Article surveillance magnetic marker having an hysteresis loop with large barkhausen discontinuities at a low field threshold level
AssigneeSensormatic Electronics Corporation
InventorsWing K. Ho, Jiro Yamasaki
Life of this patent
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Abstract
A harmonic-type EAS marker includes a wire segment formed of cobalt alloy. To form the wire segment, the cobalt alloy is cast as an amorphous wire, die-drawn to a smaller diameter, and then annealed with application of longitudinal tension. The annealed wire is cut to produce wire segments which have a magnetic hysteresis loop with a large Barkhausen discontinuity at a lower threshold level than has previously been achieved.
Claims36
A marker for use in an article surveillance system in which an alternating magnetic field is established in a surveillance region and an alarm is activated when a predetermined perturbation to said field is detected, said marker comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that exposure of said body to an external magnetic field, whose field strength in the direction opposing the magnetic polarization of said body exceeds a predetermined threshold value, results in regenerative reversal of said magnetic polarization, and means for securing said body to an article to be maintained under surveillance, characterized in that said predetermined threshold level is less than 0.04 Oe.
A marker for use in an article surveillance system in which an alternating magnetic field is established in a surveillance region and an alarm is activated when a predetermined perturbation to said field is detected, said marker comprising a body of magnetic material characterized in that the magnetic polarity thereof commences and completes reversal when the magnitude of strength of said field attains a given value, without need for increase in field strength above said given value;
- further characterized in that said given value is less than 0.04 Oe.
A method of forming a magnetic material, comprising the steps of:
- casting a metal alloy to form an amorphous metal wire which exhibits negative magnetostriction;
- die-drawing said wire to reduce a diameter thereof;
- and
- annealing the drawn wire while applying longitudinal tension to the drawn wire.
A marker for use in an article surveillance system in which an alternating magnetic field is established in a surveillance region and an alarm is activated when a predetermined perturbation to said field is detected, said marker comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that exposure of said body to an external magnetic field, whose field strength in the direction opposing the magnetic polarization of said body exceeds a predetermined threshold value, results in regenerative reversal of said magnetic polarization, and means for securing said body to an article to be maintained under surveillance;
- characterized in that said body of magnetic material is formed by casting a metal alloy to form an amorphous metal wire which exhibits negative magnetostriction, die-drawing the cast wire to reduce a diameter thereof, annealing the drawn wire while applying longitudinal tension to the drawn wire, and cutting the annealed wire to form a discrete wire segment.
An article surveillance system for detection of the presence of an article in an interrogation zone comprising:
- a. generator means for generating an incident alternating magnetic field within an interrogation zone;
- b. a marker secured to an article, said marker comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that upon exposure of said body to an external magnetic field, whose field strength in the direction opposing the instantaneous magnetic polarization of said body exceeds a predetermined threshold value, there results a regenerative reversal of said magnetic polarization, said marker characterized in that said predetermined threshold level is less than 0.04 Oe;
- and
- c. receiving means for detecting perturbations of the magnetic field in said interrogation zone when said marker is present in said interrogation zone.
A method of forming a magnetic material, comprising the steps of:
- casting a metal alloy to form an amorphous metal wire which exhibits negative magnetostriction;
- processing said wire to form longitudinal compressive stress in said wire;
- and
- annealing the processed wire to relieve some of the longitudinal compressive stress.
Dependencies are inferred from the claim text.