All of the amorphous alloys with different chemical compositions listed in Table III have excellent magnetomechanical resonance characteristics, as given in Table IV, and therefore are useful in a coded electronic identification system of an embodiment of the present invention.  
         [0055]     Furthermore, ribbons slit to about  6  mm wide width in accordance with the Example 1 were cut into strips with different lengths, and their magnetomechanical resonance properties were examined. In addition to the properties covered in Tables I, II and IV above, a complementary test to determine the effectiveness of a magnetomechanical resonance strip was performed using the following formula: 
 
 V ( t )= Vo exp  (− t/τ ),   (2) 
 
 wherein t is the time measured after termination of an AC field excitation and τ is a characteristic time constant for the resonance signal decay. The values of V 1max  in Tables I, II and IV was determined from the data for t=1 msec. The results are given in Table V, in which other parameters characterizing the resonance properties of differing strip lengths are summarized. It is noted that f r  follows the relationship of Equation (1) given above, quite well. Also noted is the increase of τ with increasing strip length. Larger value of the time constant τ is preferable if a delayed signal detection is preferred. However, in a coded electronic article identification system when the interrogation AC field is swept, the value of V 0  in Table I matters more than the value of V 1 . 
 
         [0056]     As shown in Table V, magnetomechanical resonance characteristics were determined for marker strips of an embodiment of the present invention with different lengths, l. The width and thickness of each strip were about 6 mm and about 28 μm, respectively. The resonance frequency, f r  and time constant, τ are defined in Equations (1) and (2), respectively. The definitions of V 0 max , H b0 , V 1 max , H b1  and df r ldH b  were the same as in Table 1. Marker height h is defined in  FIG. 1 , and a radius of curvature each strip was calculated using h and l.  
                                                                                     TABLE V                       Strip               Time               Radius of       Length/   f r     V 0 max     H b0     Constant T   V 1 max     H b1     df r /dH b     Curvature       (mm)   (Hz)   (mV)   (A/m)   (msec)   (mV)   (A/m)   [Hz/(A/m)]   (cm)                                18.01   120,772   73   610   0.85   23   520   6.65   26       20.16   108,536   68   550   0.92   25   370   8.07   22       24.99   87,406   94   460   1.16   42   338   6.55   22       30.02   72,284   135   461   1.35   69   342   9.44   36       35.03   61,818   143   387   1.74   79   322   8.73   29       37.95   56,782   160   389   1.86   91   337   7.89   31       41.90   51,336   184   389   2.03   109   350   6.67   43       46.95   45,992   178   330   2.49   116   320   5.21   45       52.12   41,438   197   331   2.69   132   312   5.28   35       56.99   37,900   187   292   3.30   135   291   5.93   37       62.07   34,864   197   293   3.56   148   279   4.94   34                  
 
         [0057]     In addition to the basic magnetic properties such as saturation magnetic induction and magnetostriction listed in Table III that are required to generate magnetomechanical resonance in a marker strip of an embodiment of the present invention, the direction of magnetic anisotropy which is the direction of easy magnetization in a marker strip must be essentially perpendicular to the strip&#39;s length direction. This is indeed the case, as indicated in  FIG. 4  which depicts a BH loop taken at 60 Hz using a measurement method of Example 3 on an approximately 38 mm long strip from Table V above. The BH loop of  FIG. 4  indicates that the remanent magnetic induction at H=0, i.e. B(H=0) is close to zero and the permeability defined by B/H near H=0 is linear. The shape of the BH loop shown in  FIG. 4  is typical of the BH behavior of a magnetic strip in which the average direction of the magnetic anisotropy is perpendicular to strip&#39;s length direction. A consequence of the magnetization behavior of a marker strip of an embodiment of the present invention shown in  FIG. 4  is the absence of higher harmonics generation in the strip when the strip is placed in an AC magnetic field. Thus the system “pollution problem” as mentioned in the “Background of the Invention” section, is minimized. To further check this point, a higher harmonic signal from the marker strip of  FIG. 4  was compared with that of a marker strip of an electronic article surveillance system based on magnetic harmonic generation/detection. The results of this comparison are given in Table VI below.  
         [0058]     As shown in Table VI, a magnetic higher harmonics signal comparison was made between a marker strip of an embodiment of the present invention and a marker strip based on Co-based METGLAS®2714A alloy, which is widely used in an electronic article surveillance system based on a magnetic harmonic generation/detection system. The strip size was the same for both cases and was approximately 38 mm long and approximately 6 mm wide. The fundamental excitation frequency was 2.4 kHz and the 25 th  harmonic signals were compared by using a harmonic signal detection method of Example 4.  
                                         TABLE VI                                   Marker Type   25 th  Harmonic Signal (mV)                                        Present Invention   4           Harmonic Marker   40                      
 
         [0059]     As Table VI indicates, a negligibly small harmonic signal from a marker of an embodiment of the present invention does not trigger an electronic article surveillance system based on magnetic harmonic generation/detection.  
         [0060]     Two marker-strips of an embodiment of the present invention with different lengths were selected randomly from a number of strips as characterized in Tables I, II, IV and V and were mounted on top of each other, and a marker was made as indicated by strip  110  and strip  111  in  FIG. 5A . The two marker-strips with different lengths are housed in a hollow area between non-magnetic outside casing  100  and  101 . A bias magnet  120  is attached on the outside surface of a casing  101 . For comparison, a marker configuration for two conventional marker-strips is shown by strip  210  and strip  211  in  FIG. 5B , in which a planar area available for the two strips is the same as that for the two strips of  FIG. 5A . Numerals  200 ,  201  and  220  in  FIG. 5B  correspond to items  100 ,  101  and  120  in  FIG. 5A , respectively.  
         [0061]     The magnetomechanical resonance behavior of a two-strip marker of an embodiment of the present invention corresponding to  FIG. 5A  is shown in  FIG. 6A  for a marker containing an approximately 20 mm strip and an approximately 57 mm strip from Table V, and the magnetomechanical resonance behavior of a conventional two-strip marker prepared in accordance with the &#39;490 patent, which corresponds to  FIG. 5B , is shown in  FIG. 6B  using two strips with lengths of approximately 20 mm and approximately 57 mm. It is clear from  FIGS. 6A-6B  that overall signal amplitudes from the two marker-strips of an embodiment of the present invention are considerably higher than the overall signal amplitudes from the two conventional marker-strips. For the case of a marker of an embodiment of the present invention illustrated in  FIG. 5A , the signal amplitude V 0  (illustrated in  FIG. 6A ) from the longer sized strip of an embodiment of the present invention is about 280% higher than its corresponding value V 0  (illustrated in  FIG. 6B ) for the longer sized conventional marker strip of  FIG. 5B . For the shorter sized strip, the strip of an embodiment of the present invention generates a higher signal amplitude V 1  (illustrated in  FIG. 6A ) by 370% than the signal amplitude V 1  (illustrated in  FIG. 6B ) of its corresponding conventional marker strip. An enlarged resonance amplitude profile near the lower resonance frequency, f r =38,610 Hz shown in  FIG. 6A , is depicted in  FIG. 7 , which shows the width of the magnetomechanical resonance, defined as the width in frequency at the point where the amplitude becomes ½ that of the peak amplitude, is about 420 Hz. For the upper resonance frequency region near f r =109,070 Hz, the signal amplitude has a frequency width of about 660 Hz as shown in  FIG. 8 . This frequency width, hereinafter termed resonance line width, is used to determine the minimum resonance frequency separation between the two adjacent resonance frequencies for two marker strips with slightly different lengths.  
         [0062]      FIG. 9-1  illustrates a marker of an embodiment of the present invention which contains three marker-strips,  311 ,  312  and  313 , with different lengths which were randomly selected from Tables I, II and IV above. The cavity space  302  between the two outside casings  300  and  301  is to accommodate the marker strips,  311 ,  312  and  313 , of the embodiment of the present invention, and numeral  330  indicates a bias magnet which is attached on the outside surface of casing  301 . The magnetomechanical resonance characteristics of the marker with three strips having lengths of about 25 mm, about 38 mm and about 52 mm and a width of about 6 mm are shown in  FIG. 10 . It noted in  FIG. 6A  and  FIG. 7  that the mechanical resonance observed is sharp, with a resonance line width of about 400 Hz near the lower resonance frequency region of about 40,000 Hz, and with a resonance line width of about 700 Hz near the higher resonance frequency region of about 110,000 Hz, as is indicated in  FIG. 6A  and  FIG. 8 , indicating that the magnetomechanical interference between marker strips with different lengths in a marker of an embodiment of the present invention is insignificant, which in turn allows stacking more marker-strips than three. The lack of strip-to-strip magnetomechanical interference is evident in  FIG. 9-2 , as the three marker strips with different lengths touch among themselves along a line near the center in the strips&#39; width direction. Similarly five strips with different lengths of about 30 mm, about 38 mm, about 42 mm, about 47 mm and about 52 mm and with a width of about 6 mm were selected from strips of Tables I, II, IV and V, and a marker was fabricated. The resonance characteristics of this 5-strip marker are shown in  FIG. 11 . A summary of resonance characteristics for markers of an embodiment of the present invention utilizing different length marker strips is given in Table VII.  
         [0063]     As shown in Table VII, resonance signals V 0 max  and V 1 max  are located at respective resonance frequencies f r  from coded markers of the present invention.  
                                             TABLE VII                                   Strip Length       Marker Sample   V 0 max  (mV)   V 1 max  (mV)   (mm)                                No. 1 (bias = 461 A/m)                   fr1 = 51,300   92   43   42       fr2 = 61,250   104   48   35       No. 2 (bias = 301 A/m)       fr1 = 38,070   133   90   57       fr1 = 109,070   55   10   20       No. 3 (bias = 360 A/m)       fr1 = 37,880   100   57   57       fr2 = 57,260   69   24   38       fr3 = 108,440   45   3   20       No. 4 (bias = 420 A/m)       fr1 = 46,100   65   28   47       fr2 = 57,100   53   24   38       fr3 = 72,720   61   14   30       No. 5 (bias = 399 A/m)       fr1 = 41,590   92   47   52       fr2 = 57,070   75   3   38       fr3 = 87,060   59   12   25       No. 6 (bias = 490 A/m)       fr1 = 37,640   61   20   57       fr2 = 45,740   55   12   47       fr3 = 56,680   68   21   38       fr4 = 86,280   48   4   25       No. 7 (bias = 550 A/m)       fr1 = 41,440   51   12   52       fr2 = 45,930   42   5   47       fr3 = 51,510   45   6   42       fr4 = 56,770   42   5   38       fr5 = 72,080   50   4   30                  
 
         [0064]     In Table VII, marker strip width and thickness are about 6 mm and about 28 μm, respectively.  
         [0065]     The resonance signals V 0 max  and V 1 max  given in Table VII are significant enough to be detected in an electronic article identification system in accordance with embodiments of the present invention. The data in Table V leads to a relationship between resonance frequency, f r , and strip length, which is given by 
 
 f   r =2.1906×10 6 ll (Hz), 
 
 where l is the strip length in mm. Using this relationship which is consistent with Equation (1), the variability of the resonance frequency caused by the tolerance in cutting ribbon to a predetermined length is determined as follows. The above relationship between f r  and l leads to Δf r lΔl=−2.906×10 6 l2/ 2 , where Δf r  is a change in the resonance frequency due to a variation in the strip length, Δl. The marker strip cutting tolerance achievable with a commercially available ribbon cutter is determined by comparing the nominal or targeted strip length and the actual length given in Table V. For example, the strip having a length of 18.01 mm in Table V had a targeted strip length of 18 mm, resulting in a cutting tolerance of 0.01 mm. Using the cutting machine tolerance thus obtained, the frequency variability Δf r  due to strip length variability was calculated, which ranged from about 3 Hz for shorter strips to about 400 Hz for longer strips. Since the resonance line width for a longer strip is about 400 Hz, as indicated in  FIG. 7  and is about 700 Hz for a shorter strip, as indicated in  FIG. 8 , the minimum frequency separation which is discernable in an electronic article identification system in accordance with embodiments of the present invention is determined as about 800 Hz. Thus, to assure no false identification, a resonance frequency separation of 2 kHz, which is more than twice that of the minimum discernable resonance frequency separation, was selected to determine the number of identifiable articles in a selected universe. The resonance frequency covered with the marker strips listed in Table V ranged from about 34,000 Hz to about 120,000 Hz, covering a resonance frequency span of approximately 86,000 Hz. Using a resonance frequency separation of 2 kHz for non-false identification, as determined above, the number of electronically identifiable articles becomes  43  when a marker has only one strip, which increases to about 1800, 74000, 2.96 million and 115.5 million in a given universe when a marker with two, three, four and five marker strips, respectively, with different lengths of an embodiment of the present invention is utilized in a coded electronic article identification system in accordance with the present invention. The number of the identifiable or coded articles is further increased by either adding more marker strips and/or changing the level of bias field in a marker. 
 
         [0066]     A coded marker  501  as described above is effectively utilized in an electronic article identification system in accordance with embodiments of the present invention, as is illustrated in  FIG. 12 . An article to be identified  502  carrying a coded marker  501  of an embodiment of the present invention is placed in an interrogation zone  510  in  FIG. 12 , which is flanked by a pair of interrogation coils  511 . The coils  511  emit an AC magnetic field fed by an electronic device  512  consisting of a signal generator  513  and an AC amplifier  514  with varying frequencies, which is controlled by an electronic circuit box  515  for its on-off operation, aiming at the article  502  to be identified. When the article  502  is placed in zone  510 , the electronic circuit box  515  switches on the interrogation AC field frequency sweeping from the lowest frequency to the highest frequency, the range of which depends on the marker&#39;s predetermined frequency range. In such a frequency sweep, a resonance signal from a coded marker of an embodiment of the present invention  501  is detected in a pair of signal receiving coils  516 , resulting in a resonance signal profile as exemplified in  FIG. 11 . The signal profile thus obtained by means of a signal detector  517  is stored in a computer  518  which is programmed to identify the resonance frequency sequences encoded in a coded marker  501  of an embodiment of the present invention. When this identification is complete, the computer  518  sends signal reporting results of the identification to an identifier  519  and to the electronic circuit box  515  for resetting the system. If so desired, a coded marker in accordance with embodiments of the present invention may be deactivated by demagnetizing the bias magnet in the marker after article  502  exits the interrogation zone  510 .  
         [0067]     The coded electronic article identification system provided above is used to identify an article by sweeping an AC excitation field with varying frequency. In certain cases, delayed identification is desired, which can be accomplished by tracking V 1  as depicted in  FIG. 3 ,  FIG. 5 ( a ),  FIG. 10  and  FIG. 11 . Electronically this is accomplished by programming the computer  517  in  FIG. 12  to process V 1  as a function of the sweeping frequency.  
       EXAMPLE 1  
       [0068]     A slit ribbon was cut into ductile and rectangular strips with a conventional metal ribbon cutter. The curvature of each strip was determined optically by measuring the height, h, of the curved surface over the strip length, l, as defined in  FIG. 1A .  
       EXAMPLE 2  
       [0069]     The magnetomechanical performance was determined in a set-up in which a pair of coils supplying a static bias field and the voltage appearing in a signal detecting coil compensated by a bucking coil was measured by a voltmeter and an oscilloscope. The measured voltage therefore is detecting-coil dependent and indicates a relative signal amplitude. The exciting AC field was supplied by a commercially available function generator and an AC amplifier. The signal voltage from the voltmeter was tabulated and a commercially available computer software was used to analyze and process the data collected.  
       EXAMPLE 3  
       [0070]     A commercially available DC BH loop measurement equipment was utilized to measure magnetic induction B as a function of applied field H. For an AC BH loop measurement, an exciting coil-detecting coil assembly similar to that of Example 4 was used and output signal from the detecting coil was fed into an electronic integrator. The integrated signal was then calibrated to give the value of the magnetic induction B of a sample. The resultant B was plotted against applied field H, resulting in an AC BH loop. Both AC and DC cases, the direction of the applied field and the measurement was along marker strips&#39; length direction.  
       EXAMPLE 4  
       [0071]     A marker strip prepared in accordance with Example 1 was placed in an exciting AC field at a predetermined fundamental frequency and its higher harmonics response was detected by a coil containing the strip. The exciting coil and signal detecting coil were wound on a bobbin with a diameter of about 50 mm. The number of the windings in the exciting coil and the signal detecting coil was about 180 and about 250, respectively. The fundamental frequency was chosen at 2.4 kHz and its voltage at the exciting coil was about 80 mV. The 25th harmonic voltages from the signal detecting coil were measured.  
         [0072]     Thus, in an embodiment of the present invention, a radius of curvature of the marker strip curvatures may be less than about 100 cm, or between about 20 cm and about 100 cm.  
         [0073]     Where selected, encoding is carried out by cutting an amorphous magnetostrictrive alloy ribbon having its magnetic anisotropy direction perpendicular to ribbon axis to a rectangular strip with a predetermined length having a length-to-width ratio greater than 3.  
         [0074]     Also, where selected, the strips have a strip width ranging from about 3 mm to about 15 mm.  
         [0075]     In an embodiment of the present invention, the strips have a slope of resonance frequency versus bias field ranging from about 4 Hz/(A/m) to about 14 Hz/(A/m).  
         [0076]     Where selected, the strips have a length greater than about 18 mm when a strip width is 6 mm.  
         [0077]     Also, where selected, the strips have a magnetomechanical resonance frequency less than about 120,000 Hz.  
         [0078]     In an embodiment of the present invention, the amorphous ferromagnetic alloy ribbons have a saturation magnetostriction between about 8 ppm and about 18 ppm and a saturation induction between about 0.7 tesla and about 1.1 tesla.  
         [0079]     In an embodiment of the present invention, the coded marker comprises at least two marker-strips with different lengths. Where selected, the coded marker comprises five marker-strips with different lengths.  
         [0080]     In an embodiment of the present invention, the coded marker has a magnetomechanical resonance frequency between about 30,000 and about 130,000 Hz.  
         [0081]     In an embodiment of the present invention, the coded marker has an electronic identification universe containing up to about 1800 and about 115 million separately identifiable articles for a coded marker with two and five marker strips, respectively.  
         [0082]     In an embodiment of the present invention, the coded marker has an electronic identification universe containing more than 115 million separately identifiable articles.  
         [0083]     Thus, in an embodiment of the present invention, a coded marker of a magnetomechanical resonant electronic article identification system, adapted to resonate mechanically at preselected frequencies, comprises a plurality of ductile magnetostrictive strips cut to predetermined lengths from amorphous ferromagnetic alloy ribbons that have curvatures along a ribbon length direction and exhibit magnetomechanical resonance under alternating magnetic field excitations with a static bias field, the strips having a magnetic anisotropy direction perpendicular to a ribbon axis, wherein at least two of the strips are adapted to be magnetically biased to resonate at a single, different one of the preselected frequencies.  
         [0084]     In addition, in selected embodiments of the present invention, an electronic article identification system has a capability of decoding coded information of a coded marker. The coded marker is adapted to resonate mechanically at preselected frequencies, and the coded marker comprises a plurality of ductile magnetostrictive strips cut to predetermined lengths from amorphous ferromagnetic alloy ribbons that have curvatures along a ribbon length direction and exhibit magnetomechanical resonance under alternating magnetic field excitations with a static bias field, the strips having a magnetic anisotropy direction perpendicular to a ribbon axis, and wherein at least two of the strips are adapted to be magnetically biased to resonate at a single, different one of the preselected frequencies. The electronic article identification system comprises one of: a pair of coils emitting an AC excitation field aimed at the coded marker to form an interrogation zone; a pair of signal detection coils receiving coded information from the coded marker; an electronic signal processing device with an electronic computer with a software to decode information coded on the coded marker; or an electronic device identifying the coded marker. Thus, as well as providing identification of a coded marker, the electronic article identification system may identify an article having the coded marker attached thereto.  
         [0085]     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.