GB2042979A

GB2042979A - Improvements in or Relating to Magnetic Recording

Info

Publication number: GB2042979A
Application number: GB7906573A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1979-02-23
Filing date: 1979-02-23
Publication date: 1980-10-01
Also published as: GB2042979B

Abstract

A security document includes a magnetic record comprised of anistropic magnetic particles, which in each of at least two sets of spaced regions are aligned along a different preset direction, and in a remaining set of regions are not so aligned or are aligned along a further preset direction. <IMAGE>

Description

SPECIFICATION Improvements Relating to Magnetic Recording This invention relates to magnetically recorded security documents, cards or labels, and it relates especially, although not exclusively to the recording of digitally encoded data.

The increasing use of security documents, or devices, including keys, identity passes, passports, credit cards or tokens etc., has led to the introduction of sophisticated techniques to prevent their fraudulent misuse.

In particular British Patent Specification No.

1,331,604 describes a security document formed of a layer of anisotropic magnetic particles which in spaced regions of the layer are fixedly aligned along a-predetermined direction, and in the remaining regions are not so aligned or are aligned along a substantially different direction.

By uniaxially magnetising, or "developing", the aligned particles the pattern of remanent magnetisation which results is permanent, and can only be destroyed if the document is destroyed also.

Such a pattern, therefore, which is sometimes referred to as a magnetic "watermark", or as forming a "permanent magnetic structure", may be used as a permanent identification or verification code, since the regions of differently aligned particles may be assigned different significance, e.g. a binary "1" or "O".

The structured document may be passed over a suitable reading device, for example a vertical or horizontal magneto resistive (MR) head, or a gapped inductive head.

Alternatively, the recorded data may be retrieved by means of a hand held scanning device, or a magnetic "wand". Since the data is recorded at only two levels of remanent magnetisation, however, the detected signal is dependent upon the reading rate, which particularly in the case of a hand held device may be irregular.

Especially when the density of recorded data is high therefore, it is difficult, without recourse to expensive external clocking techniques, to recover the encoded information with accuracy, and, for example, it may not be possible to retrieve with cetainty all the "1's" and "O's" of a binary signal, unless the rate of reading the information is carefully controlled or monitored.

Since, however, the use of a hand held device or a portable reader, may be particularly advantageous for reading coded packages or labels, for example, a self clocking magnetic "watermark" would be particularly useful.

It is an object of the present invention, therefore, to provide an improved form of magnetic security record, having a permanent magnetic structure, and containing information or data retrievable in a form which is independent of the rate at which it is read.

According to one aspect of the invention there is provided a secure magnetic record formed of a layer of anisotropic magnetic particles so that in each of at least two sets of spaced regions the easy axis of magnetisation of the particles is fixedly aligned along a different predetermined direction, and in a remaining set of regions of said layer is not so aligned or is aligned along a further predetermined direction.

In a particularly useful arrangement there may be three sets of regions, and preferably the predetermined directions of two of the sets of regions may be orthogonally inclined in the plane of the layer. Preferably the predetermined direction of the third set of regions may be inclined at an angle of between 250 and 650 to either of the two orthogonally inclined directions.

Advantageously, the predetermined direction of the third set of regions may be inclined at an angle of substantially 300 to one of the said orthogonally inclined directions, and in this way the particles of the third set of regions produce a remanent magnetisation halfway between that due to the particles of the other two sets of regions.

Preferably a region of said third set may separate each pair of regions from either or both of the other sets, so that the resultant magnetisation always returns to a base line, i.e. a "return to zero", (RZ) level, following a maximum (a binary "1", say), or a minimum (a binary "0", say) departure in a neighbouring region.

Consequently, irrespective of the rate at which the remanent magnetisation is sensed, all the information can be identified and received i.e. the record is self clocking.

The magnetic record may be formed of particles of y-ferric oxide, chromium dioxide, barium ferrite, or metal powders.

According to a further aspect of the invention there is provided a method of preparing a secure magnetic record including the steps of, supporting a fluid layer of anisotropic magnetic particles dispersed in a binder, and before the binder sets applying a uniform magnetic field to align the easy axis of magnetisation of the particles along a predetermined direction, and in each of at least two sets of spaced regions applying a further uniform magnetic field to realign the particles along a different predetermined direction, and permitting or causing the layer to set.

In a particularly useful arrangement there may be three sets of regions. The predetermined direction of two of the sets of regions may be orthogonally inclined. Preferably the predetermined direction in the third set of regions may be inclined at an angle of between 250 and 650 to either of the two orthogonally inclined directions.

In order that the invention may be more clearly understood, a particular embodiment thereof will be described with reference to the accompanying drawings of which, Figure 1 shows a view of a security document including a secure magnetic record of this invention, Figure 2a shows, schematically, the relative alignments of magnetic particles within the secure magnetic record, Figure 2b shows the pattern of remanent magnetisation due to the record of Figure 2a, Figure 2c shows the pattern of remanent magnetisation detected using a horizontal MR head, Figure 2d shows the rate of change of remanent magnetisation, dBr/dt, detected using either a vertical MR head or a gapped inductive head, and Figure 3 shows an arrangement for producing the secure magnetic record of this invention.

Referring to Figure 1, a secure magnetic record in the form of a strip, 10, formed on a thin plastics support web, is secured to a stiffer plastics support, 1, by hot blocking, for example.

Additional information, for example a temporary magnetic record, 14, or eye visible markings, 15, may also be included on the support, 1, if necessary. In the particular example illustrated, anisotropic particles of y-ferric oxide, chromium dioxide, barium ferrite or metal powders, for example, are aligned along one of three directions indicated by the arrows in regions 11, 12 and 13, although the invention is not limited to this particular choice of orientations.

The particles in region, 11, therefore, are aligned across the strip, 10, those of region, 12, are aligned at 300 to those of region, 11, and the particles of region, 13, are aligned along the strip, 10. The relative dispositions of each type of region can be chosen in accordance with any desired code.

For a developing field along the strip the pattern of remanent magnetisation resulting from such an alignment of particles is shown in Figure 2b, a schematic view of the corresponding directions of particle alignment being shown in Figure 2a. The high level of remanent magnetisation, due to particles aligned along the strip could for example, be arranged to represent a binary "1", as indicated in Figure 2a, and conversely the low level of remanent magnetisation, due to particles aligned across the strip, could represent a binary "0". It should be noted, however, that between each high and low level there is an intermediate "return to zero" (RZ) level, due to the regions of particles aligned at 600 to the length of the strip. These intermediate levels are indicated at Z in Figures 2b and 2c.

A convenient arrangement for preparing a record of this kind is described below.

A thin plastics support web, indicated at, 30, in Figure 3 may be coated with a dispersion of the magnetic material in a binder and a solvent therefor. Initially, of course, the particles are randomly arranged, but as illustrated in Figure 3, the support web is passed between the poles of a permanent magnet, 31, causing the particles to be aligned across the web. Before the binder sets, the web is passed over a further controlled electromagnet, 32, arranged to realign the particles along the web. If a maximum field is applied, the particles are fully realigned (i.e. to lie along the strip) as indicated at region, 33; however, if an intermediate field is used this can be of a strength sufficient to only partially realign the particles so that they lie at 600 to the length of the web, as indicated at region, 34.This field may be controlled by suitable control means in accordance with a desired code, and the relative alignments of the pole pieces can be adapted to suit a particular situation.

The web may then be passed through an oven to set the binder.

Figures 2c and 2d illustrate, respectively, the detected remanence pattern, due to the alignments shown in Figure 2a, obtained using a horizontal magneto-resistive head and a gapped inductive head respectively.

Because of the "return to zero" level shown at Z, in Figure 2c the detected waveform is readily decoded by standard techniques, and similarly the RZ signals of dBr/dt shown in Figure 2d, may also be readily decoded by integration to give the form of Figure 2c, or by detection of the cross-over points of the signal.

The three level magnetic "watermark" described, provides a means for recording data, possibly coded, which may be read without recourse to external clocking techniques, and in this respect may be convenient for use with a portable or hand held reading device.

Claims

1. A secure magnetic record comprising a layer of anistropic magnetic particles, dispersed in and fixedly retained by a binder, the easy axes of magnetisation of said particles in each of at least two sets of spaced regions being aligned along a different preset direction, and in a remaining set of regions being not so aligned or aligned along a further preset direction.

2. A secure magnetic record according to Claim 1 wherein there are three sets of regions.

3. A secure magnetic record according to Claim 2 wherein the preset directions in two of the sets of regions are orthogonally inclined in the plane of the layer.

4. A secure magnetic record according to Claim 3 wherein the preset direction in the third set of regions is inclined at an angle of between 250 and 650 to either of the two orthogonally inclined directions.

5. A secure magnetic record according to claims 3 or 4 wherein a region of the third set separates each pair of regions from either or both of the other sets.

6. A secure magnetic record according to any one of Claims 1 to 5 wherein the magnetic particles are of y-ferric oxide, or chromium dioxide or barium ferrite or a metal powder.

7. A secure magnetic record as hereinbefore described by reference to and as illustrated in the accompanying drawings.

8. A method of preparing a secure magnetic record comprising the steps of, providing a fluid layer of anisotropic magnetic particles dispersed in a binder, and before the binder sets applying a first substantially uniform magnetic field to align the easy axes of magnetisation of the particles along a preset direction, and in each of at least two sets of spaced regions of the layer applying a further substantially uniform magnetic field to realign the particle along a different preset direction, and permitting or causing the layer to set.

9. A method according to Claim 8 wherein the further magnetic field is aligned at an angle of between 250 and 650 to said first magnetic field.

10. A method substantially as hereinbefore described.