FR2818389A1 - Secure paper document incorporates polymer layer containing amorphous ferromagnetic filaments in glass sleeves - Google Patents

Abstract

The paper document incorporates a polymer layer containing amorphous ferromagnetic filaments (F) in glass sleeves, the filaments having a saturation field of 300A/m or below and either positive or negative magnetostriction. The diameter of the filaments is 20 mcm or less, and they are between 3 mm and 3 cm long. The polymer layer is of latex, and the concentration of the filaments in the area of the document is between 4 and 80 mg/sq m.

Description

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SECURE PAPER DOCUMENT AND MANUFACTURING METHOD FOR
SECURE PAPER DOCUMENT TECHNICAL FIELD AND PRIOR ART
The present invention relates to a secure paper document as well as to a method for manufacturing a secure paper document.

 The protection of sensitive information is a concern of great importance in businesses and in public or private organizations. The risk of disclosure of information can result from organized malicious acts. In this case, the panoply of available means (cameras, means of bypassing surveillance devices, etc.) only makes it possible to avoid malicious acts by empowering personnel in contact with sensitive information.

 Another risk of information disclosure is the negligence of the personnel who can use the means of reproduction available in their environment (photocopiers, scanners, fax, etc.) to reproduce confidential documents not listed in the registers kept by the company , and over which there is, in fact, no control.

 In general, rules for storage, communication and reproduction are laid down for original confidential documents. Documents originating from a total or partial pirate reproduction of an original confidential document may be treated with negligence.

 According to known art, the reproduction of documents

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 Confidentiality is prohibited through the use of paper commonly known as anti-photocopying paper. An anti-photocopying paper is a paper containing either diffraction patterns (cf. US Pat. No. 5,830,609), or luminescent pigments (cf. US Pat. No. 5,271,645). The characteristic of anti-photocopying paper is to prevent the photocopying of documents using standard photocopiers or scanners. It has the disadvantage of not allowing reproduction, even in a context of control.

 It is also known various means of marking paper intended to ensure the authenticity of a document. Among these means, labeling with magnetic pigments is used (cf. US Pat. No. 5,631,039). Magnetic pigments are of the type found for magnetic recording.

Pigment detection is only possible with a magnetic sensor placed in very close proximity (typically, the distances between sensor and pigments are less than lamm). The main goal is to avoid that we can add to an imitated document characteristics that make it take for an authentic document.

 Thin magnetic tapes or ferromagnetic wires having a Barkhausen effect are also used for marking purposes (cf. US Pat. No. 520,456). The use of thin magnetic tapes and ferromagnetic wires has been widely implemented in anti-theft surveillance systems commonly called EAS systems (EAS for Electronic

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 Article Surveillance). EAS systems are mainly intended to prevent theft of goods from stores or books from libraries (see US Pat. Nos. 4,075,618 and US 3,666,449).

 Magnetic marker detection systems for anti-theft surveillance have a number of implicit or explicit characteristics (see, inter alia, international patent application WO-99/30384 and US-5,793,289) indicated below: * the interrogation / detection system, consisting of antennas, is fixed, and the objective is to detect a moving object, * the interrogation is done using coil type antennas comprising, possibly, a soft material which serves as pole or breech, but is not carried out with a system of permanent magnets, * the interrogation zone corresponds to a relatively large volume, * one can have antennas on both sides and d 'others in the interrogation zone, * the magnetic orientation of the marker can be arbitrary, * the selectivity must be high, so as to avoid any metallic or magnetic masses, corresponding to the purchases made by the customers of s stores protected by EAS, do not generate theft detection, * the constraints of masses and electrical consumption of the antennas are not significant.

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 The invention does not have the drawbacks mentioned above.

STATEMENT OF THE INVENTION
The invention relates to a secure paper document comprising a layer of polymer containing glass-clad amorphous ferromagnetic filaments.

 The invention also relates to a method for manufacturing a secure paper document. The method includes a step during which glass-clad amorphous ferromagnetic filaments are incorporated into a polymer layer.

 The invention also relates to a process for manufacturing coated paper comprising a step of coating an aqueous colloidal suspension on the surface of a fibrous cellulosic support. Ferromagnetic filaments are incorporated into the aqueous solution during the coating step.

 Advantageously, a secure paper document according to the invention can be reproduced using standard photocopying means. The possibility for the user to photocopy documents under conditions using standard photocopying means very advantageously distinguishes the invention from the prior art mentioned above. Indeed, it is frequently useful, for service reasons, to photocopy all or part of a document only to extract extracts, transparencies for rear projection, etc. Advantageously, according to the invention, the means which authorize the reproduction

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 of the marked paper document can then be managed accordingly. The number of reproductions of the document can thus be strictly controlled. It is then possible to keep track of the number of photocopies made. More generally, an effective policy for the classification and reproduction of classified documents can be put in place.

 The reproduction of the classified document by other means than those authorized according to the invention (for example, the reproduction by photography) requires to commit a premeditated act which cannot then be qualified as negligence. It is also possible to take the classified document (s) out of the company, or reproduce it (s) with conventional means, but, again, this implies a high level of fault, which cannot be described as negligence.

 A secure paper document reproduction system according to the invention allows the organization of the company to set up efficient procedures for reproduction and traceability of classified documents insofar as the reproduction of classified documents cannot be done. by evading procedures.

 The detection of a paper document according to the invention is significantly different from the detection using the EAS type anti-theft systems mentioned above. Indeed, the paper document reproduction system according to the invention is opposed on all the points listed above to EAS type anti-theft systems. For example, according to

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 the invention, it is necessary to detect quantities of magnetic material much less than the quantities of magnetic material usually detected to produce an EAS type marker.

 The invention advantageously makes it possible to authenticate the originality of certain documents such as official documents, and to distinguish them from falsified documents or reproduced by photocopying, even with sophisticated means. The paper marking method according to the invention has the advantage of ensuring a distribution of the marker over the entire surface of the document and of being almost invisible. Furthermore, the marking advantageously remains present throughout the life of the document, even if, for example, the latter is crumpled. The secure paper according to the invention can also produce, by cutting, labels of various shapes intended to be affixed to objects for authentication.

 It is thus possible, according to the invention, to detect paper surfaces of dimensions smaller, or even much smaller, than the standard size of a sheet of paper in A4 format (A4 format = 210 x 300 nun2) so that the cutting a portion of classified document does not allow its reproduction.

 The detection of the soft magnetic inclusions of the marked paper is done using the non-linear characteristic of the applied field-magnetization response (magnetic hysteresis cycle).

This non-linearity can be exploited through the

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sinusoïdale, ou grâce à la détection des sauts dit de Barkhausen caractérisant le basculement brusque de l'aimantation.

sinusoidal, or thanks to the so-called Barkhausen jump detection characterizing the sudden tilting of the magnetization.

 The magnetic excitation must essentially allow the course of at least Yz magnetic hysteresis cycle. It is therefore necessary to generate a field passing from a saturated state to the opposite one.

 It is clear that, given the relatively small volume of soft material included in the paper, the detection system must have a high sensitivity. To overcome external electromagnetic noise, a differential device can be an advantageous solution.

 Information processing can be carried out using a circuit using analog electronics. Filters are then used to extract the useful signal from the signals picked up by the detection system. Digital electronics can also be used to process signals. Spectral processing (Fourier transform) allows the extraction of interesting harmonics.

 The device for inhibiting the reproduction capacity of the device according to the invention can, in its simplest form, be made up of a mechanical (relay type) or electronic (transistor) switch which cuts off the supply of the light source of the device. A more advanced solution may consist in generating signals capable of interacting with a control card of the device.

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la figure 2 représente l'excitation magnétique obtenue à l'aide d'une configuration selon la figure 1 ; la figure 3 représente un deuxième exemple de configuration à aimant permanent pour l'obtention d'une excitation magnétique selon le premier mode de réalisation de l'invention ; la figure 4 représente l'excitation magnétique

obtenue à l'aide d'une configuration selon la figure 3 ; la figure 5 représente une configuration de dispositif de détection selon un deuxième mode de réalisation de l'invention ; la figure 6 représente un perfectionnement de la configuration représentée en figure 5 ; la figure 7 représente une application du dispositif de détection selon l'invention à un appareil de reproduction de documents ; la figure 8 représente un premier exemple de document papier sécurisé selon l'invention ; la figure 9 représente un deuxième exemple de document papier sécurisé selon l'invention ; BRIEF DESCRIPTION OF THE FIGURES
Other characteristics and advantages of the invention will appear on reading a preferred embodiment of the invention described with reference to the accompanying figures, in which: FIG. 1 represents a first example of a configuration with permanent magnets for obtaining magnetic excitation according to a first embodiment of the invention;

FIG. 2 represents the magnetic excitation obtained using a configuration according to FIG. 1; FIG. 3 represents a second example of a permanent magnet configuration for obtaining a magnetic excitation according to the first embodiment of the invention; figure 4 represents the magnetic excitation

obtained using a configuration according to Figure 3; FIG. 5 represents a configuration of the detection device according to a second embodiment of the invention; Figure 6 shows an improvement of the configuration shown in Figure 5; FIG. 7 represents an application of the detection device according to the invention to a document reproduction apparatus; FIG. 8 represents a first example of a secure paper document according to the invention; FIG. 9 represents a second example of a secure paper document according to the invention;

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 FIG. 10 represents a block diagram of the method for reproducing a paper document according to the invention.

 In all the figures, the same references designate the same elements.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Figures 1 and 3 show, according to a first embodiment of the invention, two examples of configuration of excitation of the soft magnetic material placed in the marked paper. FIG. 2 represents the variation over time of the magnetic excitation, at a point on the marked paper, which corresponds to the configuration of FIG. 1 and FIG. 4 represents the variation over time of the magnetic excitation, at a point marked paper, which corresponds to the configuration in Figure 3.

 The configuration shown in Figure 1 comprises a set of three permanent magnets in the form of three magnetic bars A1, A2, A3 of rectangular section mounted in opposition. The axis defined by the north and south poles of each of the bars is substantially perpendicular to the plane of the marked sheet of paper (not shown in the figure). In FIG. 1, the section of the magnetized bars is substantially rectangular, the invention however also relates to the case where the section of the bars is substantially square. The three magnetic bars Al, A2, A3 are mounted on a magnetic yoke CM which allows the field lines to close so

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 limit their extension in useful areas. The width L of the bars in the direction perpendicular to the axis defined by the poles is substantially equal to the width of a sheet of marked paper.

 The configuration of FIG. 3 comprises a permanent magnet in the form of a magnetic bar A4 of substantially rectangular section. The axis defined by the north and south poles of the A4 bar is substantially parallel to the plane of the marked sheet of paper. The bar A4 is mounted on a magnetic yoke CM. The width L of the bars is also substantially equal to the width of a sheet of marked paper.

1 l'invention concerne également le cas où plusieurs barreaux participent à l'excitation magnétique. Les barreaux sont alors placés parallèlement les uns aux autres sur la culasse CM. Les pôles nord des différents barreaux sont alors tous positionnés sur un même premier côté et les pôles sud sur un même deuxième côté opposé au premier côté. According to the example shown in Figure 3, a single bar A4 participates in the magnetic excitation. According to a variant of the embodiment of FIG. 3,

1 the invention also relates to the case where several bars participate in the magnetic excitation. The bars are then placed parallel to each other on the cylinder head CM. The north poles of the various bars are then all positioned on the same first side and the south poles on the same second side opposite the first side.

 More generally, the permanent magnets can be arranged in different ways. We should endeavor to create over time, for a scanned document area, a variable magnetic excitation allowing to switch the magnetization of the soft magnetic inclusions of the marked paper. To this end, the magnetic bars are moved in a direction parallel to the plane of the document (direction AA '

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 in Figures 1 and 3).

 The maximum value Hmax of the magnetic field H produced during the displacement of the magnetic bars (cf. Figures 2 and 4) must be very appreciably greater than the value of the anisotropy field of the magnetic materials involved and also sufficiently low not to disturb the nominal operation of the original reproduction device (magnetic toner of photocopiers, spectrum of the neon-type lamp of scanners, etc.). As mentioned previously, the magnetic yoke CM allows, for this purpose, to close the field lines and to prevent their extension in non useful zones.

 The configuration shown in Figure 1 creates alternately positive and negative excitation (see Figure 2). The configuration shown in Figure 3 creates a gradient of the magnetic field: the principle is based, in this case, on the observation that the magnetic inclusions present in the paper are dispersed isotropically and that their state of magnetization is arbitrary at a given time . The positioning of magnets with different orientations perpendicular to the scanning direction prevents the device from being hacked too easily.

 FIG. 5 represents an excitation coil configuration according to a second embodiment of the invention. FIG. 6 represents an improvement of the configuration shown in FIG. 5.

 An excitation coil Be traversed by an alternating excitation current ie makes it possible to generate a magnetic field of sinusoidal amplitude. Frequency

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 excitation current ie allows excitation of magnetic inclusions. By way of nonlimiting example, the frequency of the excitation current can be equal to 1 kHz. Excessively low excitation frequencies (for example around 50 Hz) are preferably discarded in order to avoid pollution by mains supplies and their harmonics.

 The amplitude of the excitation current is adjusted in order to allow saturation of the material at each period, with, preferably, a safety margin corresponding to a disturbing external static magnetic field.

 The detection of variations in magnetization of the magnetic elements contained in the marked paper is carried out by one or more detection coils Bd.

The use of detection coils Bd concerns both the case where the magnetic excitation is carried out using excitation coils (cf. FIGS. 5 and 6) as well as the case where the magnetic excitation is carried out by magnets permanent (see Figures 1 and 3).

 In the case where the excitation is carried out by permanent magnets, placing the coils in the immediate vicinity of the permanent magnets is the most attractive solution since the dimensions of the coil (s) are then reduced. If several coils are used, it will for example be advantageous to connect them in series to simplify the information processing circuit. The number of turns of the receiving coils must be high to have an equally high sensitivity.

 According to the preferred embodiment of

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 the invention, the permanent magnets are moved opposite the marked paper document which remains fixed.

The invention relates both to the case where the detection coils move in solidarity with the permanent magnets and to the case where the detection coils are fixed. In the latter case, it is then possible, for example, to use one or more fixed coils which embrace all of the reproduction surface of the reproduction apparatus all at once. It is then possible to wind the detection coil (s) around the window of the reproduction means.

 In all cases, according to an improvement of the invention, it is advantageous to use a differential system making it possible to increase the sensitivity (signal to noise ratio) of the device. In addition to the detection coil Bd, a compensation coil Bc then makes it possible to eliminate the signal component linked to the variation of magnetic field generated by the excitation means (cf. FIG. 6).

 FIG. 7 represents an application of the detection device according to the invention to a document reproduction apparatus. The document reproduction apparatus can be, for example, a photocopier, a scanner, a fax, etc. In FIG. 7 are shown a box K, a structure M for holding the light source 1 which illuminates the document and an arm b for moving the holding structure M in the box K. According to the embodiment shown in FIG. 7, the holding structure M also comprises excitation coils Bei and detection coils Bdi. The detection head part of the

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 Detection device is here integral with the structure M which maintains the light source, which advantageously makes it possible to ensure scanning of the entire reproduction area and to place the detection head very close to the original document.

 Figures 8 and 9 show two examples of secure paper documents according to the invention. FIG. 8 represents the case where the ferromagnetic filaments are distributed isotropically on the sheet of paper and FIG. 9 represents the case where the filaments are substantially aligned in the same direction.

 Preferably, the ferromagnetic filaments have a saturation field with a value substantially less than or equal to 300 A / m.

 The secure paper document according to the invention consists of a sheet of paper P in which are incorporated, on the surface, amorphous ferromagnetic filaments sheathed with glass F. The ferromagnetic filaments F are incorporated in the surface layer of the paper at the time of l 'sleeping operation. The filaments are then dispersed in the colloidal aqueous suspension spread on the surface of the fibrous cellulosic support (coated paper).

et al (Phys. Met. Metall., nol. vol. 67, p 70 à 75, 1989). Ils peuvent avoir l'une des compositions Glass-sheathed ferromagnetic filaments can be produced by drawing a molten metal core contained in a borosilicate glass tube as described in the document referenced "Ferromagnetic resonance in amorphous magnetic wires" by SA Baranov

et al (Phys. Met. Metall., nol. vol. 67, p 70 to 75, 1989). They can have any of the compositions

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 following: CoFeSiB, or CoSiB, or FeSiB, with a proportion of Si + B substantially greater than 18% and substantially less than 35% so as to obtain an amorphous material, and Co and Fe more than 40%. We can add to these main constituents elements such as: Ni (for example from 0 to 20%), Mo, Zr, Ge, Cr, Mn, V, Ti, C or other metals or metalloids with contents for example less than 7%.

 The ferromagnetic filaments can be with positive magnetostriction, in which case, they show a significant Barkhausen jump, or else with negative magnetostriction, in which case, they do not show a Barkhausen jump. We will consider filaments which reach saturation for an external field applied along their axis less than a few hundred A / m. The properties of such materials, depending on their composition, are well known, as described in the document referenced "Magnetic hysteresis in glass-covered and waterquenched amorphous wires" by H. Chiriac, TA Ovari, M. Vasquez and A. Hernando (Journal of Magnetism and Magnetic Marerials 177-171,1998, p 205 and 206).

 Under the effect of an external excitation magnetic field of very low amplitude (typically of the order of a few hundred A / m) and of frequency greater than a few hundred hertz, the magnetization of the filaments exhibiting a jump de Barkhausen suddenly reverses twice per period producing a variation in flux easily detectable by an antenna (detection coil) located

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 proximity to the means creating the excitation field. In all cases, the applied field must be greater than the filament saturation field. The detection coil is used to detect a flow pulse.

This is due to the bistable nature of the magnetization resulting from the magnetic characteristics of the alloy and the magnetostriction actions produced by the sheath. The sudden reversal of the magnetization during the rise of the external alternating field during a half-period causes the appearance of a signal (variation of flux induced in the detection coil) comprising many harmonics of frequencies multiple of the excitation frequency (a few tens of kilohertz). Harmonics are easy to identify and allow, if necessary, to discriminate with parasitic signals of lower frequency generated by more conventional and less soft magnetic materials, in case such materials exist in the vicinity of the document to be checked. and would disrupt its magnetic environment.

 Filaments with negative magnetostriction, not showing a Barkhausen jump, but having a saturation field of less than a few hundred A / m, also create harmonics under the same conditions. This is due to the fact that their magnetization varies non-linearly over time.

 The preferential optimum length of the filaments is close to 10 mm. It can also be of the order of 7 mm, without the modification of the shape anisotropy or the effects of

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 magnetostriction do not appreciably modify the conditions of reversal of the magnetization and, consequently, the amplitude of the signal which remains essentially proportional to the quantity of matter incorporated in the paper.

 The maximum total diameter of the filaments of the composite bimaterial is most often less than 20 mm, which makes it possible to fix the thickness of the coating layer of latex polymer between 30 and 40 mm after drying, resulting in a total thickness of the paper (support cellulosic + coating) of at least 80 mm. No upper limit is however imposed on the thickness of the sheet of secure paper.

The small total diameter of the filaments allows them to be incorporated into the colloidal aqueous suspension of latex polymer spread on the cellulose support of the paper during the coating operation.

 In order to be easily detectable, the concentration of ferromagnetic material in the secure paper must be significantly greater than 1 mg / m2. The incorporation and dispersion of the filaments in the coating slip, which also contains white pigments and other additives in addition to the latex colloids, is carried out by intensive stirring using, for example, a knife mixer. The filaments, of great mechanical strength and particularly flexible, are advantageously not damaged by the stirring operation. On the other hand, because of its glass composition, the sheath allows good wetting of the surface of the filaments by the aqueous colloidal suspension, and the slip thus

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 obtained is very homogeneous, lump-free, with a fairly low viscosity state and the characteristics of which can remain stable during storage (for example, storage for a few hours at room temperature).

 The coating is carried out by a Blade method, followed by a pre-drying and possibly a calendering. Due to the presence of the glass sheath, the metal fibers do not undergo significant corrosion in the suspended liquid. It should be noted that when the fiber concentration does not exceed 80 mg / m2 and the dispersion in the coating slip has been carried out properly, the filaments are substantially elongated in the thickness of the layer. The presence of fibers in the added layer is therefore difficult to detect with the naked eye, to the point that the secured paper advantageously retains an appearance very close to the appearance of an unsecured paper, an aspect which can moreover be reinforced by the 'addition of a layer without filaments, for example of thickness 2 to 3pom, covering the layer which contains the filaments.

 In the case of relatively long filaments, for example between 1 and 2 cm, the mechanical conditions for coating the paper at high running speeds cause a certain degree of alignment of the filaments under the effect of the viscosity forces within the slip (see Figure 9). The detection of such paper then depends on the orientation in which the sheets are introduced relative to the detector, and, in the favorable case, the signal is notably

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 strengthened. Furthermore, it is noted that the filaments can cross without modifying the properties of the assembly. It should be noted that the orientation phenomenon is not encountered for short fibers (for example between 5 to 8 mm long), the orientation then being statistical (cf. FIG. 8).

 It is important to note that, when the marking takes place over the entire surface of the paper, a cut portion of smaller surface is advantageously always detectable, thus ensuring greater security as well as greater discretion of the marking material.

 Furthermore, for fibers the diameter of which does not exceed 20 μm, the penetration of the magnetic field takes place, in all cases, up to the core of the filament, even when the excitation frequency is beyond a few tens of kilohertz (the skin effect at these frequencies only manifesting for filaments with a diameter of 30 mm or more).

The entire magnetic alloy is then advantageously useful for contributing to the detected signal.

 The characteristics of secure paper advantageously allow it to be compatible with all conventional uses in printing and office automation, such as conventional paper. In particular, the secure paper according to the invention can be placed in the transmitter tray of a photocopying machine according to any type of format. The presence of metal filaments sheathed with glass does not prevent the printing of the characters on the paper. It is therefore

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 easy to produce documents which must be considered as sensitive and non-divisible.

 An example of making secure paper will now be described below. In this example, soft ferromagnetic filaments sheathed with glass are considered, the total diameter of which is 15 μm, the thickness of the glass sheath being equal to 2 m. The filaments are produced by a known method derived from the Taylor process, which consists in induction heating a certain quantity of metal introduced into a closed borosilicate glass tube. A filament is initiated and wound around a winder. The glass used is a borosilicate glass of the Pyrex type, the melting temperature of which is close to 600 ° C. The metal alloy is of the CoMnSiB type and contains an atomic proportion of Cobalt of between 75 and 80%. The filaments are negative magnetostriction and have an anisotropy field of 1 Oe, or 80 A / m. The filaments are cut to a length of 7mm and dispersed in the sleeping slip. The filament concentration is more than 5g / m2. This preparation can be put on one side or on both sides of the sheet of paper. At this concentration, the filaments are advantageously invisible when the coating thickness is between 30 and 40 μm. Writing or printing on paper is not hampered by the presence of filaments.

 FIG. 10 represents a block diagram of a method for reproducing a paper document according to the invention.

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The method comprises a step 1 of inserting the original document into the reproduction device followed by a step 2 of detecting the presence of marked paper. When the original document is presented to the reproduction device, a detection device according to the invention detects the presence or absence of marked paper. In the event that marked paper is detected, the reproduction function is inhibited (step 4). The generation of an alarm or any other means of visualization and or counting can also be integrated into the detection device to signal the presence of marked paper. According to an improvement of the invention, the inhibition function can be canceled by a specific device leading to an authorization to reproduce the document (step 3).

 Advantageously, the reproduction apparatus according to the invention comprises a detection device which is added to an existing reproduction device. The power supply of the added device can be done either autonomously or using a resource specific to the reproduction means. For portable reproduction means, supplying the device with a battery poses no problem.

 Authorization for reproduction, despite the presence of marked paper, can be done by any means which satisfies the organizational requirements of the industry or organization which uses the subject matter of the invention. This authorization can be transmitted coded or not to the control device. It is clear that for cases where this authorization is not

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 used, the added device can be deprived of any entry allowing access to any authorization function (limitation of the risks of piracy or deliberate diversion of the authorization function).

 The detection device used to detect the potential presence of marked paper ensures detection over the entire reproduction surface, even if there is a decrease in the surface of the paper to be analyzed (case of cutting of a confidential document in an attempt to reproduce the document despite the marking).

Claims (8)

 1. Secure paper document, characterized in that it comprises a layer of polymer containing amorphous ferromagnetic filaments (F) sheathed in glass.  2. Paper document according to claim 1, characterized in that the ferromagnetic filaments have a saturation field of value substantially less than or equal to 300 A / m.  3. Paper document according to claim 1 or 2, characterized in that the ferromagnetic filaments (F) are with positive magnetostriction.  4. Paper document according to claim 1 or 2, characterized in that the ferromagnetic filaments (F) are negative magnetostriction.  5. Paper document according to any one of claims 1 to 4, characterized in that the ferromagnetic filaments (F) have a diameter substantially less than or equal to 20! ! m and a length between 3mm and 3cm.  6. Paper document according to any one of claims 1 to 5, characterized in that the concentration of the filaments is situated in a range substantially between 4 mg / m2 and 80 mg / m2. <Desc / Clms Page number 24>  7. A secure paper manufacturing process, characterized in that it comprises a step during which amorphous ferromagnetic filaments (F) sheathed in glass are incorporated in a layer of polymer.  8. Method according to claim 7, characterized in that the amorphous ferromagnetic filaments (F) sheathed in glass are dispersed in an aqueous colloidal suspension spread on the surface of a fibrous cellulosic support.