AMORPHOUS ALLOY COMPOSITION FOR A MAGNETOMECHANICAL RESONATOR AND EAS MARKER CONTAINING SAME
The invention may include a novel composition for and/or processing of an active element for an EAS marker that achieves the same or better performance of existing materials while solving the problem of higher cost. It may include a magnetomechanical active element formed by planar strip of amorphous magnetostrictive alloy having a composition FeaNibMc wherein a+b+c=100, wherein a is in a range of 40-70 weight percent, b is in a range of 10-50 weight percent, and c is in a range of 10-50 weight percent, and where M is the balance of remaining elements; wherein the magnetomechanical active element is subject to batch annealing in the presence of a magnetic field that is substantially transverse to the ribbon length of the element and at a temperature of less than about 300° C. and for a time greater than at least about one hour.
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This application relates to and claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/811,280, filed Jun. 6, 2006, entitled “Amorphous Alloy Composition for a Magnetomechanical Resonator and EAS Marker Containing Same,” the entire disclosure of which is hereby incorporated by reference herein.BACKGROUND
The field of this invention is electronic article surveillance and more particularly a marker for use in an electronic article surveillance system.
There are many different sensing technologies used by retailers' anti-theft applications, known as Electronic Article Surveillance (“EAS”). Among these technologies are acoustomagnetic (“AM”) based markers (tags and labels), such as those produced by Sensormatic Electronics Corporation, which are highly desirable due to many of their unique advantages, such as high sensitivities, small size, deactivatible/reactivatible, low cost, easy deactivation, etc.
A typical AM label is measured about 45 mm long, 10 mm wide, with an overall thickness of about 1.3 mm. An example of such a label is illustrated in
The active element to be used in the marker may be produced by determining a number of factors in order to achieve an element having the desired properties. For example, this can be accomplished through careful selection of the composition from which the element is formed, and the manner in which the composition is annealed and otherwise processed, mechanically, chemically, and electromagnetically. A number of techniques for optimizing the active element of an EAS marker are known. Some of these are disclosed, for example, in U.S. Pat. Nos. 4,510,489; 5,252,144; 5,469,140; 5,469,489; 5,628,840; 6,018,296; and 6,359,563; hereby incorporated herein by reference.
U.S. Pat. Nos. 6,018,296 and 6,359,563, for example, disclose an FeNi based alloy having low or no cobalt, and with specified amounts of additional elements, M, (such as Si, B, C, P, Ge, Nb, Mo, Cr and Mn) to control how glassy the material is, its formability, and its susceptibility to changes in magnetic properties due to mechanical tension on the material during annealing. These patents focus on the use of quick (around 1 minute or less) heating of the material at higher temperatures (>300° C.) in a reel to reel annealing process to set the magnetic characteristics of the material. In the '296 patent, a batch annealing with a heating time of fifteen minutes at these higher temperatures was also used for comparison to the preferred reel-to reel annealing process.
The '296 patent also states that annealing this material in the presence of a transverse (to the length of the active element ribbon) magnetic field is undesirable because a sufficiently low slope of the frequency response characteristic of the material cannot be achieved at the desired DC bias field operating point of about 6.5 Oe. Conversely, the '563 patent, while it is able to achieve a sufficiently low slope for the frequency response characteristic even with the use of a transverse magnetic field, requires the use of reel to reel annealing at these higher temperatures and shorter annealing times.
Outside factors, such as the cost of raw materials (such as Ni) and/or the high cost of various aspects of processing materials (such as higher temperature reel to reel quick annealing) into final form can change over time, causing the problem that the cost of producing an active element for an EAS marker can become too high. Accordingly, a solution is needed in developing new ways of achieving the same or better performance in an active element for EAS markers in the face of the problem of such higher costs.SUMMARY OF THE INVENTION
Embodiments of the invention may include a novel composition for and/or processing of an active element for an EAS marker that achieves the same or better performance of existing materials while solving the problem of higher cost. It may include a marker for use in a magnetomechanical electronic article surveillance (EAS) system having a magnetomechanical active element formed by planar strip of amorphous magnetostrictive alloy having a composition FeaNibMc wherein a+b+c=100, wherein a is in a range of 40-70 weight percent, b is in a range of 10-50 weight percent, and c is in a range of 10-50 weight percent, and where M is the balance of remaining elements; wherein the magnetomechanical active element is subject to batch annealing in the presence of a magnetic field that is substantially transverse to the ribbon length of the element and at a temperature of less than about 300° C. at a temperature of greater than at least about one hour.
In one embodiment, the balance of remaining elements, M, may include one or more selected from the group consisting of Co, Si, B, C, P, Sn, Cu, Ge, Nb, Mo, Cr, Mn, and Mismetal. Particularly, the material can include about 15-30% of one or more selected from the group consisting of Si, B, C, and P in order to affect the glassy nature of the material. In order to affect magnetic properties of the material, it may include from 0 to about 10% of one or more selected from the group consisting of Sn, Cu, Ge, Nb, Mo, Cr, Mn, and Mismetal. Also, the amount of Ni, b, may particularly be less than about 25 weight percent. The batch annealing is conducted particularly at a temperature of about 250° C. for about one hour.
For a better understanding of various embodiments of the invention, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts.
For simplicity and ease of explanation, the invention will be described herein in connection with various embodiments thereof. Those skilled in the art will recognize, however, that the features and advantages of the invention may be implemented in a variety of configurations. It is to be understood therefore, that the embodiments described herein are presented by way of illustration, not of limitation.
where E, L, ρ are the Young's modulus, length, and density of the amorphous strip, and H, λ, Ms, and Hk, are the applied DC magnetic field, magnetostriction, saturation magnetization, and transverse (to the ribbon length) anisotropy field, respectively.
The amplitude characteristic of response signal 122 can be demonstrated by taking measurements at three time increments (A0, A1, A2) after marker 100 is subject to electromagnetic field 116; for example, at 0, 1, and 2 milliseconds after a burst, as shown in
To obtain a usable active element for an EAS marker, several parameters must be optimized to obtain an operating point for the active element/bias in the marker having a desired resonant frequency (such as 58 kHz for AM markers) at a desired DC bias field, typically less than Hmin. The frequency slope in Hz/Oe at this operating point should preferably be smaller than the maximum slope for the frequency characteristic curve. The A1 amplitude at the operating point should, preferably be larger than the minimum amplitude for the amplitude characteristic curve.
For proper function of a deactivatable AM marker, a semi-hard magnetic bias strip is preferably used. This strip is magnetized along the longitudinal direction, in order to provide a DC magnetic field of the desired strength to the active strip. To deactivate the marker, the bias field must be changed to achieve a proper frequency shift. This can be achieved by various methods, including AC demagnetization or magnetization by contacting a patterned magnet. The difference in resonant frequency at the DC bias field of the operating point and the resonant frequency at the DC bias field after deactivation (the deactivation frequency shift, “DFS”) should preferably be larger than a selected minimum desired to avoid unintentional detection by receiver pedestal 116.
As can be seen from Eq. 1 above, the frequency/bias field relation may be determined by at least three parameters even if the Young's Modulus and density remain relatively consistent among varying compositions. The frequency profile of the resonator should behave similarly as long as the quantity C=λ2/(MsHk3) remains relatively constant.
The anisotropy (Hk) has the strongest effect since it is cubic. C will also be affected by λ and Ms. Depending on the value of C, the anisotropic field may include contribution from field induce anisotropy as well as shape anisotropy.
The variation of Ku with Ni % at different temperatures is shown in
As shown in
The saturation magnetization is another important parameter. It is directly related to the magnetic signal strength. It also plays a role in forming the resonant frequency and DC magnetic bias relation, as shown in Eq 1.
The magnetostrictive constant (λs) of the amorphous alloys also depends on Ni percent concentration as shown in
Based on the above information, we have magnetic/mechanical information required to model the magnetomechanical resonance with a Fe—Ni-M amorphous alloy series. A sample composition was modeled comprising Fe55—Ni25—B20. The parameters related to the resonant profile for the active element used in this example are listed in Table 1 below:
With the material parameters listed in Table 1, it was determined that a value for C similar to existing active elements could be reached with an Hk value of 12.7 Oe under proper annealing conditions.
There are also other potential composition variations that can further enhance the strength of the field induce anisotropy. In addition to affecting the glassy nature of the material, a change in the percentage of Si, B, C, and/or P content could potentially increase the Curie temperature of the material which can increase induced Hk.
Another potential composition modification to improve the strength of the field induced anisotropy is the addition of low levels of Mn (or, Sn, Cu, Ge, Nb, Mo, and/or Cr). Another approach is to reduce the magnetostriction (λ) by adding traces of elements, such as chromium (Cr), Niobium (Nb), rare earth (Mismetal—mix of elements). The effect of adding Mn, Cr, or Mo can be to reduce λs, reduce Js, or reduce Hk. Mn increases the Creep effect, while Cr reduces it; and Mo has a positive creep effect
Table 2, below, shows data for sputter deposited Fe—Mn—B alloy thin films indicating a significant (3 to 4 Oe) increase in Hk for small amount (0.5%) of Mn additions to Fe—B. This increase is much greater than that obtained by Ni additions to Fe—B, which are typically less than 1 Oe for each atomic % Ni added. To further reduce the materials cost of the alloy, small amounts of Mn could be included in combination with reduced amounts of Ni.
Several samples were prepared and tested to demonstrate the invention. These samples are listed below in Table 3:
These samples were batch annealed, which is much lower in cost to reel to reel annealing. The samples were also subject to a transverse magnetic field during annealing. For different annealing temperatures, different soaking times were used. For 200° C., the soaking time was 2.5 days. At 250° C., the soaking time was at least 7 hours. For 300 and 350° C. the soaking time was 10 minutes or more. The resulting Hk for each sample at each annealing temperature is shown in Table 4 below:
The resonant performance of sample # two, batch annealed in the presence of a transverse magnetic field at 250° C. for about one hour is shown in
It is to be understood that variations and modifications of the various embodiments of the invention can be made without departing from the scope of the invention. It is also to be understood that the scope of the invention is not to be interpreted as limited to the specific embodiments disclosed herein, but only in accordance with the appended claims when read in light of the forgoing disclosure.
1. A marker for use in a magnetomechanical electronic article surveillance (EAS) system comprising:
- a magnetomechanical active element formed by planar strip of amorphous magnetostrictive alloy having a composition FeaNibMc wherein a+b+c=100, wherein a is in a range of 40-70 weight percent, b is in a range of 10-50 weight percent, and c is in a range of 10-50 weight percent, and where M is the balance of remaining elements;
- a biasing element located adjacent said resonator element;
- a housing configured to contain said resonator element and said biasing element; and
- wherein said magnetomechanical active element is subject to batch annealing in the presence of a magnetic field that is substantially transverse to the plane of said element and at a temperature of less than about 300° C. at a temperature of greater than at least about one hour.
2. The marker of claim 1, wherein M is one or more selected from the group consisting of Co, Si, B, C, P, Sn, Cu, Ge, Nb, Mo, Cr and Mn.
3. The marker of claim 1, wherein M comprises one or more selected from the group consisting of Si, B, C, and P.
4. The marker of claim 3, wherein M is from about 15 to about 30 weight percent.
5. The marker of claim 1, wherein M comprises one or more selected from the group consisting of Sn, Cu, Ge, Nb, Mo, Cr, Mn, and Mismetal.
6. The marker of claim 5, wherein M is from about 0 to about 10 weight percent.
7. The marker of claim 1, wherein M comprises one or more first elements selected from the group consisting of Si, B, C, and P to affect the glassy nature of said active element; and one or more second elements selected from the group consisting of Sn, Cu, Ge, Nb, Mo, Cr, Mn and Mismetal to affect the magnetic properties of said active element.
8. The marker of claim 7, wherein said first element comprises about 15 to about 30 weight percent of said active element, and said second element comprises about 0 to about 10 weight percent of said active element.
9. The marker of claim 1, wherein the amount of Ni, b, is less than about 25 weight percent.
10. The marker of claim 1, wherein said batch annealing is conducted at a temperature of about 250° C. for about one hour.
Filed: Jun 7, 2007
Publication Date: Nov 4, 2010
Applicant: Sensomatic Electronics Corporation (Boca Raton, FL)
Inventors: Ming-Ren Lian (Boca Raton, FL), Nen-Chin Liu (Wellington, FL), Chaitali China (Orlando, FL), Kevin R. Coffey (Oviedo, FL)
Application Number: 12/451,870
International Classification: B32B 1/04 (20060101);