Multi-layer magnetic tag

A magnetic marker or tag which comprises (a) a first magnetic material characterised by high permeability, low coercivity and a non-linear B-H characteristic;

• and (b) a second magnetic material which is capable of being permanently magnetised by conventional magnetic recording techniques, said first and second magnetic materials being superimposed one on the other, characterised in that selected regions of said second magnetic material are magnetised to carry magnetisation in the form of an erasable pattern of repeatedly reversing polarity, said magnetisation being such as to couple magnetically with contiguous regions of said first magnetic material so as to magnetically divide the first magnetic material into a series of magnetically discrete zones, the length of said magnetically discrete zones and/or the lengths of said magnetised regions (“gaps”) of said second material constituting elements of a code.