In previous patent applications, in particular PCT/GB96/00823 (WO 96/31790) and PCT/GB96/00367 (WO 97/04338), we have described and claimed novel techniques for spatial magnetic interrogation and novel tags. The technology described in WO 96/31790 is based on exploiting the behaviour of magnetic materials as they pass through a region of space containing a magnetic null. In particular, these earlier applications describe, inter alia, how passive tags containing one or more magnetic elements can perform as remotely-readable data carriers, the number and spatial arrangement of the elements representing information.

In the above applications we described a number of possible system embodiments employing either permanent magnets or electromagnets to create the magnetic null.

We also described several system implementations some of which are particularly appropriate for tags employing very low coercivity, high permeability magnetic elements. These implementations work by detecting harmonics of a superimposed low-amplitude alternating interrogation field.

In a later application, GB9612831.9, and its successor PCT/GB97/01662 (W097/48990), we describe arrangements which work by detecting the baseband signals generated by the passage of the tag through the magnetic null,

without the need for any superimposed alternating interrogation field. Specific designs for readers are described in PCT/GB97/02772. The content of these above-mentioned patent applications (hereinafter termed "the prior FN applications) is incorporated herein by reference thereto.

The present invention utilises techniques and materials such as described in the prior FN applications, or components of such techniques and materials, in conjunction with the known technique of magnetic watermarking (termed simply"watermarking" hereinafter).

Watermarking is based upon the formation, within a document, of a permanent magnetically readable element formed (e. g. by deposition) of magnetisable particles of predetermined shape (e. g. acicular) in a predetermined orientation or in several spaced orientations. Thus certain regions within the magnetic material may have zones comprising acicular magnetic particles aligned in one given direction; and other zones comprising acicular magnetic particles aligned in another given direction; and yet other zones comprising acicular magnetic particles in random orientations. In this way, a variety of magnetic patterns may be generated. These patterns can represent information which, by virtue of the watermark being incorporated into the document during its manufacture, can be regarded as quasi-permanent. Watermarking is described, for example, in British Patent Specifications Nos. 1,331,604; 1,519,142; 1,529,937; 1,529,938 and 1,529,939.

GB 1,529,938 describes a magnetic recording medium in which the magnetic watermark layer has, overlying it, a

second magnetic layer which comprises a magnetically anisotropic material, the easy axis of magnetisation of which has a permanent predominant alignment. As disclosed, the overlying layer consists of magnetic particles, e. g. gamma-Fe203 particles, in a binder; the coercivity of this material is preferably 200-250 Oersted. The easy axis of magnetisation of the overlying layer is preferably perpendicular to that or those of regions making up the watermark layer.

We have now found, surprisingly, that if a document containing a magnetic watermark has, in addition, a layer (or at least regions of) a further magnetic material of low coercivity deposited over the watermark, either during the manufacture of the watermark or afterwards, the composite magnetic structure may be susceptible to interrogation in the manner proposed in the prior FN applications.

Accordingly, the present invention provides a document which incorporates (1) a magnetic watermark and (2) a further magnetically anisotropic material in proximity to the magnetic watermark, said further magnetic material being in the form of a continuous layer of a metallic material having a coercivity of 10 Oersted or less.

Said further magnetic material is advantageously a thin film magnetic material. Preferably, the continuous layer of magnetic material is not more than 10 microns in thickness, and more preferably is not more than one micron in thickness. The coercivity of said further magnetic material is preferably less than 2 Oersted, and typically will be in the range of 0.1 to 1.0 Oersted.

Said further magnetic material may be an amorphous metal glass or a magnetic alloy, e. g. permalloy.

The further magnetic has low coercivity-i. e. is a soft magnetic material. Additionally, it is anisotropic, and possesses an easy axis of magnetisation. This may be a shape-dependant property and/or a material dependant property.

Preferably, the document of this invention incorporate a magnetic watermark which is overlaid with a layer of a soft magnetic material such as is currently used in magnetic EAS tags. This will generally be a thin film material which can easily be magnetised along one axis, e. g. its major axis, but which cannot effectively be magnetised in a direction through the thickness of the material. Such a material may be deposited directly over the magnetic watermark or there may be intervening material between the two. Also, the further magnetic material may be coextensive with the watermark; or it may be smaller than, but contained wholly with the area of, the watermark; or it may have a part which is superjacent with respect to the watermark and another part which lies outside the area of the watermark.

A document in accordance with this invention may be a security document, e. g. a banknote, cheque, bond, credit card, share certificate or security key.