SECURITY DEVICE COMPRISING SOFT MAGNETIC THIN FILM

Abstract: A security device such as an anti-pilferage label or authentication label comprises as security feature a (first) soft-magnetic thin film having a thickness ranging from 100 nm to 2000 nm. The thin film is composed of an amorphous CoxZryNbz alloy, where x, y and z are atomic percentages and fulfil following conditions: (a) x ranging between 80 and 95; (b) y and z ranging between 0 and 20; (c) y + z = 100 - x. Due to the high effective relative permeability mu r, and the direction of its easy axis, a label with a CoZrNb thin film is relatively independent of its geometry.

Description: SECURITY DEVICE COMPRISING SOFT MAGNETIC THIN FILM Field of the invention. The present invention relates to a security device comprising as security feature a (first) soft-magnetic thin film with a thickness ranging from 100 nm (nanometer) to 2000 nm. The terms "security device" are herein defined as comprising anti- pilferage labels, authentication labels and data carrying labels. The term "soft-magnetic" generally refers to material having a rather low coercive force H c ranging from 3 A/m (0.037 Oersted) to 500 A/m (6.28 Oersted). In distinction herewith, hard or semi-hard magnetic material has a much higher coercive force H c , for example the coercive force H c of semi-hard magnetic material ranges from 2000 A/m (about 25 Oe) to 8000 A/m (about 100 Oe), and the coercive force H c of hard magnetic material is higher than 25000 A/m (about 312 Oe). The magnetization of a soft-magnetic material is easily affected by a small external magnetic field, whereas for a hard- magnetic material a strong external magnetic field is necessary to change its magnetization. Background of the invention. Security devices with a soft-magnetic thin film as security feature are known in the art. US-A-4,960,651 discloses an anti-pilferage label comprising a soft- magnetic thin film with a preferable composition along following lines : Co a Fe Ni c Mo d Si e B f , where a to f are atomic percentages and a ranges between 35 % and 70 %, b between 0 % and 8 %, c between 0 % and 40 %, d between 0 % and 4 %, e between 0 % and 30 %, f between 0 % and 30 %, with at least one element of each of the groups (b, c, d) and (e, f) being non zero. Such a Co a Fe Ni c Mo d Si e B f composition is hereinafter referred to as a CoFeNiMoSiB composition. Although very suitable as a security device, this type of label has some drawbacks. First of all, the soft-magnetic thin film must have a thickness greater than 500 nm. Below 500 nm, surface pinning effects become dominant and the signal obtained from the security device in an interrogating gate is rather poor. Another drawback is that the soft-magnetic thin film has its easy axis in a direction perpendicular to the direction of winding the magnetic thin film. This makes it difficult to make and/or use labels with a high aspect ratio, e.g. with dimensions 40 mm x 1 mm, where the long axis is in the roll- direction. For example, use of such labels at high speed in source- tagging or using application guns is causing more failures and production stand stills. Summary of the invention. It is an object of the present invention to avoid the drawbacks of the prior art. It is another object of the present invention to provide for a security device where the security feature is less bound to geometrical limitations than is the case with prior art security devices. It is still another object of the present invention to provide for a security device with a soft-magnetic thin film of a reduced thickness. According to the present invention, there is provided a security device which comprises as security feature a soft-magnetic thin film having a thickness ranging from 100 nm to 2000 nm. The thin film is composed of an amorphous Co x Zr y Nb z alloy, where x, y and z are atomic percentages and fulfil following conditions : (a) x ranging between 80 and 95 in order to obtain a saturation flux density B s above 9000 Gauss, e.g. above 10000 Gauss ; (b) y and z ranging between 0 and 20 ; (c) y + z = 100 - x. These Co x Zr y Nb z alloys are referred to as CZN compositions. Following CZN compositions are given as examples : Co 87 Zr s Nb 8 Co 89 Zr 45 Nb θ 5 CosgZrn Co 93 Zr 7 CZN compositions are known as such for use in magnetic recording applications (e.g. as materials used in recording heads) and for use in antennas. However, the effective relative permeabilities μ e reported in these applications are significantly lower in magnitude than those found in the present invention. Thin films made of CZN compositions have an effective relative magnetic permeability μ e ranging between 20000 and 200000, of course very dependent on sample dimensions. The effective relative magnetic permeability μ e is determined as the slope of a hysteresis curve as measured by means of a magneto meter at the coercive point, i.e. the point where the magnetic induction flux B is zero. The above values of μ e are higher than the effective relative magnetic permeability μ e of comparable CoFeNiMoSiB thin films, i.e. with an equal thickness. The CoFeNiMoSiB thin films have an effective relative magnetic permeability μ e ranging between 20000 and 60000. This means that, in comparison with such CoFeNiMoSiB compositions, either less material can be used, for example by reducing the thickness of the thin film or by using other label geometries which have not been possible up to now, or that the signals measured in anti-pilferage detection systems or in authentication detection systems have a larger amplitude resulting in the possibility of going to larger detection distances e.g. wider exit gates in electronic article surveillance applications. It has proved to be possible to reduce the thickness of a CZN thin film to below 500 nm, without experiencing the above-mentioned surface pinning effects and without experiencing responsive signals which are too low. This is important not only with respect to a potential saving of material but also with respect to its influence on the effective relative permeability μ e of a magnetic security feature. Indeed the effective relative permeability μ e of a magnetic security tag, i.e. the magnetic permeability as seen from outside the tag, may be derived approximately from the formula : 1 / μ e = 1 / μ r + N where μ r is the intrinsic relative magnetic permeability and where N is the demagnetisation factor. The demagnetisation factor N or its inverse, 1 / N, the shape factor, can be calculated as function of the geometrical shape of the security tag. Take, for example, a security tag having a fixed length L of 30 mm and a fixed width W of 30 mm. The demagnetisation factor N and the shape factor 1 / N change as follows with decreasing thickness T of the security tag : Decreasing the thickness T of the security tag increases the shape factor and, as a consequence, also the effective relative magnetic permeability Me- The increased shape factor also results in an improved non-linear behaviour of the hysteresis curve to small interrogating magnetic fields so that the security device is not only better detected in security detecting devices, but this is also the case in anti-pilferage systems based on harmonics as in "Barkhausen" anti-pilferage systems (as commercialized by Sensormatic and Certus). The fact that with CZN thin films one is able to reduce the thin film thickness to below 500 nm also means that there will be - in comparison with CoFeNiMoSiB thin films a reduced amount of eddy-currents and a reduced hysteresis loss with high frequencies. Still another advantage of the thin CZN thin films is that they also allow to reduce the thickness of the substrate, due to lower heat-load of the deposition process on the substrate. This results in the advantage of lower cost, possibility of longer production runs and a more environment- friendly process. So it represents a significant advantage that in case of CZN thin films the thickness can be reduced to lower than 500 nm. The CZN thin films further have following magnetic properties : a coercive field H c ranging between 5 A/m and 80 A/m ; an anisotropy field H k ranging between 500 A m and 5000 A/m. CZN thin films have proved to give an easy axis in the direction of the winding of the thin film during manufacturing by means of a webcoater. This makes a CZN thin film suitable for elongated labels with e.g. a length to width ratio being higher than 2, for example ranging from 10 to 100. This makes a CZN thin film also suitable for combination with other thin films such as CoFeNiMoSiB thin films which have their easy axis in a direction perpendicular to the direction of the winding of the thin film. By combining a CZN thin film with a CoFeNiMoSiB thin film a 360° detection label can be obtained. Brief description of the drawings. The invention will now be described into more detail with reference to the accompanying drawings wherein FIGURE 1 schematically illustrates how a CZN soft-magnetic thin film is applied to a substrate ; FIGURE 2A, FIGURE 2B, FIGURE 2C, FIGURE 2D, FIGURE 2E and FIGURE 2F illustrate various embodiments of a 360° label by combining a CZN thin film with a CoFeNiMoSiB thin film. Description of the preferred embodiments of the invention. Referring to FIGURE 1 , reference number 10 represents a cross-section of a high vacuum chamber, i.e. a chamber at 10 '1 to 10 "5 Pa. Vacuum pumps (not shown) are connected to chamber 10 by means of conduit pipes 11. A flexible substrate 12 such as a polymer film is unwound from a storage drum 14 and guided to a coating or cooling drum 16. When passing along magnetrons 18, 20, 22 and 24, ultra-thin layers of the CZN target material are sputtered onto the surface of the substrate 12. The coated substrate 26 is then rewound on the other side of the coating drum 16 on another storage drum 28. The sputtering starts when a negative voltage is applied to the CZN target material and when an inert gas flow is introduced into the vacuum chamber 10 up to a total pressure of between 8.10 "4 mbar and 5.10 "2 mbar. Positive argon ions, which originate due to the electric and magnetic fields, are accelerated towards the negatively biased target, impinge on it with high energy and release atoms from the target. These atoms have enough energy to travel to the substrate where they are stopped and bonded, forming a high precision coating. The CZN soft-magnetic coating according to the invention can be made by means of a single, double or multiple pass vacuum magnetron- sputtering operation. The anisotropic behaviour of the magnetic properties can be tailored by proper arrangement of magnetic flux lines. Furthermore, the deposition power, cooling of drum and type of substrate are important factors that influence the anisotropic behaviour and magnetic properties in general. This anisotropic behaviour makes the soft-magnetic coating according to the invention different from soft-magnetic coatings of similar CZN composition, as used in magnetic recording heads. The deposition process according to the invention results in softer magnetic properties in the direction of the easy axis, than soft-magnetic coatings of similar composition, as used in magnetic recording heads. This is achieved by the nature of the webcoating process. Referring both to FIGURE 1 and to FIGURES 2 (A through F), a 360° label can be obtained by combinations of CZN thin films with easy axis in the direction of winding and CoFeNiMoSiB thin films with easy axis perpendicular to the direction of winding. These combinations can be obtained by lamination or in-situ deposition of the two different materials. For example the following combinations can be deposited in-situ, in the vacuum-coating chamber as given in FIGURE 1. FIGURE 2A : A double layer comprising a CZN layer 30 and a CoFeNiMoSiB layer 32 can be obtained with CZN targets on magnetron positions 18 and 20, and CoFeNiMoSiB targets on magnetron positions 22 and 24. FIGURE 2B : A multi-layer comprising alternating CZN layers 30 and CoFeNiMoSiB layers 32 can be obtained with CZN targets on magnetron positions 18 and 22, and CoFeNiMoSiB targets on magnetron positions 20 and 24. FIGURE 2C : A double layer comprising a CZN layer 30 and a CoFeNiMoSiB layer 32 can be obtained with a CZN target on magnetron position 18, and CoFeNiMoSiB targets on magnetron positions 20, 22 and 24. FIGURE 2D : A double layer comprising a CZN layer 30 and a CoFeNiMoSiB layer 32 can be obtained with CZN targets on magnetron positions 18, 20 and 22 and a CoFeNiMoSiB target on magnetron position 24. FIGURE 2E : A double layer comprising a CZN layer 30 and a CoFeNiMoSiB layer 32 can be obtained with a CoFeNiMoSiB target on magnetron position 18, and CZN targets on magnetron positions 20, 22 and 24. FIGURE 2F : A double layer comprising a CZN layer 30 and a CoFeNiMoSiB layer 32 can be obtained with CoFeNiMoSiB targets on magnetron positions 18, 20 and 22, and a CZN target on magnetron position 24. Of course, other combinations are possible depending on the number of magnetrons available in the webcoater and on the required magnetic properties. As a matter of example, following values of magnetic properties have been measured on 30 mm x 30 mm soft-magnetic thin films :

Claims: CLAIMS 1. A security device comprising as security feature a first soft-magnetic thin film (30) having a thickness ranging from 100 nm to 2000 nm, characterized in that said thin film is composed of an amorphous Co x Zr y Nb z alloy, where x, y and z are atomic percentages and fulfil following conditions : (a) x ranging between 80 and 95 ; (b) y and z ranging between 0 and 20 ; (c) y + z = 100 - x. 2. A security device according to claim 1 wherein said first thin film has an effective relative magnetic permeability μ e measured at DC ranging between 20000 and 200000. 3. A security device according to any one of the preceding claims wherein said first thin film has a coercive field H c ranging between 8 A/m and 80 A/m. 4. A security device according to any one of the preceding claims wherein said first thin film has an anisotropy field H κ ranging between 500 A/m and 5000 A/m. 5. A security device according to any one of the preceding claims wherein said first thin film has a thickness smaller than 500 nm. 6. A security device according to any one of the preceding claims wherein said first thin film has its easy axis in the direction of winding of the film. 7. A security device according to claim 6 wherein said device further comprises at least one other thin film having an easy axis perpendicular to the direction of the easy axis of the first thin film. 8. A security device according to any one of the preceding claims wherein said device is an anti-pilferage label. 9. A security device according to any one of claims 1 to 7 wherein said device is an authentication label. 10. A security device according to any one of claims 1 to 7 wherein said device is a data carrying label. 11. A security device according to any one of claim 8 to 10 wherein said label has a length to width ratio greater than 2.

Inventors: TE LINTELO JOHANNES, TE LINTELO, JOHANNES
Applicants: INNOVATIVE SPUTTERING TECH, TE LINTELO JOHANNES, INNOVATIVE SPUTTERING TECHNOLOGY, TE LINTELO, JOHANNES
Application year: 1999
Main classification: G08B13
IPC codes: G08B13, H01F10