The invention is principally (but not exclusively) concerned with such magnetic materials.

Extrinsic magnetic properties of magnetic elements are determined not just by the nature of the magnetic material, but also by their shape. In particular, permeability is directly related to the aspect ratio of the element (i. e. the ratio of its length to its cross sectional area), and this typically needs to be greater than several thousand to one if the potential performance of high intrinsic permeability materials is not to be significantly compromised.

High aspect ratio is usually achieved by forming magnetic elements into long thin strips, fine wires or patches made from very thin film material. Suitable small cross-section strip and wire materials for retail security and other applications are commercially available from Vacuumschmeltze (Germany), Allied Signal Corporation (USA) and Unitika (Japan). Typical lengths required to give a permeability sufficiently high for retail security applications are 30-60 mm. Thin film

material in the form of a 0.8 micron thin film sputter-deposited coating on a 23 micron PET backing is commercially available from IST (Belgium). Thin film material of the type manufactured by IST may be cut into very small patches, 5 square mm or less, and still achieve an extrinsic permeability of several thousand.

This makes the material very suitable for compact, multi-element tags.

For certain applications in retail security and multi-element magnetic tagging, it is necessary to place one or more patches of thin film material onto a carrier or substrate. At present this is done by cutting a thin film coated PET into the desired form, and bonding it into place. This process involves a number of processing steps, and can lead to a composite label which is excessively thick for some applications.

For some other applications where multi-element tags are required, the desired format can be achieved by chemically removing thin film material from selected areas of a continuous layer. This is similar to processes used in the printed circuit board industry.

It has the advantage that no carrier layer is required.

However, the chemical process involves aggressive chemicals (e. g. ferric chloride) and the effluent produced is toxic and expensive to dispose of.

Hot foil transfer is a process routinely used in the printing industry. It is also commonly employed in the manufacture of holograms. In essence, a thin plastic backing layer, usually in web form, is coated with a thermally-softenable release layer and a bonding layer.

Metal (frequently aluminium) is then vacuum deposited onto the bonding layer, and subsequently overcoated with an adhesive layer. To transfer the decorative

metal film onto a substrate (e. g. a document), a suitably-shaped heated die is used to stamp the metal film against the substrate. The die breaks out a correspondingly-shaped segment of the metal layer, and the heat releases it from the plastic carrier film, and bonds it down to the substrate.

Cold foil transfer is used as an alternative to hot foil transfer and is widely used for adding foil features to printed packaging and the like. The main difference from hot foil transfer is that non of the layers are heat-sensitive; and the tool used to effect transfer is not heated. Typically, the metal layer is deposited onto a backing layer to which it bonds only weakly; and the metal layer (foil) is coated with a pressure-sensitive adhesive. Tool pressure applied to the back of the carrier layer (again, typically PET) breaks out a segment of the metal foil, which is caused to bond to a substrate through the adhesive which has been activated by the pressure from the cold tool.

Rolling is sometimes used to transfer lines or bands of foil instead of stamping, In this case, the tool is a roller of appropriate width. Rolling is normally carried out as a cold process, but could in principle also be used as a hot-transfer process.

Despite the widespread use of such techniques in the printing industry, where the transfer effected is that of a thin metal layer, there have been no suggestions hitherto (so far as we are aware) that such a technique could successfully be employed when the material to be transferred is an amorphous metal alloy, e. g. an amorphous metal glass, or a similar material having magnetic properties.

The invention The present invention combines technology of the kind already employed for transfer printing, e. g. hot-foil transfer printing and cold-transfer processes such as rolling and stamping, with the technique of magnetic thin film deposition to produce a thin film magnetic layer which can be transferred in discrete elements from a backing layer onto a carrier or substrate.

Accordingly, in one aspect the invention provides a transfer web or tape for transferring discrete areas of a magnetic film from the tape to a substrate, characterised in that the web or tape comprises: (i) a backing layer; (ii) a release layer carried on one face of said backing layer; (iii) a bonding layer overlying said release layer; (iv) a magnetic layer overlying said bonding layer; and an an adhesive layer overlying said magnetic layer.

Advantageously, the magnetic layer is in the form of a thin film of a high permeability, low coercivity amorphous magnetic alloy.

The particular choice of material for the release layer, the bonding layer and the adhesive layer will be determined by the nature of the transfer process intended, i. e. whether a hot-or cold-transfer process is to be used. Materials suitable for both types of transfer process are well known.

Conveniently, the backing layer is a polyethylene

terephthalate film, preferably of a thickness in the range from 10 to 40 microns.

According to another aspect of the invention, there is provided a method of depositing a discrete area of an amorphous magnetic material onto a substrate, which comprises transferring the magnetic material by a transfer process from a web or tape as defined above onto the substrate.

In the present invention a backing layer (typically a plastics material) is coated first with a release layer on which is superimposed a bonding layer, and the resulting intermediate is sputter coated with a thin film of high permeability, low coercivity amorphous magnetic alloy. After sputter-coating, a layer of an adhesive (typically a heat-sensitive adhesive) is coated onto the sputtered metal layer. The resultant article is then ready for use as a transfer web in a hot stamping process, whereby elements of magnetic film of preselected shape, number and mutual disposition may be transferred from the article onto a final substrate in the desired pattern by using a heated stamping die or dies.

Compared to existing processes, this method produces a very small increase in thickness (typically c. 1 micron) of the substrate, achieves good material utilisation, and avoids the production of toxic effluents.

Figures 1-4 of the accompanying drawings illustrate the process of this invention.

Referring now to the drawings, a polyethylene terephthalate backing film 1 is first coated in a

manner known per se with a conventional heat-sensitive release layer 2. Next, a bonding layer 3 is formed over the release layer 2 in a manner known per se. The resulting intermediate (comprising layers 1,2 and 3) is then used in a sputter coating machine (not shown) to deposit a layer of an amorphous magnetic metal alloy 4 over the bonding layer 3. Sputter coating machines and their operation are well known and do not need to be described in detail herein. The sputter-coated pre- web thereby formed is next coated with a heat-sensitive adhesive layer 5 which bonds to and overcoats the magnetic layer 4 to generate thereby a transfer web 6.

Web 6 is then placed over a selected substrate 8 at a controlled separation therefrom (as indicated by air gap 7 in Figure 2) and then a heated stamping die 10 of suitable shape and size is used to bring web 6 and substrate 8 together. The heat and pressure causes bonding layer 3 to separate from release layer 2, and simultaneously activates the heat-sensitive adhesive 5 which thus bonds to substrate 8. Thus die-shaped metal film fragments 9 are transfer-bonded to the surface of substrate 8, as shown in Figures 3 and 4. It will be evident that the shape, size and number of fragments or patches transferred to substrate 8 can be selected at will; it is only necessary to use a suitable stamping die, or two or more dies, to achieve the desired result.

Where the invention utilises a cold-transfer process, e. g. rolling or stamping, the transfer process is effective in substantially the same manner as described above, except that the adhesive layer will be a pressure-activated material instead of a heat-activated material; similarly, the release layer will be a pressure-sensitive release material.

A thermally transferable magnetic film such as described above constitutes one aspect of this invention. The process for producing a magnetic film bonded to a substrate via such a thermally transferable magnetic film constitutes another embodiment of this invention. In both of these embodiments, it is possible to make routine modifications without departing from the scope of the invention.