Amorphous alloys for magneto-acoustic markers in electronic article surveillance having reduced, low or zero co-content and method of annealing the same
View Patent ↗A ferromagnetic resonator for use in a marker in a magnetomechanical electronic article surveillance system is manufactured at reduced cost by being continuously annealed with a tensile stress applied along the ribbon axis and by providing an amorphous magnetic alloy containing iron, cobalt and nickel and in which the portion of cobalt is less than about 4 at %.
1. A method of annealing a magnetic amorphous alloy article comprising the steps of:
(a) providing an unannealed amorphous alloy article having an alloy composition and a longitudinal axis;
(b) disposing said unannealed amorphous alloy article in a zone of elevated temperature while subjecting said amorphous alloy to a tensile stress σ along said longitudinal axis to produce an annealed article; and
(c) selecting said alloy composition to comprise at least an Fe content larger than about 30 at %, Ni, Si, B and at least one element from the group consisting of Mo and Nb so that dH k /dσ>0, where H k is an induced isotropy field, and so that df r /dH<1000 Hz/Oe, where f r is the resonant frequency of the annealed article and H is a magnetization field strength, and the annealed article has an induced magnetic easy plane perpendicular to said longitudinal axis due to a combination of said composition, said temperature and said tensile stress.
2. A method of making a marker for use in magnetomechanical electronic article surveillance system comprising the steps of:
(a) providing at least one unannealed amorphous alloy article having an alloy composition and a longitudinal axis;
(b) disposing said at least one unannealed amorphous alloy article in a zone of elevated temperature while subjecting said at least one amorphous alloy to a tensile stress σ along said longitudinal axis to produce at least one annealed article;
(c) selecting said alloy composition to comprise at least an Fe content larger than about 30 at %, Ni, Si, B and at least one element from the group consisting of Mo and Nb so that dH k /dσ>0, where H k is an induced isotropy field, and so that df r /dH<1000 Hz/Oe, where f r is the resonant frequency of the annealed article and H is a magnetization field strength, and said at least one annealed article has an induced magnetic easy plane perpendicular to said longitudinal axis due to a combination of said composition, said temperature and said tensile stress;
(d) placing said at least one annealed article adjacent a magnetized ferromagnetic bias element which produces a bias magnetic field; and
(e) encapsulating said at least one annealed article and said bias element in a housing.
3. A method as claimed in claim 1 wherein step (c) comprises selecting said alloy composition to additionally comprise Cr.
4. A method as claimed in claim 1 wherein step (c) comprises selecting said alloy composition to comprise Co in an amount less than or equal to about 4 at %.
5. A method as claimed in claim 2 wherein step (c) comprises selecting said alloy composition to additionally comprise Cr.
6. A method as claimed in claim 2 wherein step (c) comprises selecting said alloy composition to comprise Co in an amount less than or equal to about 4 at %.
7. A method as claimed in claim 1 comprising applying a magnetic field to said amorphous alloy article in a direction perpendicular to the longitudinal axis during step (b).
8. A method as claimed in claim 2 comprising applying a magnetic field to said amorphous alloy article in a direction perpendicular to the longitudinal axis during step (b).
9. A method as claimed in claim 7 wherein said amorphous alloy article has an article plane and comprising applying said magnetic field with a magnitude of at least 2 kOe and a significant component perpendicular to the article plane.
10. A method as claimed in claim 8 wherein said at least one amorphous alloy article has an article plane and comprising applying said magnetic field with a magnitude of at least 2 kOe and a significant component perpendicular to the article plane.