Marker for use in an electronic article surveillance system

• US 6,137,412 A
• Filed: 06/21/1999
• Issued: 10/24/2000
• Est. Priority Date: 12/20/1996
• Status: Expired due to Term

• Current Assignee: Vacuumschmelze GmbH & Company KG (Vectra Corp.)
• Original Assignee: Vacuumschmelze GmbH & Company KG (Vectra Corp.)
• Inventors: Herzer, Giselher
• Primary Examiner(s): Lee, Benjamin C.
• Application Number:
  US09/331,445
• Time in Patent Office: 491 Days
• Field of Search: 340/572.6, 340/572.1, 340/551, 148/121
• US Class Current: 340/572.6
• CPC Class Codes: G01V 15/00 Tags attached to, or associated with, an object, in order to enable detection of the object record carriers for use with machines having a detectable tag or marker G06K19/00
• CPC Class Codes: G08B 13/2408 using ferromagnetic tags
• CPC Class Codes: G08B 13/2442 Tag materials and material properties thereof, e.g. magnetic material details

Abstract

A marker for employment in a magnetic merchandise monitoring system is composed of one or more oblong, ductile, magnetostrictive strips composed of amorphous ferromagnetic material. These strips experience a change in resonant frequency due to a change of a pre-magnetization field and are excited to longitudinal, mechanical resonant oscillation at the resonant frequency f_r due to an alternating magnetic field, whereby the mechanical stresses resulting from the resonant oscillations cause a change in magnetization of the strips and, thus, a detectable change of the alternating magnetic field. The material of which the strips are composed exhibits a flat B-H loop that proceeds optimally linearly into the range of saturation; further, the strips exhibit a magnetic anisotropy transverse to the longitudinal strip direction, whereby the anisotropy field strength is greater than the pre-magnetization field strength. The strips experience a change of the resonant frequency compared to the change of the pre-magnetization field strength of df_r /dH_Bias ≧1500 Hz/Oe and thereby exhibit the resonator quality Q≧100 in a field range dH_Bias ≧1 Oe.

Claims

1. A marker for a magnetic electronic article surveillance system, comprising at least one oblong, ductile, magnetostrictive strip composed of an amorphous ferromagnetic material that exhibits a change in resonant frequency df_r /dH_Bias due to variation of a pre-magnetization field H_Bias, and which is excitable to longitudinal, mechanical resonant oscillations at the resonant frequency f_r by an alternating magnetic field, with mechanical stresses resulting from the resonant oscillations causing a change in magnetization of the strip and a detectable variation of the alternating magnetic field, said strip of the amorphous ferromagnetic material exhibiting a flat B-H loop that proceeds linearly into a range of saturation, said strip of the amorphous ferromagnetic material exhibiting a magnetic anisotropy at a right angle to a longitudinal strip direction, with an anisotropy field strength H_aniso that is greater than the pre-magnetization field strength H_Bias said strip exhibiting a change of the resonant frequency compared to a change of the pre-magnetization field strength df_r /dH_Bias ≧

1500 Hz/Oe and a resonator quality Q≧

100 in a field range dH_Bias ≧

80 A/m.

2. A marker according to claim 1, wherein the amorphous ferromagnetic material exhibits a saturation magnetostriction 20 ppm≧

λ

_s ≦

40 ppm and an anisotropy field strength 200 A/m≦

H_aniso ≦

800 A/m.

3. A marker according to claim 1 wherein the amorphous ferromagnetic material has a composition having the formula

4. A method for manufacturing a marker for use in an electronic article surveillance system, comprising the steps of:

(a) producing a melt of a ferromagnetic alloy with a composition having the formula Fe_a Co_b Ni_c Si_d B_e M_f wherein M is at least one element of at least one of the groups III_A through V_A and III_B through V_B of the periodic table, and wherein a through f are in atomic % and wherein 20≦

a≦

74, 1≦

b≦

8, 1≦

c≦

50, 0≦

d≦

10, e≦

20, 0≦

f≦

5, with a+b+<

88 and 12≦

d+e+f≦

21;

(b) casting a continuous amorphous ferromagnetic ribbon, having a ribbon plane and a longitudinal axis, from said melt;

(c) thermally treating said ribbon in a magnetic field oriented in said ribbon plane and transverse to said longitudinal axis, to produce a thermally treated ribbon; and

(d) cutting a plurality of oblong, ductile magnetorestrictive strips from said thermally treated ribbon, each strip exhibiting a change in resonant frequency df_r /dH_Bias due to variation of a pre-magnetization field H_Bias, and being excitable to longitudinal, mechanical resonant oscillations at the resonant frequency f_r by an alternating magnetic field, with mechanical stresses resulting from the resonant oscillations causing a change in magnetization of each strip and a detectable variation of the alternating magnetic field, each strip further exhibiting a flat B-H loop that proceeds linearly into a range of saturation, each strip further exhibiting a magnetic anisotropy at a right angle to said longitudinal axis with an anisotropy field strength H_aniso that is greater than said premagnetization field strength H_Bias, and each strip exhibiting a change of the resonant frequency compared to a change of the premagnetization field strength df_r /dH_Bias ≧

1500 Hz/Oe and a resonator quality Q≧

100 in a field range of dH_Bias ≧

80 A/m.

5. A method as claimed in claim 4 wherein step (c) comprises continuously passing said ribbon through an annealing oven.

6. A method as claimed in claim 5 wherein step (c) comprises continuously passing said ribbon through an annealing oven at a throughput speed to heat said ribbon to a temperature T of 300°

C.≦

T≦

440°

C. for a time t of 0.5 sec≦

t≦

30 sec.

7. A method as claimed in claim 6 wherein step (c) further comprises:

measuring a B-H loop of said continuous ribbon while said continuous ribbon is being thermally treated and controlling at least one of T, t and said throughput speed dependent on said B-H loop of said continuous ribbon while said continuous ribbon is being thermally treated.

8. A method as claimed in claim 4 wherein said continuous amorphous ferromagnetic ribbon is cast with a ribbon width, and wherein said strips are cut from said continuous ribbon so as to also have said width, and wherein step (a) comprises selecting the composition of said ferromagnetic alloy dependent on said width.