HK1099051B - Annealed amorphous alloys for magneto-acoustic markers - Google Patents

Description

Method for annealing amorphous alloy workpiece

Technical Field

The present invention relates to a magnetic amorphous alloy and a method of annealing such an alloy. The present invention also relates to an amorphous magnetostrictive alloy used in magnetomechanical electronic article surveillance or identification. The invention also relates to a magnetomechanical electronic article surveillance or identification system using such a marker, a method of manufacturing amorphous magnetostrictive alloy, and a method of manufacturing such a marker.

Background

U.S. Pat. No. 3,820,040 discloses that transverse field annealing of amorphous iron-based metal produces a large change in young's modulus by application of a magnetic field, and this action provides an effective means of enabling control of the oscillation frequency of a magnetic resonator in conjunction with the application of a magnetic field.

It has been found in european application 0093281 that the method of controlling the frequency of oscillation with an applied magnetic field is particularly suitable for use in markers used in electronic article surveillance. A magnetic field can be generated for this purpose by placing a magnetized ferromagnetic ribbon bias magnet in proximity to a magnetoelastic resonator and housing the ribbon and resonator in a marker or tag housing. The change in the effective permeability of the marker at the resonant frequency provides the marker with a signal characteristic. The resonance frequency can be changed by changing the applied magnetic field, thereby eliminating the signal signature. In this way, the marker may be activated, for example, by magnetizing the bias tape, and accordingly deactivated by demagnetizing the bias magnet to remove the applied magnetic field and appropriately changing the resonant frequency. Such systems initially (see european application 00923281 and PCT application WO 90/03652) used markers made of an "as prepared" amorphous tape which also produced a suitable change in young's modulus under the action of an applied magnetic field due to uniaxial anisotropy with respect to the intrinsic mechanical stress of the product. The typical composition used in this prior art marker is Fe₄₀Ni₃₈Mo₄B₁₈。

US patent US5,459,140 discloses that the use of transverse field annealed amorphous magnetic elements in electronic article surveillance systems eliminates many of the drawbacks associated with prior art markers using specially treated amorphous material. One reason is that the linear hysteresis loop associated with lateral field annealing avoids the generation of harmonics (i.e., harmonic systems) that could produce undesirable alarms in other types of EAS systems. Another advantage of such annealed resonators is their higher resonance amplitude. Another advantage is that heat treatment in a magnetic field can greatly improve the uniformity in the resonant frequency of the magnetostrictive strip.

For example, resonators or properties such as resonant frequency, amplitude or ringing time are determined primarily by the saturated stretch properties and the strength of the induced anisotropy as described in Livingston J.D.1982, "magnetic Properties of Amorphous Metals", Phys.Stat sol (a) vol.70 pp 591 596 and Herzer G.1997 magnetic damping in atomic resonance free area amplification, Materials Science and engineering A226-228 p.631. The amounts of both depend mainly on the alloy composition. The induced Anisotropy also depends on the Annealing conditions, i.e., Annealing time and temperature, and the tensile Stress applied during Annealing (see Fujimori H.1983, "Magnetic Anisotropy" in F.E.Luborsky (ed) atomic metals Alloys, butterworks, London pp.300-316 and references therein, Nielsen O.1985 Effect of Long atomic and Stress analysis Materials, IEEE transactions on magnetism, vol.Mag-21, No.5, Hilzine H.R.1981. induced Anisotropy in Non-Magnetic Alloys, Stress 4.4^thInt. Conf. on Rapid queue Metals (Sendai 1981) pp.791). Thus, resonator performance is primarily dependent on these parameters.

Thus, the above-mentioned US patent US5,459,140 discloses that the preferred material is an Fe-Co based alloy with a Co content of at least 30 at% (atomic percentage). According to this patent, a high Co content is necessary to maintain a long ringing time of the signal. German Gebrauchsmuter G9412456.6 discloses that long ring-down times are achieved by selecting alloy compositions that exhibit high magnetic induction anisotropy, and therefore, such alloys are particularly well suited for EAS markers. The Gebrauchsmuter discloses that lower Co content may also achieve such functionality if the iron content reaches about 50 at% and/or nickel is used instead of cobalt for Fe-Co based alloys. Again, the research work described in US5,628,840 confirms that a linear hysteresis loop (B-H loop) is required to achieve a high anisotropy field of at least 8Oe (oersted) and the benefit of using Ni to reduce the Co content of such a magnetoelastic marker, US5,628,840 discloses that alloys with an iron content between about 30 at% and less than 45 at% and a Co content between about 4 at% and about 40 at% are particularly suitable. US patent US5,728,237 discloses that another component with a Co content below 23 at% is characterized by a small change in the resonance frequency and resulting signal amplitude due to changes in the orientation of the marker in the earth's magnetic field, while reliably deactivating it. US patent US5,841,348 discloses that Fe-Co-Ni based alloys with Co content of at least about 12 at% have an anisotropic magnetic field of at least 10Oe and optimize the ringing characteristics of the signal due to the iron content below about 30 at%.

The field annealing in the above example was performed in a direction transverse to the strip width, i.e. the magnetic field direction was perpendicular to the strip axis (longitudinal axis) and in the strip plane. This type of annealing is known and is referred to herein as lateral field annealing. The magnetic field strength must be strong enough to ferromagnetically saturate the ribbon in a direction transverse to the width of the ribbon. This effect can be achieved in a magnetic field of several hundred Oe. For example, US patent US5,469,140 discloses magnetic fields with magnetic field strengths in excess of 500Oe or 800 Oe. PCT application WO 96/32518 discloses a magnetic field having a magnetic field strength of 1kOe to 1.5 kOe. PCT application WO 99/02748 and PCT application WO 99/24950 disclose that applying a magnetic field perpendicular to the plane of the ribbon enhances (or can enhance) the signal amplitude.

For example, field annealing can be performed in batches on an annularly coiled core or on pre-cut straight strip. Alternatively, as disclosed in detail in european application EP 0737986 (U.S. Pat. No.5,676,767), the strip is annealed in a continuous manner by transferring the alloy strip from one reel to another reel through a furnace in which a transverse saturation magnetic field is applied to the strip.

Conventional annealing conditions disclosed in the above patents are annealing temperatures between about 300 ℃ and 400 ℃; the annealing time ranges from a few seconds to a few hours. For example, PCT application WO 97/132358 discloses that for a furnace 1.8 meters long, the annealing rate is between 0.3 and 12 meters per minute.

The general functional requirements of a magneto-acoustic marker can be summarized as follows:

1. a linear hysteresis loop of the applied field of typically a minimum of 8Oe is achieved.

2. In the activated state, the resonance frequency f_rLess sensitivity to the applied bias field H, i.e. typically | df_rand/dH is less than 1200 Hz/Oe.

3. The ringing of the signal is sufficiently long, i.e. the signal amplitude is high for a time interval of at least 1-2ms after the excitation driving magnetic field is switched off.

All of these requirements can be met by inducing a higher magnetic anisotropy in the appropriate resonator alloy in the direction perpendicular to the ribbon axis. It has been traditionally considered that these requirements can only be met when the resonator alloy contains a suitable amount of Co, i.e. according to US patents US5,469,140, US5,728,237, US5,628,840 and US5,841,348, such as Fe₄₀Ni₃₈Mo₄B₁₈The prior art compositions of (a) are not suitable for this purpose. However, due to the high material cost of cobalt, it is highly desirable to reduce its content in the alloy.

The above-mentioned PCT application WO 96/32518 also discloses that tensile stresses in the range of about 0 to about 70Mpa can be applied during the annealing process. As a result of this tensile stress, the resonator amplitude and frequency slope | df_rthe/dH is slightly increased, kept constant or slightly reduced, i.e. when a tensile stress limited to a maximum of about 70Mpa is applied, there are no significant advantages or disadvantages for the resonator performance.

However, it is well known (see Nielsen O.1985 Effects of longitudinal and Torque Stress Annealing on the magnetic Annealing in Amorphous Ribben Materials)，IEEE Transitions onMagnetics，vol.Mag-21，No.5，Hilzinger H.R.1981 StressInduced Anisotropy in a Non-Magnetostrictive Amorphous Alloy，Proc.4^thOn rapid Quenched Metals (Sendai 1981) pp.791) during annealing a tensile stress induced magnetic anisotropy is applied. The magnitude of this anisotropy is proportional to the magnitude of the applied stress and depends on the annealing temperature, annealing time and alloy composition. Oriented to either the easy axis of the strip or the hard axis of the strip (-the easy plane of magnetization perpendicular to the strip axis), which reduces or increases the field induced anisotropy, respectively, depending on the alloy composition.

One of the present inventors is a co-inventor's co-pending application (an unexamined patent application entitled "Method Employing Tension Control and Lower-Cost alloy composition for Annealing with short Annealing Time", filed on 13.8.1998 by Herzer et al, serial No.09/133,172, and issued patent No. US 6,254,695 at the Time of filing) which discloses a Method of Annealing an Amorphous ribbon in the presence of both a magnetic field perpendicular to the ribbon axis and a tensile stress applied parallel to the ribbon axis. It has been found that for compositions having an iron content below about 30 at%, the applied tensile stress can enhance the induced anisotropy. Thus, the desired resonator performance can be achieved with a low Co content, in a preferred embodiment between about 5 at% and 18 at%.

Disclosure of Invention

In light of the above background, it is highly desirable to provide another method capable of reducing the Co content of an amorphous magnetoacoustic resonator. The present invention is based on the recognition that all of this can be achieved by selecting specific alloy compositions with little or no Co content and applying controlled tensile stress along the strip during annealing.

An object of the present invention is to provide a method of annealing an amorphous alloy workpiece, comprising the steps of:

providing an unannealed amorphous alloy article having a longitudinal axis and an alloy composition, the alloy composition being selected so as to produce a stress-induced anisotropy in the amorphous alloy article of greater than 0.04Oe/MPa when the amorphous alloy article is annealed at 360 ℃ for 6 seconds and a magnetization easy axis perpendicular to the longitudinal axis when a tensile stress is applied along the longitudinal axis during annealing; and

the amorphous alloy article is placed in a high temperature region and is free of a magnetic field other than an ambient magnetic field, while subjecting the amorphous alloy to a tensile force along the longitudinal axis to produce the anisotropy and the easy magnetization axis of more than 0.04Oe/MPa in the amorphous alloy article.

It is an object of the present invention to provide a magnetostrictive alloy and a method of annealing such an alloy to produce a resonator with lower raw material costs and properties suitable for use in electronic article surveillance.

It is another object of the present invention to provide an annealing method wherein the annealing parameters, in particular the tensile stress, are adjusted in a feedback process to obtain a high degree of uniformity of the magnetic properties in the annealed amorphous ribbon.

It is another object of the present invention to provide a magnetostrictive amorphous metal alloy which is used for a marker in a magnetometric monitoring system and can be cut into long, ductile magnetostrictive strips, which can be activated and deactivated by applying or removing a pre-magnetization magnetic field H, and which can be excited by an alternating magnetic field in an activated state to have a resonance frequency f_rThe lower is represented by a longitudinal mechanical resonance, which has a high signal amplitude after excitation.

It is another object of the present invention to provide such an alloy wherein the resonant frequency changes only slightly upon a change in the bias magnetic field, but changes significantly when the marker resonator switches from an activated to a deactivated state.

It is another object of the present invention to provide such an alloy wherein when used in a marker of a magnetic force monitoring system, no alarm is triggered in a harmonic monitoring system.

It is another object of the present invention to provide a marker suitable for use in a magnetometric monitoring system.

It is another object of the present invention to provide a magnetomechanical electronic article surveillance system that is operable with a marker having a resonator formed from such an amorphous magnetostrictive alloy.

The above objects are achieved when an amorphous magnetostrictive alloy is continuously annealed under a tensile stress of at least between about 30Mpa and about 400Mpa, or while applying a magnetic field perpendicular to the axis of the ribbon. The alloy composition must be selected such that the tensile stress applied during annealing includes the hard axis of the strip, in other words the easy magnetization plane perpendicular to the strip axis. This enables the same magnitude of induced anisotropy to be achieved, which is only achieved at higher Co content and/or slower annealing rates without providing tensile stress. Thus, the annealing treatment of the present invention enables production of a magnetoelastic resonator at a comparatively low raw material cost and annealing treatment cost as compared with the prior art.

For this purpose, it is preferable to select an Fe-Ni based alloy having a Co content of less than 4 at%. The general formula for the alloy composition, when subjected to the annealing process described above, is capable of producing a resonator suitable for performance in a marker in an electronic article surveillance or identification system,

Fe_aCo_bNi_cM_dCu_eSi_xB_yZ_z

wherein a, b, C, d, e, x, y and Z are represented by a t%, M is at least one element selected from the group consisting of Mo, Nb, Ta, C r and V, Z is at least one element selected from the group consisting of C, P and Ge, wherein 20. ltoreq. a.ltoreq.50, 0. ltoreq. b.ltoreq.4, 30. ltoreq. c.ltoreq.60, 1. ltoreq. d.ltoreq.5, 0. ltoreq. e.ltoreq.2, 0. ltoreq. x.ltoreq.4, 10. ltoreq. y.ltoreq.20, 0. ltoreq. z.ltoreq.3, and 14. ltoreq. d + x + y + z.ltoreq.25, and a + b + C + d + e + x + y + z.ltoreq.100.

In a preferred embodiment, M is an element selected from the group consisting of Mo, Nb and Ta, wherein the ranges are as follows: a is more than or equal to 30 and less than or equal to 45, b is more than or equal to 0 and less than or equal to 3, c is more than or equal to 30 and less than or equal to 55, d is more than or equal to 1 and less than or equal to 4, e is more than or equal to 0 and less than or equal to 1, x is more than or equal to 0 and less than or equal to 3, y is more than or equal to 14 and less than or equal to 18.

An example of such an alloy particularly suitable for EAS applications is Fe₃₃Co₂Ni₄₃Mo₂B₂₀、Fe₃₅Ni₄₃Mo₄B₁₈、Fe₃₆Co₂Ni₄₄Mo₂B₁₆、Fe₃₆Ni₄₆Mo₂B₁₆、Fe₄₀Ni₃₈Cu₁Mo₃B₁₈、Fe₄₀Ni₃₈Mo₄B₁₈、Fe₄₀Ni₄₀Mo₄B₁₆、Fe₄₀Ni₃₈Nb₄B₁₈、Fe₄₀Ni₄₀Mo₂Nb₂B₁₆、Fe₄₁Ni₄₁Mo₂B₁₆And Fe₄₅Ni₃₃Mo₄B₁₈。

In a preferred embodiment, M is an element selected from the group consisting of Mo, Nb and Ta only, and other ranges are as follows: a is more than or equal to 20 and less than or equal to 30, b is more than or equal to 0 and less than or equal to 4, c is more than or equal to 45 and less than or equal to 60, d is more than or equal to 1 and less than or equal to 3, e is more than or equal to 0 and less than or equal to 1, x is more than or equal to 0 and less than or equal to 3, y is more than or equal to 14 and less than or equal to 18.

An example of such a component is Fe₃₀Ni₅₂Mo₂B₁₆、Fe₃₀Ni₅₂Nb₁Mo₁B₁₆、Fe₂₉Ni₅₂Nb₁Mo₁Cu₁B₁₆、Fe₂₈Ni₅₄Mo₂B₁₆、Fe₂₈Ni₅₄Nb₁Mo₁B₁₆、Fe₂₆Ni₅₆Mo₂B₁₆、Fe₂₆Ni₅₄Co₂Mo₂B₁₆、Fe₂₄Ni₅₆Co₂Mo₂B₁₆And other similar situations.

Such alloy compositions are characterized by an induced anisotropic magnetic field H when a tensile stress sigma is applied during annealing_kIncrease, when annealed at 360 ℃ for 6s, of at least about dH_k/dσ≈0.02Oe/Mpa。

Suitable alloy compositions have a saturation magnetostriction greater than about 3ppm and less than about 20 ppm. Particularly suitable resonators have an anisotropic magnetic field H of between about 60e and 14Oe when subjected to the above-described annealing_kAnd when the saturation magnetostriction is decreased, H_kAnd correspondingly decreases. Such an anisotropic magnetic field is sufficiently high to cause the moving resonator to exhibit the property that if a change occurs in the intensity of the magnetizing field, | df_rA resonance frequency f when/dH is less than 1200Hz/Oe_rOnly a slight change occurs, but at the same time when the marker resonator switches from the active state to the inactive state, the resonance frequency f_rLarge variations occur, at least up to about 1.6 kHz. In a preferred embodiment, such resonator strips have a thickness of less than about 30 microns, a length of between about 35 mm and 40 mm, and a width of less than about 13 mm, preferably between about 4 mm and 8 mm, i.e. for example 6 mm.

The annealing process produces a linear hysteresis loop up to the magnetic field that ferromagnetically saturates the magnetic alloy. The material therefore produces virtually no harmonics when excited in an alternating magnetic field, which does not trigger an alarm in a harmonic monitoring system.

Changes in induced anisotropy due to the action of tensile stress and corresponding changes in magneto-acoustic properties can also be advantageously used to control the annealing process. For this purpose, the magnetic properties (e.g. anisotropic magnetic field, magnetic permeability or sound velocity achieved under known bias) are measured after the strip has passed through the furnace. During the measurement, the strip should be under a predetermined stress or preferably stress-free, and may be arranged with an empty loop (dead loop). The results of the measurement can be adjusted to incorporate its degaussing effect on the short resonator. If the resulting test parameter deviates from its predetermined value, the tension is increased or decreased to produce the desired magnetic properties. The feedback system can effectively compensate the influence of composition fluctuation, thickness fluctuation and deviation of annealing time and annealing temperature on magnetism and magnetoelasticity. The result is a very good consistency and reproducibility of the annealed strip, which otherwise would be subject to strong fluctuations due to the influencing parameters.

Drawings

Figure 1 shows a typical hysteresis loop for annealing an amorphous ribbon under tensile stress or a magnetic field perpendicular to the ribbon axis.

FIG. 2 shows the bias magnetic field H at the resonant frequency f as a function of the amorphous magnetostrictive band annealed under tensile stress and/or in a magnetic field perpendicular to the ribbon axis_rAnd the typical behavior of the resonance amplitude a 1.

FIG. 3 illustrates a marker with an upper portion of the housing partially pulled away to show internal components with a resonator made in accordance with the principles of the present invention, wherein the magnetomechanical article surveillance system is schematically illustrated.

Detailed Description

The invention will be described in detail below with reference to the attached drawing figures, wherein:

figure 1 shows a typical hysteresis loop for annealing an amorphous ribbon under tensile stress or a magnetic field perpendicular to the ribbon axis. The specific example shown in FIG. 1 is of the inventionAn embodiment and corresponds to a dual resonator made of amorphous Fe₄₀Ni₄₀Mo₄B₁₆Made of an alloy strip which has been continuously annealed at 360 ℃ at an annealing speed of 2 m/min (annealing time of about 6s) under the simultaneous application of a 2kOe magnetic field oriented substantially perpendicular to the plane of the strip and a tensile force of about 19N, two thin strips 38 mm long, 6 mm wide and 25 μm thick being cut out of the alloy strip in sequence, thus making said double resonator.

FIG. 2 shows the bias magnetic field H at the resonant frequency f as a function of the amorphous magnetostrictive band annealed under tensile stress and/or in a magnetic field perpendicular to the ribbon axis_rAnd the typical behavior of the resonance amplitude a 1. The specific example shown in fig. 2 is an embodiment of the present invention and corresponds to a double resonator made of amorphous Fe₄₀Ni₄₀Mo₄B₁₆Made of an alloy strip which has been continuously annealed at 360 ℃ at an annealing speed of 2 m/min (annealing time 6s) under the simultaneous application of a 2kOe magnetic field oriented substantially perpendicular to the plane of the strip and a tensile force of about 19N, from which strip two thin strips 38 mm long, 6 mm wide and 25 μm thick are cut continuously, thus making said double resonator.

FIG. 3 illustrates a marker with an upper portion of the housing partially pulled away to show internal components with a resonator made in accordance with the principles of the present invention, wherein the magnetomechanical article surveillance system is schematically illustrated.

EAS system

The magnetometric monitoring system shown in figure 3 operates in a known manner. The system comprises, in addition to the marker 1, a transmitter circuit 5, the transmitter circuit 5 having a coil or antenna 6, the coil or antenna 6 transmitting (transmitting) RF bursts of a predetermined frequency, such as 58kHz, at a repetition rate of, for example, 60Hz, with pauses between successive bursts. The transmitter circuit 5 is controlled by means of a synchronization circuit 9 for transmitting the above-mentioned RF pulse trains, which synchronization circuit 9 also controls a receiver circuit 7 with a receiving coil or antenna 8. If there is an activated marker 1 (i.e. a bias element 4 with a magnetization) between the coils 6 and 8 when the transmitter circuit 5 is activated, the RF pulse train emitted by the coil 6 will drive the resonator 3 to oscillate at a resonance frequency of 58kHz (in this example), thus generating a signal with an initially very high amplitude which decays exponentially.

The synchronization circuit 9 controls the receiver circuit 7 to activate the receiver circuit 7 to find a signal of a predetermined frequency 58kHz (in this example) within a first and a second detection window (detection window). Typically, the synchronization circuit 9 will control the transmitter circuit 5 to transmit an RF burst of duration about 1.6ms, in which case the synchronization circuit 9 will activate the receiver circuit 7 in a first demodulation window of duration about 1.7ms, starting from about 0.4ms after the end of the RF burst. In the first detection window, the receiver circuit 7 rectifies any signal present at a predetermined frequency (e.g., 58 kHz). In order to produce an regularization result in this first detection window (if present) that can be reliably compared to the regularized signal in the second detection window, the signal transmitted by the marker 1 should have a higher amplitude.

When a resonator 3 made according to the invention is driven with 18mOe using a transmitter circuit 5, the receiver coil 8 is a 100 turn, tightly coupled pick-up coil and the signal amplitude is measured about 1ms after an a.c. excitation burst of duration about 1.6ms, which produces an amplitude of at least 1.5nWb in a first detection window. Typically, A1 ℃. N.W.Hac, where N is the number of turns of the receiver coil, W is the width of the resonator and Hac is the field strength of the excitation (driving) magnetic field. The particular combination of these factors that produces a1 is not important.

The synchronization circuit 9 then stops activating the receiver circuit 7 and then reactivates the receiver circuit 7 in a second detection window that begins approximately 6ms after the end of the RF burst. In the second detection window, the receiver circuit 7 again looks for a signal with a suitable amplitude at a predetermined frequency (58 kHz). Since the signal emanating from the marker 1, if present, is known to have an attenuated amplitude, the receiver circuit 7 compares the amplitude of any 58kHz signal detected in the second detection window with the amplitude of the signal detected in the first detection window. If the amplitude difference coincides with the amplitude difference of the exponentially decaying signal, the receiver circuit 7 activates the alarm 10, assuming that the signal is actually indeed emitted from the marker 1 present between the coils 6 and 8.

This method reliably avoids false alarms due to spurious RF signals emanating from RF sources other than the marker 1. It is assumed that such spurious signals will exhibit relatively constant amplitudes, and therefore even if such signals are rectified in each of the first and second detection windows, they do not meet the comparison criterion and do not cause the receiver circuit 7 to trigger the alarm 10.

In addition, when the bias magnetic field Hb is removed, the resonance frequency f of the resonator 3 is reduced_rThe above-mentioned large variation occurs, at least 1.2kHz, so that it is assumed that, when the marker 1 is deactivated, the marker 1 does not signal the predetermined resonance frequency to which the receiver circuit 7 has been tuned, even if it is activated by the transmitter circuit 5, even if the deactivation is not fully effective.

Alloy preparation

Amorphous metal alloys in Fe-Co-Ni-M-Cu-Si-B (where M ═ Mo, Nb, Ta, Cr system) are prepared by rapidly cooling the melt to thin ribbons typically 20 to 25 microns thick. Amorphous as used herein refers to ribbons exhibiting a crystalline fraction of less than 50 at%. Table 1 lists the ingredients studied and their basic properties. The compositions shown are only nominal and the respective concentrations may deviate slightly from these nominal values, the alloy may contain impurities such as carbon due to the melting process and the purity of the raw materials. In addition, for example up to 1.5 at% boron may be replaced by carbon.

All castings were prepared from at least 3kg ingots using commercially available raw materials. The strips used for the tests were 6 mm wide and were either cast directly to their final width or cut from a wider strip. The strip is strong, hard and tough and has a glossy top surface and a less glossy bottom surface.

Annealing

The strip is annealed in a continuous manner by transferring the alloy strip from one spool to another through a furnace and applying a tension in the range of about 0.5N to about 20N along the axis of the strip.

A magnetic field of 2kOe generated by the permanent magnets is provided simultaneously in a direction perpendicular to the long strip axis during annealing. According to the teachings of the prior art, the magnetic field is oriented transverse to the strip axis, i.e. transverse to the strip width, or the magnetic field adopts an orientation which exhibits a substantial component in a direction perpendicular to the strip plane. The latter technique provides the advantage of higher signal amplitude. In both cases, the annealing field is perpendicular to the long strip axis.

Although most of the examples given below are obtained with annealing magnetic fields oriented substantially perpendicular to the plane of the strip, the main conclusion also applies to conventional "transverse" annealing and to annealing carried out without the provision of a magnetic field.

Annealing is performed in ambient atmosphere. The annealing temperature is selected to be in the range of about 300 ℃ to about 420 ℃. The lower limit of the annealing temperature is about 300 c, and at least about 300 c is required to relieve a portion of the inherent stress generated and to provide sufficient thermal energy to produce the magnetic anisotropy. The upper limit of the annealing temperature is due to the crystallization temperature. Another upper limit of the annealing temperature results from the requirement that the strip after heat treatment has sufficient ductility to be cut into short strips. The maximum annealing temperature should preferably be below the lowest of these material characteristic temperatures. Thus, the upper limit of the annealing temperature is typically about 420 ℃.

The furnace used to treat the strip was approximately 40 cm long and had a heated zone of approximately 20 cm in length, where the strip was subjected to the annealing temperature. The annealing speed was 2 m/min, corresponding to an annealing time of 6 seconds.

The strip is conveyed through the furnace in a straight path and supported by elongated annealing fixtures to avoid bending and twisting of the strip due to forces and torques applied thereto by the magnetic field.

Testing

The annealed strip is cut into short pieces, typically 38 mm long. These samples were used to measure hysteresis loops and magnetoelastic properties. To this end, two resonator plates are arranged together to form a double resonator. Such a dual resonator has essentially the same performance as a single co-resonator with a width of twice the electromagnetic width, but with the advantage of a smaller Size (see Herzer's co-pending application serial No.09/247,688, entitled "magnetic-Acoustic Marker for electronic surveilling and High Amplitude", published as PCT publication No. WO 00/48152, filed 10.2.1999). Although this form of resonator is used in this embodiment, the invention is not limited to this particular type of resonator. Other types of resonators, such as resonator(s) having a length of between 20 mm and 100 mm and a width of between 1 and 15 mm, may also be used.

The hysteresis loop was measured at a frequency of 60Hz in a sinusoidal field with a peak amplitude of 30 Oe. The anisotropy field is defined as the magnetic field Hk at which the hysteresis loop behaves linearly and the magnetization reaches its saturation value. For an easy axis of magnetization (or easy plane of magnetization) perpendicular to the ribbon axis, the relationship between the transverse anisotropy field and the anisotropy constant Ku is:

H_k＝2Ku/Js

where Js is the saturation magnetization and Ku is the energy per unit volume required to turn the magnetization vector from a direction parallel to the easy axis to a direction perpendicular to the easy axis.

The anisotropy field essentially comprises two components, namely,

H_k＝H_demag+Ha

wherein H_demagDue to the demagnetization effect, and Ha is characterized by anisotropy caused by heat treatment. A prerequisite for reasonable resonator performance is that Ha > 0, corresponding to H_k＞H_demag。

The demagnetizing field to be investigated is 38 mm long and 6 mm wide, the double resonator sample is usually H_demag3-3.5Oe。

Longitudinal resonance excited by acoustic pulse train with a small alternating magnetic field oscillating at a resonance frequency with a peak amplitude of 18mOe, such as the resonance frequency f_rAnd the resonance amplitude a1 as a function of the superimposed d.c. bias field H along the strip axis. The on-time of the bursts is 1.6ms and there is an 18ms pause between the bursts.

The resonance frequency of the longitudinal mechanical vibration of the sliver is given by the following formula:

wherein L is the specimen length, E_HIs the Young's modulus at the bias field H, and ρ is the mass density. For a 38 mm long sample, the resonance frequency is typically between 50kHz and 60kHz, depending on the bias field strength.

Mechanical stresses associated with mechanical vibrations cause the magnetization J to assume its average value J determined by the bias field H via magnetoelastic interaction_HThe surroundings are periodically changed. Electromagnetic force (electricity) induced by related magnetic flux changesMomentum) that is measured in a tightly coupled pick-up coil of 100 turns around the tape.

In an EAS system, the magneto-acoustic response of the marker is preferably detected between acoustic bursts, which can reduce the noise level, thereby enabling, for example, the construction of a wider threshold. After excitation, i.e. the end of the acoustic burst, the signal decays exponentially. The decay (or "ringing") time depends on the alloy composition and heat treatment and can range from about a few hundred microseconds to a few milliseconds. A sufficiently long decay time of at least about 1ms is important to provide sufficient signal identity between the acoustic bursts.

Thus, the reduced resonance signal amplitude is measured about 1ms after excitation; this resonance signal amplitude will be referred to as a1 below. Thus, a high a1 amplitude measured here indicates a good magnetoacoustic response and simultaneously low signal attenuation.

To characterize the resonator performance, the following is for f_rRelative to H_biasThe characteristic parameters of the curve have been calculated:

-Hmax: bias field with amplitude of A1 appearing as its maximum

-A1_Hmax: amplitude of A1 at H-Hmax

-t_R.Hmax: ringing time at Hmax, i.e. the time interval during which the signal decreases to about 10% of its initial value

-|df_r[ dH ] |: f when H is Hmax_r(H) Slope of (2)

-Hmin: resonant frequency f_rExhibits its minimum value, i.e. at | df_rBias field when/dH | ═ 0

-A1_Hmin: amplitude of A1 at H-Hmin

-t_R.Hmin: ringing time at Hmin, i.e. the time interval during which the signal is reduced to about 10% of its initial value

Results

Table II lists amorphous Fe for conventional magneto-acoustic markers₄₀Ni₃₈Mo₄B₁₈The properties of the alloy in the as-cast condition. The disadvantage in the as-cast state is a non-linear hysteresis loop, which triggers an undesired alarm in harmonic systems. The latter drawback can be overcome by annealing in a magnetic field perpendicular to the strip axis that can produce a linear hysteresis loop. However, resonator performance is greatly reduced by such conventional heat treatments. Thus, the ringing time of the signal is greatly reduced, resulting in a low a1 amplitude. In addition, the slope | df at the bias field Hmax at which the amplitude of a1 has its maximum value_rthe/dH | increases to undesirably high values of several thousand Hz/Oe.

The inventors have found that the above difficulties can be overcome if a tension of, for example, 20N is applied during the annealing process. Tension may be applied in addition to or instead of a magnetic field. In either case, for the same Fe₄₀Ni₃₈Mo₄B₁₈The result is a linear hysteresis loop with excellent resonator performance, as listed in table III. Annealing under tensile stress produced a high signal amplitude a1 (manifested as a long ringing time) that greatly exceeded that of conventional markers utilizing as-cast alloys, as compared to pure field annealing. In addition, the stress annealed specimens exhibited suitably low slopes of less than about 1000 Hz/Oe.

The results for Fe are given in Table I V₄₀Ni₄₀Mo₄B₁₆Another example of an alloy. Also, the tensile stress during annealing greatly improved the resonator performance (i.e., higher amplitude and lower slope) compared to field annealed samples. Anisotropy field H_kIs linearly increased with respect to the applied tensile stress, i.e.,

therefore, the relationship between tensile stress σ and tension F is:

where t is the strip thickness and w is the strip width (example: for a strip 6 mm wide and 25 μm thick, a tension of 10N corresponds to a tensile stress of 67 Mpa).

As an example, fig. 1 shows a typical linear hysteresis loop characteristic of an annealed resonator according to the present invention. The corresponding magneto-acoustic response is given in fig. 2. These figures are intended to describe the basic mechanism that affects the magnetoacoustic performance of the resonator. Thus, the resonant frequency f_rThe change in the relative bias field H and the corresponding change in the resonance amplitude a1 is closely related to the change in the magnetization J relative to the magnetic field. Thus, f_rThe bias field Hmin with its minimum value is located close to the anisotropy field H_k. In addition, f_rThe bias field Hmax having its maximum value is also associated with the anisotropy field H_kAnd (4) correlating. For the present examples, typically Hmax ≈ 0.4-0.8H_kAnd Hmin is approximately equal to 0.8-0.9H_k. In addition, the slope | df_rdH | with anisotropy field H_kIs increased and decreased. In addition, high H_kThe signal amplitude A1 is favored because the ringing time is over H_kA significant increase (see table IV). When the anisotropy field H_kAbove about 6-7Oe, suitable resonator performance was found.

By suitably adjustingThe selection of stress levels utilizes the dependence of resonator performance on tensile stress to set a particular resonator performance. In particular, tension may be used to control the annealing process in a closed loop process. For example, if H is measured continuously after annealing_kThe results can then be fed back to adjust the tensile stress to achieve the desired resonator performance in a most consistent manner.

From the results discussed so far, it is evident that the stress annealing is only in the anisotropy field H_kAs the annealing stress increases, i.e., if dH_kThe beneficial effect is achieved when the/d sigma is more than 0. It has been found that with respect to an amorphous alloy of the type Fe-Co-Ni-Si-B, if the iron content is less than 30 at%, such a situation exists (see Japanese unexamined patent application, serial No.09/133,172, filed on 8/13 of 1998, and granted under the patent No. US 6,254,695). Table V shows the results of some of these comparative examples (alloy No. 1 and alloy No. 2 in Table I). The results shown for alloy No. 1 and alloy No. 2, when used in current electronic article surveillance markers, generally appear as linear resonators (unexamined application serial No.09/133,172 (U.S. Pat. No. 6,254,695 and serial No.09/247,688(PCT publication No. WO 00/48152), however, these alloys are outside the scope of the present invention because their appreciable Co content is greater than about 10 at%, increasing the cost of raw materials.

Alloy No. 3 and alloy No. 4 of table I give other examples beyond the scope of the present invention. As can be seen from Table V, for alloy No. 3, dH_kThe/d σ is negative, i.e., stress annealing produces unsuitable resonator performance (low ringing time, hence low amplitude for this example). Alloy No. 4 is not suitable because it still has a non-linear hysteresis loop even after annealing.

Table VI lists other inventive examples (alloy No.5 to alloy No. 21 in table I). All these examples show that after annealing under stress, H_kAre all greatly increased (dH)_k/d σ > 0), and therefore, have suitable resonatorsThe performance is shown by a relatively low slope at Hmax and a high signal amplitude a 1. These alloys are characterized by an iron content greater than about 30 at%, a low or zero Co content, and contain, in addition to Fe, Co, Ni, Si and B, at least one element selected from group Vb and/or group V1B of the periodic table, such as Mo, Nb and/or Cr. In particular, the latter case is reliable, dH_kThe/d σ > 0, i.e., the resonator performance can be greatly improved to a suitable value by tensile stress annealing, although the alloy does not contain Co or the Co content is negligible. When a suitable alloy No.5 to 21 is used, for example, with alloy No. 3 (Fe)₄₀Ni₃₈S₁₄B₁₈) The beneficial effects of these elements of Vb group and/or V1b group are clearly seen when compared.

Alloys 7 through 21 are particularly suitable because they exhibit a slope at Hmax of less than 1000 Hz/Oe. It is clear that the use of Mo and Nb is more effective in reducing the slope than the addition of Cr alone. In addition, reducing the B content is also beneficial for resonator performance.

In all the examples given in table VI, in addition to the tensile stress, a magnetic field perpendicular to the plane of the strip was applied. In addition, similar results can be obtained without applying a magnetic field. This is beneficial for the investment in annealing equipment (no expensive magnets are required). Another advantage of stress annealing is that the annealing temperature can be higher than the alloy curie temperature (in which case field annealing produces no or only low anisotropy), thereby aiding in the optimization of the alloy. In addition, on the other hand, the simultaneous presence of magnetic fields also provides the advantage of reducing the amount of stress required to achieve the desired resonator performance.

One problem caused by the alloys containing high amounts of Mo, about 4 at%, is that these alloys are difficult to cast. These difficulties can be largely eliminated when the Mo content is reduced to about 2 at% and/or replaced with Nb. In addition, low Mo and/or Nb content can lower raw material cost, but a reduction in Mo content can reduce susceptibility to annealing stress, e.g., produce higher slopes. This can be a disadvantage if the resonator requires a slope of less than about 600-700 Hz/Oe. The effect of the increased slope caused by decreasing the Mo content can be compensated by decreasing the Fe content to 30 at% and below. The alloy system Fe corresponding to examples 18 to 21 in Table I and Table VI, respectively, can be utilized_30-xNi_52+xMo₂B₁₆(x ═ 0, 2, 4, and 6 at%) was confirmed. These low iron alloys have a high sensitivity to annealing stress, i.e., dH_kThe content of the iron is higher than that of the Mo and/or the Nb, and the dH can be achieved only under the condition that the content of the Mo and/or the Nb is very high_k, (/ d σ) is 0.050Oe/MPa (see examples 13 and 15 in tables I and VI, respectively). Therefore, stress annealing of these low iron containing alloys produces a low slope well below 700Hz/Oe, which can produce a particularly suitable resonator. Sensitivity to annealing stress dH_kThe/d σ can even be so high that a low slope can be achieved without increasing the field induced anisotropy. (it should be noted that the Curie temperatures of these alloys range from about 230 ℃ to about 310 ℃ well below the annealing temperature. Therefore, these low iron alloys are preferred because they can produce a suitably low slope without the simultaneous presence of a magnetic field during annealing, which can greatly reduce the cost of the annealing equipment.

In general, such as Fe_30+xNi_52-y-xCo_yMo₂B₁₆Or Fe_30+xNi_52-y-xCo_yMo₁B₁₆(where x-10 to 3, y-0 to 4) are particularly suitable because of their good castability, low raw material cost and high sensitivity to stress annealing (i.e., when annealed at 360 ℃ for 6 seconds, dH_k0.05Oe/Mpa) so that, even without the application of an additional magnetic field, a particularly low slope can be obtained with moderate annealing stress. All these factors contribute to a reduction in the investment in annealing equipment.

Watch (A)

TABLE I

The alloy compositions studied and their basic magnetic properties (Js saturation magnetization,. lambda.s saturation magnetostriction, Tc Curie temperature)

Number (I)	Ingredient (at%)	Js(T)	λs(ppm)	Tc(℃)
					123456789101112131415161718192021	Fe24Co12.5Ni45.5Si2B16Fe24Co11Ni47Mo1Si0.5B16.5Fe40Ni38Si4B16Fe40Ni38B22Fe40Ni38Mo2B20Fe40Ni38Cr4B18Fe33Co2Ni43Mo2B20Fe35Ni43Mo4B18Fe36Co2Ni44Mo2B16Fe36Ni46Mo2B16Fe40Ni38Mo3Cu1B18Fe40Ni38Mo4B18Fe40Ni40Mo4B16Fe40Ni38Nb4B18Fe40Ni40Mo2Nb2B16Fe41Ni41Mo2B16Fe45Ni33Mo4B18Fe30Ni52Mo2B16Fe28Ni54Mo2B16Fe26Ni56Mo2B16Fe24Ni58Mo2B16	0.860.820.960.990.930.890.810.840.960.940.940.900.910.850.911.040.970.800.750.700.64	11.410.214.915.114.714.511.112.616.416.015.013.915.013.215.119.015.812.1108927.9	388353362360342333293313374358346328341314339393347309288261229

TABLE II (prior art)

Fe₄₀Ni₃₈Mo₄B₁₈Magneto-acoustic properties after annealing at 360 ℃ for 6s in the as-cast state and in a magnetic field oriented across the width of the strip (transverse magnetic field) and in a magnetic field oriented perpendicular to the plane of the strip (perpendicular magnetic field).

Annealing conditions

Hk(Oe)

Hmax(Oe)

A1Hmax(nWb)

|dfr/dH|(Hz/Oe)

Hmin(Oe)

A1Hmin(nWb)

Vertical magnetic field without (as-cast) transverse magnetic field

(*)4043

4.35.35.0

2.20.91.2

14526123192

4.83.83.6

2.10.51.1

^*Non-linear hysteresis loop

TABLE III

Fe₄₀Ni₃₈Mo₄B₁₈Magneto-acoustic properties after annealing at 360 ℃ for 6s in the absence of an applied magnetic field and under a magnetic field oriented transverse to the strip width (transverse magnetic field) and in a magnetic field oriented perpendicular to the strip plane (perpendicular magnetic field) at a tension of about 20N.

Annealing conditions	Hk(Oe)	Hmax(Oe)	A1Hmax(nWb)	|dfr/dH|(Hz/Oe)	Hmin(Oe)	A1Hmin(nWb)
							Transverse magnetic field of vertical magnetic field without magnetic field	9.310.510.7	6.26.56.3	3.53.43.3	700795805	8.09.09.0	32.71.8

TABLE IV

Fe₄₀Ni₄₀Mo₄B₁₆Magneto-acoustic properties after annealing at 360 ℃ for 6s in a magnetic field oriented perpendicular to the plane of the strip (perpendicular magnetic field) under tension of strength F.

F(N)	Hk(Oe)	Hmax(Oe)	A1Hmax(nWb)	tR，Hmax(ms)	|dfr/H|(Hz/Oe)	Hmin(Oe)	A1Hmin(nWb)	tr，Hmin(ms)
									011131920	4.68.99.912.212.9	5.35.56.38.38.8	1.03.83.73.33.3	2.34.14.85.56.0	31321121944665599	4.17.88.810.511.0	0.92.72.42.62.7	1.22.62.73.54.1

TABLE V (comparative example)

The magnetoacoustic properties of alloys No. 1 to 4 listed in table I after annealing at 360 ℃ for 6s in a magnetic field oriented perpendicular to the plane of the strip (perpendicular magnetic field) under tension of strength F.

Alloy number

Hk(Oe)＜0.5N

F(N)

Hk(Oe)atF

dHk/dσ(Oe/MPa)

Hmax(Oe)

A1Hmax(nWb)

|df/dH|(Hz/Oe)

Hmin(Oe)

A1Hmin(nWb)

1234

7.44.24.8(*)

13181111

9.99.74.3(*)

0.0280.032-0.005(*)

6.56.56.05.5

3.83.30.60.55

622490142316

8.57.94.05.8

3.12.80.30.53

(^*) Non-linear hysteresis loop

TABLE VI (examples of the invention)

The magnetoacoustic properties of alloys No.5 to 17 listed in table I after annealing at 360 ℃ for 6s in a magnetic field oriented perpendicular to the plane of the strip (perpendicular magnetic field) under a tension of 20N.

Alloy number	Hk(Oe)＜0.5N	Hk(Oe)20N	|dHk/dσ|(Oe/MPa)	Hmax(Oe)	A1Hmax(nWb)	|df/dH|(Hz/Oe)	Hmin(Oe)	A1Hmin(nWb)
									56789101112131415161718192021	4.33.73.33.66.45.54.44.34.63.95.17.74.83.63.43.02.9	6.46.76.410.311.410.98.610.512.99.512.412.110.61111.511.511.2	0.0140.0170.0200.0420.0360.0370.0270.0420.0560.0360.0520.0330.0370.0500.0540.0580.057	3.32.84.06.57.56.54.56.58.86.89.87.36.57.07.57.88.0	1.72.42.12.94.03.73.43.43.33.32.64.13.53.12.72.21.7	12251271728632755853996795599614177867765634505351182	5.55.85.48.810.09.37.59.011.08.311.310.39.09.29.710.010.0	1.01.31.82.02.72.21.72.72.72.92.42.42.91.81.81.71.2

Claims (6)

1. A method of annealing an amorphous alloy article, comprising the steps of:

providing an unannealed amorphous alloy article having a longitudinal axis and an alloy composition selected in such a manner that:

i) the amorphous alloy is selected from: fe₃₅Ni₄₃Mo₄B₁₈；Fe₄₀Ni₃₈Mo₄B₁₈；Fe₄₀Ni₄₀Mo₄B₁₆；Fe₄₀Ni₄₀Mo₂Nb₂B₁₆；Fe_30-xNi_52+xMo₂B₁₆Wherein x is 0, 2, 4 or 6; fe_30+xNi_52-y-xCo_yMo₂B₁₆Wherein x is-10 to 3 and y is 0 to 4; fe_30+xNi_52-y-xCo_yMo₁B₁₆Wherein x is-10 to 3 and y is 0 to 4; and is

ii) a stress-induced anisotropy of greater than 0.04Oe/MPa is generated in the amorphous alloy article when the amorphous alloy article is annealed at 360 ℃ for 6 seconds and a magnetization easy axis perpendicular to the longitudinal axis is generated when a tensile stress is applied along the longitudinal axis during annealing; and

the amorphous alloy article is placed in a high temperature region and is free of a magnetic field other than an ambient magnetic field, while subjecting the amorphous alloy to a tensile force along the longitudinal axis to produce the anisotropy and the easy magnetization axis of more than 0.04Oe/MPa in the amorphous alloy article.

2. A method as claimed in claim 1, comprising the step of selecting said alloy composition in such a manner as to produce a stress induced anisotropy in said amorphous alloy article of greater than 0.05Oe/MPa when said amorphous alloy article is annealed at 360 ℃ for 6 seconds.

3. The method of claim 1 wherein the step of positioning the amorphous alloy article in a high temperature zone comprises placing the amorphous alloy article in a high temperature zone having a temperature profile with a maximum temperature between 300 ℃ and 420 ℃ in less than 1 minute.

4. The method of claim 1, wherein the step of selecting the alloy composition comprises: from the inclusion of Fe₃₅Ni₄₃Mo₄B₁₈、Fe₄₀Ni₃₈Mo₄B₁₈、Fe₄₀Ni₄₀Mo₄B₁₆、Fe₄₀Ni₄₀Mo₂Nb₂B₁₆Wherein the subscript represents at% and up to 1.5 at% of B is replaced with C.

5. The method of claim 1, wherein the step of selecting the alloy composition comprises: from the inclusion of Fe₃₀Ni₅₂Mo₂B₁₆、Fe₂₈Ni₅₄Mo₂B₁₆、Fe₂₆Ni₅₆Mo₂B₁₆、Fe₂₆Ni₅₄Co₂Mo₂B₁₆、Fe₂₄Ni₅₆Co₂Mo₂B₁₆Wherein the subscript represents at% and up to 1.5 at% of B is replaced with C.

6. The method of claim 1, wherein providing an amorphous alloy article that has not been annealed comprises: an unannealed amorphous alloy ribbon is provided such that the unannealed amorphous alloy ribbon has a width of between 1 millimeter and 14 millimeters and a thickness of between 15 microns and 40 microns.