Magnetic Tagging Techniques

A method of writing information onto a magnetic tag flowing within a microfluidic channel, the said tag having a plurality of magnetically switchable regions for storing said information, said regions being separately switchable at different strengths of an external applied field, the method comprising either:

• i) controlling said external applied field to set each said region in turn into a bistable magnetic state, in order of decreasing external applied field strength;
• or
• ii) applying a set of external applied fields to said tag, each field of said set of applied fields comprising a field of sufficient strength to switch ones of said regions having switching strengths equal to or less than the field strength, said applying comprising applying said fields in turn from a strongest field to a weakest field, whereby after each said field application one or more of said regions is switched to a magnetic state which is substantially unaffected by subsequently applied fields of the set.

Apparatus for writing information onto a magnetic tag having a plurality of magnetically switchable regions for storing said information, said regions being separately switchable at different strengths of an external applied field, the apparatus comprising either:

• i) means for controlling said external applied field to set each said region in turn into a said bistable magnetic state, in order of decreasing external applied field strength;
• or
• ii) means for applying a set of external applied fields to said tag, each field of said set of applied fields comprising a field of sufficient strength to switch ones of said regions having switching strengths equal to or less than the field strength, said applying comprising applying said fields in turn from a strongest field to a weakest field, whereby after each said field application one or more of said regions is switched to a magnetic state which is substantially unaffected by subsequently applied fields of the set.

Apparatus for reading information from a magnetic tag having a plurality of magnetically switchable regions storing said information, said regions being separately switchable at different strengths of an external applied field, the apparatus comprising:

• means for applying an external applied field to said plurality of switchable regions simultaneously in increasing strength from a strength below a smallest switching strength of a said region to above a greatest switching strength of a said region;
• and
• means for monitoring switching of said regions as said field is increased by monitoring the magnetic moment of said tag.

A miniature planar magnetic tag for tagging a chemical or biological molecule or moiety or a biological entity, the tag being configured to store a plurality of bits of binary information, the tag comprising:

• a plurality of physically separate regions of switchable magnetic material;
• and
• wherein said switchable magnetic material regions are bistably switchable between at least two different magnetised states by means of an external applied field.