US3878367A

US3878367A - Magnetic security document and method for making same

Info

Publication number: US3878367A
Application number: US356603A
Authority: US
Inventors: Richard E Fayling; Douglas D Campbell
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1973-05-02
Filing date: 1973-05-02
Publication date: 1975-04-15
Anticipated expiration: 1992-04-15
Also published as: DE2421469A1; DE2421469C2; JPS5015511A; FR2228265A1; FR2228265B1; GB1466329A; IT1011317B; CA1031858A

Abstract

A security document having a magnetic recording layer containing uniformly dispersed magnetizable material having magnetic anisotropy wherein the material at a plurality of selected locations is differently physically aligned with respect to a reference location to provide a magnetically detectable permanent fixed information pattern such as a code pattern useful for authenticating the document. The document is authenticated in the following manner. A magnetic field is applied to the document to cause the material within the plurality of selected locations to be differently magnetized according to the physical alignment thereof. The differently magnetized selected locations are subsequently sensed by traversing the document past a sensor device to provide a signal representative of the plurality of selected locations. This signal is compared with a predetermined signal pattern to authenticate the document. A method for making such a security document includes, providing a sheet having a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are temporarily free to rotate, physically aligning the magnetizable particles at selected locations by applying magnetic fields to form a fixed information pattern; and then immobilizing the particles to provide a permanent magnetically detectable fixed information pattern. Visible indicia characteristic of an intended use of the document, are applied to the sheet.

Description

United States Patent 1191 Fayling et al.

1 MAGNETIC SECURITY DOCUMENT AND METHOD FOR MAKING SAME 175] Inventors: Richard E. Fayling. White Bear Lake; Douglas D. Campbell. Minneapolis, both of Minn.

Minnesota Mining and Manufacturing Company, St. Paul. Minn.

1221 Filed: May 2. 1973 21 App1.No.:356,603

[73] Assignee:

[52] 1.1.5. Cl.235/6l.l2 M; 235/6l.1l D; 346/74 M;

179/1002 CH; 235/6l.l2 M, 61.12 R. 61.11 D. 61.7 B; 346/74 M; 360/56. 131. 134

[56] References Cited UN1TED STATES PATENTS 2.796.359 6/1957 Speed 360/131 3.052.567 9/1962 Gabor 360/131 3.092.813 6/1963 Broadbcnt 340/174 CB 3.117.065 1/1964 Wootten 235/6l.12 M

3.441.884 4/1969 Eppe 360/131 3.460.116 8/1969 Bobeck 340/174 CB 3.471.862 10/1969 Barney... 346/74 3.503.054 3/1970 Bobeck... 340/174 CB 3.508.215 4/1970 Cohler.... 235/61.l2 M 3.531.627 9/1970 Ham 235/6l.l2 M 3.588.771 6/1971 Martin 360/131 3.643.064 2/1972 Hudson.. 235/6l.7 B 3 641. 16 2/1972 Nagata 235/6112 M 14 1 Apr. 15, 1975 Primary ExaminerDaryl W. Cook Assistant Examiner-Robert M. Kilgore Attorney. Agent, or FirmA1exander. Sell. Stcldt & DeLaHunt 1 57] ABSTRACT A security document having a magnetic recording layer containing uniformly dispersed magnetizable material having magnetic anisotropy wherein the material at a plurality of selected locations is differently physically aligned with respect to a reference location to provide a magnetically detectable permanent fixed information pattern such as a code pattern useful for authenticating the document. The document is authenticated in the following manner. A magnetic field is applied to the document to cause the material within the plurality of selected locations to be differently magnetized according to the physical alignment thereof. The differently magnetized selected locations are subsequently sensed by traversing the document past a sensor device to provide a signal representative of the plurality of selected locations. This signal is compared with a predetermined signal pattern to authenticate the document.

A method for making such a security document includes. providing a sheet having a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are temporarily free to rotate. physically aligning the magnetizable particles at selected locations by applying magnetic fields to form a fixed information pattern; and then immobilizing the particles to provide a permanent magnetically detectable fixed information pattern. Visible indicia characteristic of an intended use of the document, are applied to the sheet.

12 Claims, 5 Drawing Figures t/O/VE5 /Z 7 MAGNETIC SECURITY DOCUMENT AND METHOD FOR MAKING SAME CROSS REFERENCE TO RELATED APPLICATIONS This application is related to the copending applicalions of Richard L. Miklos and Jack E. Blackburn. entitled Method of Making a Magnetic Record Medium for Use In Information Processing System. U.S. Ser. No. 356.604. and Magnetic Record Medium and Information Processing System U.S. Ser. No. 356.605 and to the application of Richard E. Fayling entitled Magnetic Record Medium Authentication System. U.S. Ser. No. 356.602. all of which applications were filed on May 2. I973 and are assigned to the same assignee as this application.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention pertains to magnetic recording. and especially to the adaptation of magnetic recording techniques to provide a security document having a permanent magnetically detectable fixed information pattern. such as a code pattern. useful for authentication of the document in particular and for data processing in general.

2. Description of the Prior Art The expanded use of credit cards. airline tickets. stock certificates and like security documents has created a great need for documents which can be readily authenticated and which cannot be easily altered or counterfeited.

As used herein. a security document is a type of record medium which contains at least two forms of information: visible indicia relating to the intended use of the document; and a permanent fixed information pattern. such as a code pattern. usually concealed and difficult to reproduce so as to inhibit or prevent counterfeiting. Many such security documents include mag netic recording material to enable recording of data for subsequent machine processing. The magnetic material may be employed for conveying temporary information as well as document authentication and/or other fixed information. A multi-purpose magnetic record medium security document. which is disclosed in U.S. Pat. No. 3.566.356. contains a layer or layers of a composite of magnetizable material having particular hysteretic response characteristics. Authentication of the document is accomplished by comparing the signal response produced upon subjecting the document to a demagnetizing field with a predetermined range of signal values typical of the particular hysteretic response characteristics of the material. Temporary data may be recorded in the magnetic material according to conventional magnetic recording techniques. Such document. however. requires the use of specially prepared magnetic materials.

Other multi-layer magnetic recording media. which are disclosed in U.S. Pat. Nos. 3.052.567. 3.2l9.353 and 3.328.I95 contain multiple layers designed, to respond to different frequency ranges or to provide easily erased temporary information on one layer and more difficultly erased fixed information on another layer. Although not suggested by the prior art. such media could be used for security documents. wherein authentication would be effected by observing the presence of a particular fixed information pattern such as a code pattern. recorded on the layer from which information is more difficultly erased. However. such authentication can be thwarted by the use of conventional record ing techniques to erase or alter such a recorded fixed information code pattern.

SUMMARY OF THE INVENTION The present invention provides a security document in which magnetically detectable fixed information. such as a code pattern. is permanently implanted in a manner precluding alteration of such fixed information by conventional recording techniques. The security document has visible indicia characteristic of an intended use. and includes a magnetic recording layer comprising uniformly dispersed magnetizable material having magnetic anisotropy. The magnetizable material at a plurality of selected locations is differently physically aligned with respect to the physical alignment of the magnetizable material at a reference location to provide the fixed information pattern.

The present invention also provides a method for making a security document having a magnetically dctectable permanent fixed information pattern. In this embodiment. a sheet is first provided which comprises a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are temporarily free to rotate. The magnetizable particles at selected locations defining a permanent fixed information code pattern in the layer are caused to be differently physically aligned from the physical alignment of the magnetizable particles at a reference location. after which the particles are permanently immobilized. Visible indicia characteristic of an intended use ofthe secu rity document are also applied to the sheet.

In a preferred embodiment. the different physical alignment ofthe particles at the plurality of selected locations in the layer is achieved by applying magnetic fields at such locations.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a three-dimensional view of a security document according to the present invention;

FIG. 2 is a cross-sectional view of a segment of a security document having a layer containing differently physically aligned acicular magnetizable particles;

FIG. 3 is a cross-sectional view of a segment of another security document having a layer containing magnetizable platelet particles sandwiched between inner printable layers and outer protective layers;

FIG. 4 is a three-dimensional illustration showing the formation of a security document according to the present invention; and

FIG. 5 is a cross section expanded schematic view illustrating the different physical alignment of magnetizable particles within a sheet to be used as a portion of a security document.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I shows a security document 10 which comprises a substrate 12, a recording layer I4 containing uniformly dispersed magnetically anisotropic magnetizable particles and an outer layer 15 upon which visible indicia l7 are presented. By uniformly dispersed. it is herein meant that the particle density. i.e. the number of particles per unit area. is approximately constant throughout the layer. even though the particles may be differently physically aligned at various locations. The particles within a plurality of selected locations 16 and I8 are differently physically aligned with respect to a reference location. which may. for example. be along an edge of the document 10. Double headed arrows such as at the selected locations [6 and [8 are used herein to indicate the easy direction of magnetization produced by the physical alignment of the magnetizable particles. Anisotropic particles are readily magnetized in either direction parallel to their easy direction of magnetization and retain a higher level of remanent magnetization after having been magnetized with a given applied field than is retained after having been magnetized with the same applied field in a direction other than the easy direction. Single headed arrows are used herein to depict various directions of magnetization or magnetic field.

in one embodiment. the magnetizable particles within those portions of the recording layer l4 which are not contained within the selected locations l6 and 18 may be unaligned. Alternatively. the magnetizable particles within all portions of the recording layer 14 not contained within the selected locations may be physically aligned to provide a single easy direction of magnetization such as indicated by the arrow 22. and the magnetizable material at more than two selected locations [6 and [8 in a region not to be used for re cording is differently aligned from the single direction.

Magnetic anisotropy in particulate magnetizable materials is most commonly associated with either shape anisotropy or crystalline anisotropy. For example. hexagonal ferrite materials such as barium ferrite are characterized by a high degree of crystalline anisotropy.

Such materials are readily available in the form of minute platelets in which the crystalline anisotropy results in an easy direction of magnetization normal to the plane of the platelets. The platelets are readily physically aligned by mechanical and/or magnetic forces to have the plane ofthe platelets parallel to the surface of the layer [4. In such an event. the easy direction of magnetization would then be as shown by the arrow 22. The selected locations 16 and I8 represent regions wherein the platelets are set on edge within the layer [4 and rotated in a desired direction as indicated by the double headed arrows. The selected

locations

16 and 18 may be uniformly spaced in the layer 14 to form a repetitive pattern which defines a permanent signal track from which a predetermined repetitive signal may be produced. Such a repetitive pattern is presented in the security document described in conjunction with figures 4 and S hereinafter. Alternatively. the selected locations l6 and [8 may be spaced at non-repetitive predetermined positions. with varying intervals therebetween or may have the particles therein physically aligned in different directions. as depicted in FIG. 1. A

predetermined non-repetitive signal may be produced.

representing the positions of the non-repetitive selected locations.

FIG. 2 is an enlarged cross section of a portion of anotehr security document 24 comprising a nonmagnetic backing 26. a recording layer 28 and a printable layer 29 upon which visible indicia are applied. The particles 30 within the recording layer 28 are further enlarged for graphic clarity. The layer 28 contains acicular particles of gamma-R2 0 uniformly dispersed within a flexible binder 31. The layer 28 is shown to have

background portions

32 and 34 wherein the particles 30 are uniformly physically aligned in one direction parallel to both the surface and to a long dimension of the document 24. Since shape anisotropy is paramount in gamma-R 0 particles. the easy direction of magnetization is parallel to the long dimension of the particles. The easy direction of magnetization in the

background portions

32 and 34 is. therefore. also parallel to both the surface and to a long dimension of the document 24. Portion 36 represents a selected location within which the particles 30 are further physically aligned. still parallel to the surface of the document 24. but also normal to the physically aligned particles within the

background portions

32 and 34. The delineation between the

portions

32 and 34 and the selected location 36 is shown for clarity as an abrupt transition in the direction of physical alignment of the particlesv Due to the normal divergence of magnetic flux. such a transition will generally extend over a distance dictated by the characteristics of the aligning magnetic field.

When the document 24 has applied thereto a conventional magnetic recording field applied along the long dimension of the document. i.e. along the direction of alignment of the particles within the

background portions

32 and 34, the particles within those portions will be readily magnetized and will retain a higher state of remanent magnetization than is produced within the selected location 36 where a direction other than the easy direction of magnetization is presented to the magnetizing field. Upon playback. a high amplitude signal will be produced corresponding to the

background portions

32 and 34 while a lower amplitude signal is produced corresponding to the selected location 36.

The magnetic recording layers used in the security documents such as shown in FIG. 2 may conveniently be a stripe of conventional magnetic recording media formed imbedded or affixed to a substrate such as a standard 30 mil. (0.76 mm) credit card stock. Such stock is readily obtained as 26 mil. (0.66 mm) thick sheets of wt. 71 polyvinyl chloride-5 wt. 94 polyvinyl acetate. If desired. the printable layer 29 may be eliminated by adding titanium dioxide pigment to the substrate composition to provide a printable surface. If further desired. an outer protective 2 mil. (0.05l mm) thick layer of 95 wt. k polyvinyl chloride5 wt. A polyvinyl acetate may be heat-fused to the pigmented layer after the visible indicia has been applied thereto. The magnetic recording layers are typically formed of a mixture of the magnetizable material and a nonmagnetic flexible organic binder together with a suitable solvent which are coated onto the substrate and subjected to appropriate aligning magnetic fields. In a typical case. such a coating comprises a uniform dispersion of 65 wt. 94 gamma-Fe- O acicular particles (typically 500 nm long and I00 nm in diameter) and 35 wt. Z thermoplastic polyurethane binder together with a suitable solvent. Other formulations may similarly be employed consistent with known magnetic recording media formulations.

FIG. 3 is an enlarged cross section of a portion of another security document 38 comprising a magnetic recording layer 40, printable layers 4! and 42. and protective layers 43. In this embodiment. the magnetizable layer 40 contains minute platelets 44 of barium ferrite uniformly dispersed within a flexible binder 45. The particles 44 within the layer 40 are further enlarged for graphic clarity. The particles 44 in the

background portions

46 and 48 are shown physically aligned parallel to the surface of the document 38 such that the easy direction of magnetization thereof is perpendicular to the plane of the document as shown by arrow 22 in FIG. I. The particles within the selected location 50 are shown physically aligned to have the particles turned on edge so that the easy direction of magnetization within the selected location 50 is parallel to the plane of the document 38. The greater anisotropy normally present in barium ferrite particles than is present in gamma-H O acicular particles and greater ease with which the particles become physically aligned results in an even higher difference in the remanent magnetization resulting from applying a uniform magnetic field upon the magnetic recording layer 40. As discussed in conjunction with FIG. 2. the delineation between the

background portions

46 and 48 and the selected location 50 may extend over a distance dictated by the characteristics of the aligning magnetic field.

FIG. 4 shows a method of making a security document such as depicted in FIGS. 1. 2 and 3, wherein a nonmagnetic backing 52 from a roll 5-! is passed beneath a coater 56 within which is a dispersion 57 of magnetizable anisotropic particles. binder and appropriate solvents. A coating 58 is thereby applied to the backing 52. A section of the coating 58 is then exposed to a magnetic field produced by an aligning device 60. In the embodiment shown. the aligning device 60 is conveniently a section of a premagnetized polymer based permanent magnet material containing barium ferrite platelets such as manufactured by Minnesota Mining and Manufacturing Company under the trade name "Plastiform." This permanent magnet material is described in L'.S. Pat. No. 2.999.275. Bands of oppositely magnetized

material

62 and 64 extend across the aligning device 60. When the coating 58 is stationary adjacent the aligning device 60. the magnetizable particles become physically aligned with the magnetic fields in a manner to be hereinafter further described. The aligning device 60 is thereafter removed to prevent smearing of the physically aligned particles as the backing 52 and coating 58 is then passed adjacent a heated bar 66 to heat the coating 58 and evaporate the solvent. thereby permanently immobilizing the magnetizable particles. The heated bar 66 is conveniently electrically heated in a conventional manner. If desired. the align ing device 60 may be left adjacent the backing 52 and coating 58 until the particles are permanently immobilized such as by heating. thereby preventing any inadvertent smearing of the physically aligned particles. After the particles are permanently immobilized. a printable layer 68 containing visible indicia 70 characteristic of an intended use ofthe document is affixed to the coating 58. The layer 68 may conveniently be a section of pressure-sensitive adhesive tape on which has been preprinted the desired indicia. Similar layers may be heat-fused or otherwise affixed as desired.

In one embodiment. a security document may convi ently be made in the following manner:

A 4 mil. (0.10 mm) thick sheet of Ti pigmented 95 wt. '14 polyvinyl chloridewt. 7? polyvinyl acetate is positioned adjacent an aligning magnetic field source such that appreciable flux from the field source extends through the sheet. A dispersion of 50 wt. 9i barium ferrite platelets (approximately 1 to 5 micrometers average diameter and a length to thickness ratio of approximately 7/] blended with 50 wt. 1 thrmosetting resin such as Adiprene L-l67 (E. l. Dupont DeNemours. lnc. isocyanate terminated urethane prepolymer). diamine curing agent and an appropriate catalyst is cast onto the sheet to a thickness of about 23 mil. (0.58 mm) and allowed to cure in the presence of the aligning field. thereby forming a layer having a uniform dispersion of the barium ferrite platelets selectively physically aligned along the direction ofthe flux lines of the field. With a binder of sufficient viscosity it is not necessary to leave the coating in the presence of the field during the remainder of the curing step. Since no substantial forces causing disalignment are normally present. the particles will tend to remain as physically aligned. A typical cure time of IO hours at 25C is required before the casting is fully cured. After the cure is complete. a top pigmented layer may be applied by coating a dispersion of TiO pigment in Adiprene L-l67 onto the layer to a thickness of about 1% to 2 mil. (0.0l2 to 0.050 mm). Visible indicia may then be printed on either surface following which a l mil. (0.025 mml transparent protective layer may be affixed to protect and prevent alteration ofthe printed indicia. The amount of barium ferrite in the layer may be as high as \\'t. while still allowing particle rotation to occur.

FIG. 5 illustrates an enlarged view of the physical alignment of magnetizable particles within a sheet 72 having a uniform dispersion of magnetically anisotropic particles within a flexible and unset binder such that the particles are free to rotate in response to an applied magnetic field.

The sheet 72 is positioned adjacent an orienting mag netic field source 76 which contains a repetitive pattern of alternating

magnetic field polarities

77 and 78. of sufficient field strength to cause rotation of the parti cles within the sheet 72 along the directions ofthe magnetic flux lines. The magnetic field source 76 is com eniently a section of premagnetized polymer based permanent magnetic material. such as described above. The particles within the sheet 72 are thus physically aligned along their easy direction of magnetization as shown by the double headed arrows along the lines of flux pressed therein. In the event that barium ferrite platelets are used. the plane of the platelets will. of course. be normal to the double headed arrows 74. After the particles in the sheet 72 are physically aligned. the binder is caused to set and thereafter permanently immobilize the particles in their respective physical alignments. The encoded sheet may then be assembled together with printable and protective layers to form a security document. If desired. regions of magnetization with relatively narrow boundaries between adjacent regions such as depicted in FIGS. 2 and 3 may be formed by reducing the spacing 73 between the magnetic field source 76 and the sheet 72. A still further reduction in boundary widths may be achieved by placing the sheet between matching magnetic field sources or by using a flux concentrating base plate.

A document such as described hereinabove is desirably used in an authentication system such as that described and claimed in the above cross-referenced patent application of Richard E. Fayling. entitled Magnetic Record Medium Authentication System. the disclosure of which is incorporated herein by reference. In the authentication described therein. the document is subjected to a magnetic field to cause the particles within the plurality of selected locations to be differently magnetized according to the physical alignment thereof. The differently magnetized selected locations are subsequently sensed by traversing the document past a sensor device to provide a signal representative of the selected locations. The signal is compared with predetermined signal pattern to authenticate the document.

What is claimed is:

l. A security document having visible indicia characteristic of an intended use thereof. which document includes a magnetic recording layer comprising uniformly dispersed magnetizable material having magnetic anisotropy. wherein the magnetizable material at a plurality of selected locations in the layer is differently physically aligned from the physical alignment of the magnetizable material at a reference location in the layer to provide a magnetically detectable permanent fixed information code pattern.

2. A security document according to claim I. wherein the magnetizable material is particulate barium ferrite.

3. A security document according to claim I. wherein the selected locations form a repetitive pattern defining a permanent signal track from which a predetermined repetitive signal may be produced.

4. A security document according to claim 3. wherein the repetitive pattern consists of adjacent selected locations within each of which the magnetizable material is differently physically aligned to define a continuously varying permanent signal track from which a predetermined continously varying signal may be produced.

5. A security document according to claim I. wherein the magnetizable material in the remainder of the layer outside said selected locations is physically aligned in a single direction and the magnetizable material at more than two selected locations in a region not to be used for recording is differently aligned from said single direction.

6. A security document according to claim 5. wherein each of said selected locations consists of a discrete section within which the magnetizable material is physically aligned perpendicular to the direction of physical alignment of the material in the remainder of the layer.

7. A security document according to claim I. wherein the selected locations form a non-repetitive pattern defining a permanent signal track from which a predetermined non-repetitive signal may be produced.

8. A method for making a security document having a magnetically detectable permanent fixed information code pattern comprising providing a sheet having a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are temporarily free to rotate.

causing the magnetizable particles at selected locations defining a permanent fixed information code pattern in the layer to be differently physically aligned from the physical alignment of the magne tizable particles at a reference location; and subsequently permanently immobilizing the particles. and

applying to said sheet visible indicia characteristic of an intended use of the security document.

9. A method according to claim 8. wherein the magnetizable particles are physically aligned at the selected locations by applying magnetic fields at the selected locations,

N]. A method according to claim 8. wherein said sheet is provided with a layer containing magnetically anisotropic magnetizable particles of barium ferrite.

11. A system for making a security document having a magnetically detectable permanent fixed information code pattern comprising means for providing a sheet having a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are at least temporarily free to rotate.

means for causing the magnetizable particles at selected locations defining a permanent fixed information code pattern in the layer to be differently physically aligned from the physical alignment of the magnetizable particles at a reference location and for subsequently permanently immobilizing the particles. and

means for applying to said sheet visible indicia characteristic of an intended use of the security document.

l2. A system according to claim ll, wherein said means for causing said particles to be differently physically aligned comprises means for applying magnetic fields upon the layer at selected locations to physically align the magnetizable particles within said locations. i t i t i

Claims

1. A security document having visible indicia characterisTic of an intended use thereof, which document includes a magnetic recording layer comprising uniformly dispersed magnetizable material having magnetic anisotropy, wherein the magnetizable material at a plurality of selected locations in the layer is differently physically aligned from the physical alignment of the magnetizable material at a reference location in the layer to provide a magnetically detectable permanent fixed information code pattern.

2. A security document according to claim 1, wherein the magnetizable material is particulate barium ferrite.

3. A security document according to claim 1, wherein the selected locations form a repetitive pattern defining a permanent signal track from which a predetermined repetitive signal may be produced.

4. A security document according to claim 3, wherein the repetitive pattern consists of adjacent selected locations within each of which the magnetizable material is differently physically aligned to define a continuously varying permanent signal track from which a predetermined continously varying signal may be produced.

5. A security document according to claim 1, wherein the magnetizable material in the remainder of the layer outside said selected locations is physically aligned in a single direction and the magnetizable material at more than two selected locations in a region not to be used for recording is differently aligned from said single direction.

6. A security document according to claim 5, wherein each of said selected locations consists of a discrete section within which the magnetizable material is physically aligned perpendicular to the direction of physical alignment of the material in the remainder of the layer.

7. A security document according to claim 1, wherein the selected locations form a non-repetitive pattern defining a permanent signal track from which a predetermined non-repetitive signal may be produced.

8. A method for making a security document having a magnetically detectable permanent fixed information code pattern comprising providing a sheet having a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are temporarily free to rotate, causing the magnetizable particles at selected locations defining a permanent fixed information code pattern in the layer to be differently physically aligned from the physical alignment of the magnetizable particles at a reference location; and subsequently permanently immobilizing the particles, and applying to said sheet visible indicia characteristic of an intended use of the security document.

9. A method according to claim 8, wherein the magnetizable particles are physically aligned at the selected locations by applying magnetic fields at the selected locations.

10. A method according to claim 8, wherein said sheet is provided with a layer containing magnetically anisotropic magnetizable particles of barium ferrite.

11. A system for making a security document having a magnetically detectable permanent fixed information code pattern comprising means for providing a sheet having a nonmagnetic backing and a layer thereon of a substantially uniform dispersion of magnetically anisotropic magnetizable particles wherein the particles are at least temporarily free to rotate, means for causing the magnetizable particles at selected locations defining a permanent fixed information code pattern in the layer to be differently physically aligned from the physical alignment of the magnetizable particles at a reference location and for subsequently permanently immobilizing the particles, and means for applying to said sheet visible indicia characteristic of an intended use of the security document.

12. A system according to claim 11, wherein said means for causing said particles to be differently physically aligned comprises means for applying magnetic fields upon the layer at selected locations to physicallY align the magnetizable particles within said locations.