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Magnetic tags or markers

  • US 6,371,379 B1
  • Filed: 10/09/1998
  • Issued: 04/16/2002
  • Est. Priority Date: 07/17/1995
  • Status: Expired due to Fees
First Claim
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1. A magnetic marker or tag, which comprises:

  • a first layer of magnetic material characterised by high permeability, low coercivity and a non-linear B-H characteristic;

    wherein said first layer of magnetic material is coated with a second layer of magnetic material which is capable of being permanently magnetised, so as to have a non-uniform magnetic field pattern, said second layer of magnetic material comprising at least three discrete regions of magnetic bias material, wherein each of said discrete regions exhibits a different level and direction, such that during interrogation by a constant frequency alternating magnetic field, the magnetic bias levels of each of said discrete regions are overcome at different times in the interrogation cycle.

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