Magnetic powder, magnetic ink, printed member and manufacturing method therefor

Abstract: The magnetic powder for validity determining ink is composed of magnetic oxide powder having a Curie temperature between -50°C and 150°C and a mean powder particle diameter of 10µm or less.

Description: The present invention relates to a magnetic powder for validity determining ink, a magnetic ink for validity determination, a printing member for validity determination, and methods for making a printing member. 2. Description of the Related Art Forgery preventive measures for notes as money, securities, and cards having the value equal to cash are taken. Particularly, an art for printing a certain kind of information on a paper sheet with magnetized ink and magnetically detecting the information is easy in recording and erasing information and used widely. Further, recently, for example, as disclosed in U.S. Patent No. 5,533,759 (Jeffers, July 9, 1996), an objective document is printed using magnetic ink including a magnetic pigment having a Curie temperature lower than 130°C and the printed part is magnetized in an optional magnetic pattern. The magnetized part is heated at least up to 130°C using a heat lamp. The validity of the document is determined depending on whether the magnetic pattern is destroyed by heat in the temperature region beyond the Curie point or not. However, in U.S. Patent No. 5,533,759 mentioned above, the particle diameter of the magnetic pigment included in the magnetic ink is not disclosed. When the particle diameter of the magnetic pigment is larger than a predetermined value, the magnetic pigment cannot respond sufficiently to the high resolution like printing by an ink jet printer. Further, when the particle diameter of the magnetic pigment is larger than a predetermined value and the magnetic pigment is printed on a paper sheet, particularly the magnetic information recorded on the surface is gradually torn off due to friction with the magnetic detection head during reading and it is anxious that the SN ratio may be reduced during reading of the information. On the other hand, as input-output devices such as scanners, printers, and copying machines, personal computers, and image processing software have been highly advanced recently, even devices on sale can commit highly precise forge. In order to respond to this situation, various forgery preventive arts are applied to securities and individual authentication ID cards. Particularly, from the viewpoint of that information is invisible to a human, arts using a magnetic material are widely used. For example, in securities, an art for printing a predetermined area with magnetic ink with magnetic powder mixed and determining the validity by detecting the existence of magnetism or the magnetic pattern itself is known. Further, in IC cards, it is known to magnetically record information on a magnetic stripe, reproduce it, and authenticate an individual. As mentioned above, generally, output detection by magnetism can respond to determination of the validity by high-speed reading comparatively easily, so that it has been used in various fields. However, the conventional method determines the validity depending on judgment of whether there is magnetic information in a predetermined position or not, so that using a material of Fe 3 O 4 or others which can be obtained comparatively easily, forging arts using the latest printing art are generated frequently. In the aforementioned magnetic forgery preventive art, a recording and reproducing apparatus can be prepared comparatively simply and can read recorded information easily, so that an art having weak resisting force to forgery and a high security property is required. European patent publication no. 0 702 339 A1 discloses an ink containing chromium oxide or MnZn ferrite for determining the validity of a banknote or other valuable document. German patent publication no. 41 03 263 A1 discloses needle shaped ferrite materials having a diameter of less than 0.5 microns, a coercive strength of less than 1 KA/m and Curie temperatures in the range between room temperature and about 200°C. Japanese Patent Publication No. 60-210801 discloses a method for making fine hexagonal ferrite crystal particles. A vitreous solid oxide mixture is melted and heat treated at about 800°C, thereby causing fine magnetic crystal particles of a given shape to be deposited. The particles are separated from the matrix by acid washing. U.S Patent No. 4,081,132 discloses a security document having a magnetizable information layer disposed on a carrier and a magnetizable verification layer deposited on the information layer. The information layer is formed by a magnetic tape material comprising magnetic particles. The verification layer comprises a metal or alloy and exhibits a Curie temperature that is below the Curie temperature of the information layer. An object of the present invention is to provide a magnetic powder and a magnetic ink for validity determination that is satisfactory in durability, applicable to various printing arts, highly reliable, quick determining speed and effective for forgery prevention. This object is achieved by the magnetic powder of claim 1 and the magnetic ink of claim 4. Additional embodiments of the magnetic powder are provided in sub-claims 2 and 3. Another object of the present invention is to provide a printed (printing) member for validity determination that is highly reliable, quick determining speed and effective for forgery prevention. This object is achieved by the printed member of claim 5. Additional embodiments of this invention are provided in sub-claims 6-8. Another object of the present invention is to provide methods for making a printed member. This object is achieved by the methods of claim 9 and 16. Additional embodiments of this method are provided in sub-claims 10-15. Fig. 1 is a graph showing the relationship between the Zn substitution ratio X of NiZn ferrite series magnetic powder used in the present invention with the Curie temperature; Fig. 2 is a schematic view showing an example of a manufacturing apparatus used in a preferred manufacturing method for the magnetic powder of the present invention; Fig. 3 is a schematic plan view of an individual authentication card that is an example of a printed-paper of the present invention; Fig. 4 is a graph showing the relationship between the temperature and the magnetization intensity as a magnetic property of the first magnetic ink and a magnetic property of the second magnetic ink; Fig. 5 is a schematic view showing a detecting device capable of use with the printed paper of the present invention; Fig. 6 is a graph showing a detected record at normal temperature; Fig. 7 is a graph showing a detected record at a temperature between the Curie temperature of the first magnetic ink and the Curie temperature of the second magnetic ink; Fig. 8 is a plan view showing another example of a printed-paper for validity determination relating to the present invention; Fig. 9 is a graph showing the waveform of a detected signal obtained by a sensor; and Fig. 10 is a graph showing the waveform of a detected signal obtained by a sensor. The magnetic powder for validity determining ink of the present invention is practically composed of magnetic oxide powder and the magnetic oxide powder has a Curie temperature between -50°C and 150°C and a mean crystal particle diameter of 10µm or less. Further, an example of a manufacturing method for the aforementioned magnetic powder includes a step of mixing and dissolving a magnetic oxide and a glass forming material, a step of cooling the obtained mixture rapidly and making the amorphous magnetic oxide, a step of heat-treating the cooled mixture thereafter and crystallizing the magnetic oxide, and a step of removing the glass forming material from the mixture and obtaining magnetic oxide powder with a mean powder particle diameter of 10µm or less. Furthermore, the ink for validity determination of the present invention contains the aforementioned magnetic powder, that is, magnetic oxide powder having a Curie temperature between -50°C and 150°C and a mean powder particle diameter of 10µm or less. As mentioned above, the present invention uses magnetic oxide powder with a mean powder particle diameter of 10µm or less. When the particle diameter of magnetic powder for validity determination is 10µm or less, it is easily mixed in the fibers of the print base, for example, paper during printing and the amount of magnetic powder existing on the paper surface is reduced. By doing this, the omission of magnetic powder due to magnetic detection is greatly reduced and the durability is greatly improved. The mean powder particle diameter of magnetic powder is preferably 5 nm to 5µm and most preferably 5 nm to 1µm. Further, according to the present invention, when the particle diameter is decreased, the color depth due to a pigment becomes light, so that the color can be adjusted by a combination of various pigments. Further, the dispersibility of pigments is satisfactory, so that magnetic powder in ink can be dispersed uniformly and the detection output becomes larger. According to the present invention, as a magnetic material, from the viewpoint of durability, an oxide is used. As a constitution of an oxide, the crystal structures such as the perovskite type, garnet type, hexagonal type, and spinel type may be cited. The mean powder particle diameter can be easily obtained by setting the maximum length of each particle as a particle diameter and averaging those of 20 or more particles obtained from the TEM observation. Or, when a calibration curve of the value and specific surface area can be obtained, the mean particle diameter can be obtained from the specific surface area. Further, according to the present invention, the Curie temperature of magnetic powder can be used for validity determination and in the present invention, at least two kinds of magnetic powder having a Curie temperature within the range from -50 to 150°C are used. The reason is that when magnetic powder having a Curie temperature within the range from -50 to 150°C is used, by changing the temperature comparatively easily, the magnetic detection output is greatly changed and best reversibility of detection output is obtained. By doing this, reliable validity determination can be executed simply. Furthermore, within this temperature range, the magnetic permeability at just the Curie temperature is very high and the detection sensitivity is extremely satisfactory. On the other hand, when the Curie temperature is higher than 150°C, the surface temperature is easily varied and a place where the output is changed and a place where no output is changed may be generated due to it, so that accurate binary coding becomes difficult. On the other hand, when the Curie temperature is lower than -50°C, the magnetic permeability of magnetic powder is reduced, so that the output itself is reduced and the variation in the neighborhood of the Curie temperature is made smaller. In the magnetic ink for validity determination of the present invention, at least one kind of another magnetic powder different in the Curie temperature from the magnetic powder of the present invention is mixed additionally. The setting of Curie temperature can be realized in the same component system under control of the composition. A different composition system having a different Curie temperature can be mixed. Further, in the magnetic ink for validity determination of the present invention, at least one kind of still another magnetic powder different in the coercive force from the magnetic powder of the present invention can be mixed additionally. The setting of coercive force can also be realized in the same component system under control of the composition, though a different composition system having a different coercive force can be mixed. Magnetic oxide powder includes ferrite series magnetic powder having coercive force of 20,000 A/m or less. It is also possible to use a combination of another magnetic powder different in the Curie temperature and still another magnetic powder different in the coercive force. It is also possible to prepare various types of ink for validity determination including another magnetic powder and still another magnetic powder and print using them respectively. As mentioned above, by use of a combination of several kinds of magnetic powder, printed papers having a higher security property can be provided. As magnetic oxide powder, soft magnetic ferrite such as NiZn ferrite, MnZn ferrite, and CuZn ferrite is desirable. Further, it is desirable to replace a part of Ni ferrite and Mn ferrite with Zn so as to control the Curie temperature. Particularly, since the coercive force of magnetic powder including Ni oxide is low and the detection sensitivity is increased, it is desirable. In Fig. 1, as an example, the relationship between the Zn substitution ratio X of NiZn ferrite (Ni 1-x Zn x Fe 2 O 4 ) series magnetic powder preferably used in the present invention with the Curie temperature is shown. As shown in the drawing, it is found that even if the same NiZn ferrite is used, the Curie temperature greatly varies with the component constitution. Adjusting the component constitution of an element having a Curie temperature and coercive force within the desired ranges can use the magnetic powder used in the present invention. At the time of detection of magnetic output, by heating by a heater lamp or cooling by spraying cooling gas such as dry ice, the detection output can be obtained at every desired temperature. Changing the composition can control the Curie temperature of magnetic powder, for example, by partially replacing Ni or Mn of Ni ferrite or Mn ferrite that is a basic component with Zn or Cd, preferably Zn. The manufacturing method for magnetic powder for validity determining ink preferably has a step of mixing and dissolving a magnetic oxide and a glass forming material, then cooling the mixture rapidly, and making the magnetic oxide among the mixture amorphous, a step of heat-treating the amorphous magnetic oxide and crystallizing the amorphous magnetic oxide among the mixture, and a step of removing the glass forming material from the crystallized mixture and obtaining magnetic oxide powder with a mean powder particle diameter of 10µm or less. As a glass forming material, B 2 O 3 or P 2 O 5 can be used. Fig. 2 is a schematic view showing an example of a preferred manufacturing apparatus used in the manufacturing method for the magnetic powder. As shown in Fig. 2, the manufacturing apparatus has a platinum crucible 40 having a nozzle 43 at its lower end, a high frequency induction-heating coil 41 arranged around the crucible 40, and a rapid cooler 47 composed of a pair of iron rollers 45 and 46 installed under the nozzle 43. In an example of the manufacturing method, in the crucible 40, both B 2 O 3 as a glass forming material and a magnetic oxide material such as NiZn ferrite are housed. By heating up to about 1,400°C to 1,500°C by the high frequency induction heating coil 41, the glass forming material and magnetic oxide material are dissolved and mixed. After being dissolved and mixed, in the neighborhood of a press contact portion 48 on the rollers 45 and 46 of the rapid cooler 47, the dissolved mixture is ejected. The pair of rollers 45 and 46 are pressed in contact with each other and rotated in the directions of the arrows at high speed so that the rotational direction of the press contact portion 48 is synchronized with the ejection direction of the dissolved mixture. The ejected dissolved mixture is rapidly cooled on the rollers 45 and 46, passes the press contact portion, and is formed as a ribbon-shaped or flake-shaped amorphous material. Then, the obtained amorphous material is heat-treated and crystallized to a magnetic oxide. The material of the cooler 47 used for rapid cooling of the dissolved mixture is preferable to be, for example, Fe or Cu and the material of the pair of rollers is particularly preferable to be an Fe alloy from the viewpoint of durability. The peripheral speed of the rollers, although depending on the feed amount of a molten material, is preferable to be within the range from 0.1 to 30 m/s. The heat-treating condition, although depending on the composition, is, for example, 10 minutes to 10 hours at 650 to 900°C. Hereafter, the glass-forming component is removed from the heat-treated mixture by cleaning it using a weak acid solution; for example, a dilute acetic acid and magnetic powder can be taken out. According to this method, magnetic oxide fine particles are well dispersed in the crystallized mixture because the mutual interfaces of magnetic oxide fine particles are isolated by the glassy phase and after cleaning, magnetic oxide fine particles having an equal particle diameter can be obtained easily. The mean powder particle diameter of magnetic powder can be controlled, for example, by properly changing the composition ratio of a magnetic oxide and a glass forming material, the peripheral speed of the cooler, and the heat-treating temperature after rapid cooling, and the heat-treating time. The printing member for validity determination of the present invention is used to detect a magnetic image indicating magnetic characteristics at a temperature higher than the first Curie temperature of the first magnetic powder and lower than the second Curie temperature of the second magnetic powder and has the first magnetic image printed with the first magnetic ink including the first magnetic powder having the first Curie temperature and the second magnetic image printed with the second magnetic ink including the second magnetic powder having the second Curie temperature higher than that of the first magnetic powder. The first and second magnetic images may be such that for example, when one of them is a magnetic background image, the other is a magnetic data image. Further, on the first magnetic image, the second magnetic image can be overprinted. Furthermore, a preferred detecting device for the printing member for validity determination has the aforementioned printing member for validity determination, a heater for heating the printing member for validity determination to a temperature higher than the first Curie temperature and lower than the second Curie temperature, and a means for detecting a magnetic data image of the heated printing member for validity determination. In this case, the first magnetic powder and second magnetic powder are preferable to be magnetic powder mainly composed of an iron oxide from the viewpoint of the environmental adaptability and detection. As such an iron oxide, for example, NiZn ferrite, CuZn ferrite, MnZn ferrite, and CuZuMg ferrite may be cited. Particularly, MnZn ferrite, CuZn ferrite, and NiZn ferrite can easily control the Curie temperature and the detection sensitivity thereof is high. Further, as at least one of the first magnetic powder and second magnetic powder, it is preferable to use oxide magnetic powder practically composed of oxide magnetic powder having a Curie temperature between -50°C and 150°C and a mean powder particle diameter of 10µm or less relating to the present invention. Such magnetic powder has characteristics that the dispersibility in magnetic ink is satisfactory, and necessary information can be precisely written in a fine position in a predetermined place, and satisfactory durability, high output, and high sensitivity are realized, and the reliability is high. Particularly, as such an iron oxide, magnetic powder having a mean powder particle diameter of 5 nm to 5 µ m is preferable to be used and in this magnetic powder, the aforementioned characteristics are more satisfactory. The mean powder particle diameter is more preferably 5 nm to 1µm. Furthermore, by changing the amount of magnetic powder in two kinds of ink to be printed, the detection pattern can be changed. Further, a preferred means for detecting a magnetic data image can be composed of the first magnetic detecting section and second magnetic detecting section installed at the preceding stage and later stage of the heater respectively. Furthermore, in the preferred validity determining device, a validity determining section for determining the validity from the first detected magnetic pattern by the first magnetic detection section and the second detected magnetic pattern by the second magnetic detection section is additionally installed in the detecting device. The present invention will be explained in detail hereunder with reference to the accompanying drawings. Fig. 3 is a schematic view of an individual authentication card that is an example of a printed-paper of the present invention. An individual authentication card 11 has a magnetic background image 12 printed on a card base 10 at random with the first magnetic ink including the first magnetic powder having a low Curie temperature higher than the room temperature, a magnetic data image 13 in a bar code pattern shape printed on the magnetic background image 12 with the second magnetic ink including the second magnetic powder having a Curie temperature higher than that of the first magnetic powder in correspondence with predetermined information, a face photograph 14 of the said person printed with ordinary color ink, and an authentication number not shown in the drawing. As mentioned above, on the individual authentication card 11, the face photographs of the said person and authentication number are printed and a security art composed of the magnetic background image 12 and the magnetic data image 13 is additionally provided. Fig. 4 shows a graph indicating the relationship between the temperature and the magnetization intensity as a magnetic property 21 of the first magnetic ink and a magnetic property 22 of the second magnetic ink. Here, Ta indicates a standard room temperature (20 to 30°C), and T1 indicates the Curie temperature of the first magnetic ink, that is, the temperature at which the magnetization is eliminated, and T2 indicates the Curie temperature of the second magnetic ink, and the first magnetic ink and second magnetic ink are designed so that Ta < T1 < T2 is held. The magnetization intensity of the first magnetic ink at the room temperature Ta is preferably higher than the magnetization intensity of the second magnetic ink. As a combination having such a magnetic property, for example, in Ni 1-x Zn x Fe 2 O 4 , there are two kinds of combinations such as x = 0.7 and x = 0.8. By use of it, two Curie temperatures can be set. Further, in Mn 1-x Zn y Fe 2 O 4 , even when y = 0.80 and y = 0.90 are set, magnetic powder having a different Curie temperature can be set. Furthermore, a combination of different constituent elements, for example, a combination of NiZn ferrite and MnZn ferrite is also acceptable. When these materials are used as magnetic powder, particularly high effects can be obtained. Figs. 5 to 7 are drawings for explaining a preferred detecting method for the printing member for validity determination. Fig. 5 is a schematic view showing a preferred detecting device, Fig. 6 is a graph showing a detected record at the normal temperature Ta, and Fig. 7 is a graph showing a detected record at a temperature To between T1 and T2. As shown in Fig. 5, a detecting device 31 comprises a conveyor 35 composed of, for example, a belt-shaped member for conveying the same individual authentication card 11 as that shown in Fig. 3, a first sensor 32 composed of a magnetic detecting section, a heater 34 composed of a halogen lamp, and a second sensor 33 composed of a magnetic detecting section. Furthermore, the detecting device 31 has a validity determining section 36 that is connected to the first sensor 32 and the second sensor 33, receives respective detection signals from the first sensor 32 and the second sensor 33, and determines the validity. The first sensor 32 detects magnetic output at the rough room temperature Ta in the area where the magnetic data image 13 printed with the second magnetic ink in correspondence to predetermined information is overwritten on the magnetic background image 12 printed on the individual authentication card 11 with the first magnetic ink. Thereafter, the area is heated up to a temperature of To between the first Curie temperature and the second Curie temperature by the heater 34 and the magnetic output at that time is detected by the second sensor 33. In this case, the first sensor 32 and the second sensor 33 are arranged at two positions above and below the conveyor 35 respectively so as to increase the SN ratio. As a heater 34, in addition to the halogen lamp, a predetermined heater or a heat roller may be used. Figs. 6 and 7 show the detected records by the first sensor 32 and the second sensor 33 in the area A-A' shown in Fig. 3 as graphs indicating the relationship between the time and the magnetic output respectively. The output of the first sensor 32, since the detection temperature is the room temperature Ta, detects the magnetic background image 12 printed with the first magnetic ink at random and the shape of the graph, as shown in Fig. 6, is detected as a noise-shape pattern of high output. On the other hand, since the temperature in the detection area is increased to a temperature of To higher than the Curie temperature T1 of the first magnetic ink, the magnetization of the magnetic background image 12 becomes zero, so that the output of the second sensor 33 can detect the bar code pattern of the magnetic data image 13 overwritten in this area at a high SN ratio. Furthermore, the magnetic output of the first sensor 32 and the magnetic output of the second sensor 33 are input to the validity determining section 36. In this case, at the room temperature Ta, the noise pattern of high output is increased to a temperature of To, so that from the change in the magnetic output that it can be detected as a predetermined bar code pattern, the validity determining section 36 determines whether the individual authentication card 11 is a true card based on a predetermined specification or not and can send a validity determining signal 37 to a system not shown in the drawing. Magnetic powder Ni 0.25 Zn 0.75 Fe 2 O 4 having a mean powder particle diameter of 0.1 µ m and a Curie temperature of 80°C, resin, and dispersant are mixed so as to form ink. A paper is prepared as a base and a bar code is printed on the paper using the obtained magnetic ink. The coercive force of the used magnetic powder is 7110 A/m. The obtained printed-paper is applied to a validity-determining device having the same constitution as that shown in Fig. 5. Firstly, a signal of the obtained printed-paper is detected using the first sensor 32 composed of a non-contact reading head at room temperature. Thereafter, the printed paper is heated up to 130°C or more by the heater 34 composed of a heater lamp and immediately after, a signal is detected again using the second sensor 33 composed of a non-contact reading head having the same constitution. As a result, a signal of 22 mVp-p is obtained at room temperature, though in the latter case, no signal is obtained. Even if the operation is repeated 1000 times in a short time, no change is observed in the detected signal. As a Comparison, the same evaluation is made using magnetic ink produced using CrO 2 as a magnetic pigment. The obtained output is extremely small and cannot be detected unless it is amplified considerably. When it is evaluated by "3M Viewer" after the temperature is raised up to the Curie temperature or more once, although data erasure can be ascertained surely, it is found that writing and erasure confirmation require a lot of time, thereby validity determination at high speed is difficult. Further details concerning the present invention will be provided by the following concrete examples. Embodiment 1 and Comparison example 1: Magnetic powder Ni 0.2 Zn 0.8 Fe 2 O 4 having a mean powder particle diameter of 50 nm, a Curie temperature of 40°C, and coercive force of 9480 A/m and magnetic powder Ni 0.25 Zn 0.75 Fe 2 O 4 having a mean powder particle diameter of 70 nm, a Curie temperature of 80°C, and coercive force of 8000 A/m at a rate of 1:7, resin, and dispersant are mixed so as to form ink. Using the obtained magnetic ink, a bar code is printed on a paper. The magnetic powder used is a one produced by the glass crystallization method. A signal of the obtained printed-paper is detected in the same way as described above. As a result, a signal of 32 mVp-p is obtained at room temperature and a signal of 15 mVp-p is obtained at 60°C, though no signal is obtained under the heating condition. Even if the operation is repeated 1000 times in a short time, no change is observed in the detected signal. As Comparison example 1, the same evaluation is made using magnetic ink produced using CrO 2 with a particle diameter of 20 µm as a magnetic pigment. In this case, the output is small, such as about 0.1 mVp-p, and even if the operation is repeated 1000 times, the output is extremely small and cannot be measured. As mentioned above, high reliability that validity determination can be executed easily for the magnetic ink of the present invention and an article using it and can sufficiently withstand the repetition can be obtained. Embodiment 2 and Comparison example 2: Magnetic ink A (Embodiment 2) not forming part of the invention obtained by mixing Ni 0.3 Zn 0.7 Fe 2 O 4 having a mean crystal particle diameter of 80 nm and a Curie temperature of 120°C, resin, and dispersant and magnetic ink B (Comparison example 2) obtained by mixing Ni 0.7 Zn 0.3 Fe 2 O 4 having a Curie temperature of 430°C or more, a mean crystal particle diameter of 14µm, and coercive force of 790 A/m and the same resin and dispersant are prepared respectively. Papers are printed using the obtained two kinds of magnetic ink respectively. The magnetic powder of this embodiment is a one produced by the glass crystallization method and the magnetic powder of this comparison example is a one produced by a method for obtaining magnetic powder by preparing and calcining iron oxide, zinc oxide, and nickel oxide so as to obtain a predetermined ratio. Fig. 8 is a plan view showing another example of a printed-paper for validity determination viewed from above. As shown in the drawing, the printed-paper has a predetermined pattern 111 printed on a paper 120 using the magnetic ink B having a high Curie temperature and predetermined patterns 112 and 113 printed using the magnetic ink A having a low Curie temperature. A signal of the obtained printed paper is detected by the first sensor 32 at normal temperature, and then the magnetic ink is heated up to about 150°C by the heater 34 composed of a heater lamp, and a signal is detected by the second sensor 33 again. In Figs. 9 and 10, the waveform of the detected signal obtained by the first sensor 32 and the waveform of the detected signal obtained by the second sensor 33 are shown respectively. In the drawing, numeral 111a indicates a peak of the pattern 111 using the magnetic ink B having a high Curie temperature, and numeral 112a indicates a peak of the pattern 112 using the magnetic ink A having a low Curie temperature, and numeral 113a indicates a peak of the pattern 113 using the magnetic ink A having a low Curie temperature. As shown in the drawings, the peaks 112a and 113a of the magnetic ink A having a low Curie temperature obtained by the first sensor 32 disappear from the waveform of the detected signal obtained by the second sensor 33. The detected signals obtained in the aforementioned embodiment can be determined as indicated below. For example, with respect to the detected waveforms shown in Figs. 9 and 10, a high-pass filter removes the DC component and the signal waveform in a pulse shape is taken out. From the taken out signal waveforms, the number of pulses at a fixed voltage or higher is counted for the signals before and after heating. By ascertaining that the respective counts are the intrinsic predetermined numbers of the article for validity determination, that is, the value before heating is 3 and the value after heating is 1, the validity can be determined. Or, after the high-pass filter removes the DC component and the signal waveform in a pulse shape is taken out, the signal is rectified to a DC signal. This DC signal is integrated and compared with the intrinsic predetermined numbers of the article for validity determination in magnitude. Namely, by ascertaining that the value before heating is larger and the value after heating is smaller, the validity can be determined. Embodiments 3 and 4: As a magnetic powder, Ni ferrite is selected, and Zn is selected so as to control the Curie temperature, and B 2 O 3 is combined and used as a glass forming material, and the composition is changed, and a (Ni, Zn) Fe 2 O 4 series is produced by way of trial. Firstly, the raw materials are sufficiently mixed and the mixture is put into a platinum vessel having a nozzle at its end. Next, the mixture is heated up to 1450°C by high frequency induction heating, pressured from above the platinum vessel, and put and suddenly cooled on the dual iron rollers with a diameter of 500 cm and a number of revolutions of 500 rpm and an amorphous material with a thickness of about 50 µ m is obtained. The obtained amorphous material is heat-treated in the air at 750°C for one hour and target fine particles of ferrite are crystallized. The glass forming material of the sample is dissolved and removed by a dilute acetic acid and the remaining powder is cleaned with water and dried. Among magnetic powder with a particle diameter of 50 to 100 nm expressed by Ni 1-x Zn x Fe 2 O 4 , three kinds of X = 0.7, 0.75, and 0.8 are mixed at a rate of 1:1:1 so as to form ink. A high-resolution ink jet printer prints a paper using this ink as Embodiment 3. Respective kinds of magnetic powder of X = 0.7, 0.75, and 0.8 are formed as ink and individual papers are printed by the same method in a stripe shape at different positions as Embodiment 4. These samples are detected repeatedly by a contact type magnetic head and the durability is ascertained. It is ascertained that no change is found in the output by detection of 1000 times. Furthermore, the samples of Embodiments 3 and 4 are detected repeatedly by a contact type magnetic head, and the durability is ascertained, and it is found that when the detection is repeated 1000 times, the output is reduced to about 2/3 of the initial value. The reason is considered as that since the particle diameter of the magnetic powder is comparatively large, powder existing on the surface without entering between fibers of the paper is omitted due to friction with the head caused by high-speed movement. The magnetic powder for validity determining ink of the present invention is satisfactory in the output and durability, applicable to various printing arts, and high in the reliability, determining speed, and forgery preventive effect. By the preferred manufacturing method for magnetic powder for validity determining ink, magnetic powder having a desired small particle diameter that is satisfactory in the output and durability, applicable to various printing arts, and high in reliability, determining speed, and forgery preventive effect can be obtained easily. Furthermore, when the magnetic ink for validity determination of the present invention is used, a printing member for validity determination which is satisfactory in output and durability, applicable to various printing arts, and high in reliability, determining speed, and forgery preventive effect can be provided easily. Furthermore, the printing member for validity determination of the present invention is satisfactory in output and durability and high in reliability, determining speed, and forgery preventive effect. Further, when the preferred detecting device for the printing member for validity determination is used, magnetic information that is high in reliability and forgery preventive effect can be detected easily. Furthermore, when the preferred validity-determining device is used, magnetic information that is high in reliability and forgery preventive effect is detected and the validity can be determined quickly.

Claims: A magnetic powder for validity determining ink comprising a first magnetic oxide powder having a first Curie temperature between -50°C and 150°C, characterized in that : the first magnetic oxide powder has a mean powder particle diameter of 10 µm or less and the first magnetic oxide powder is a ferrite series magnetic powder having a first coercive force of 20,000 A/m or less, and the magnetic powder further comprises: a second magnetic oxide powder having a second Curie temperature different from the first Curie temperature, a mean powder particle diameter of 10 µm or less, wherein the magnetic oxide powder is a ferrite series magnetic powder having a second coercive force different from the first coercive force. The magnetic powder according to claim 1, wherein the second magnetic oxide powder has a Curie temperature between -50°C and 150°C. The magnetic powder according to claim 1 or 2, wherein at least one of the first magnetic oxide powder and the second magnetic oxide powder includes at least one of nickel ferrite, copper ferrite and manganese ferrite as a main component. A magnetic ink comprising the magnetic powder of one of claims 1-3. A printed member for validity determination, comprising: a base (10), a first magnetic image (12) formed by a first magnetic oxide powder comprising a ferrite series magnetic oxide powder having a first coercive force of 20,000 A/m or less, and a second magnetic image (13), characterized in that : the first magnetic image (12) is obtainable by printing a first magnetic ink on the base (10), wherein the first magnetic oxide powder has a first Curie temperature between -50°C and 150°C and a mean powder particle diameter of 10 µm or less, and the second magnetic image (13) is obtainable by printing a second magnetic ink on the base, the second magnetic ink including a second magnetic oxide powder having a second Curie temperature higher than the Curie temperature of the first magnetic oxide powder, a mean powder particle diameter of 10 µm or less and a second coercive force different from the first coercive force. A printed member according to claim 5, wherein the first magnetic oxide powder and the second magnetic oxide powder have an iron oxide as a main component. A printed member according to claim 5 or 6, wherein at least one of the first magnetic oxide powder and the second magnetic oxide powder includes at least one of nickel ferrite, copper ferrite and manganese ferrite as a main component. A printing member according to one of claims 5-7, wherein the second magnetic image (13) is printed over the first magnetic image (12). A method for making a printed member comprising: forming a first magnetic image (12) by printing a first magnetic ink comprising a first magnetic oxide powder on a base (10), wherein the first magnetic oxide powder has a first Curie temperature between -50°C and 150°C, a mean powder particle diameter of 10 µm or less and a first coercive force of 20,000 A/m or less, wherein the first magnetic oxide powder is a ferrite series magnetic oxide powder, and forming a second magnetic image (13) by printing a second magnetic ink comprising a second magnetic oxide powder on the base (10), wherein the second magnetic oxide powder has a second Curie temperature higher than the Curie temperature of the first magnetic oxide powder, a mean powder particle diameter of 10 µm or less and a second coercive force different from the first coercive force. A method according to claim 9, wherein at least one of the first magnetic oxide powder and the second magnetic oxide powder includes at least one of nickel ferrite, copper ferrite and manganese ferrite as a main component. A method according to claim 9 or 10, wherein the second magnetic image (13) is printed over the first magnetic image (12). A method according to one of claims 9-11, wherein the base (10) is paper. A method according to one of claims 9-12, wherein the first and second magnetic images (12, 13) are printed using an ink jet printer. A method according to one of claims 9-13, further comprising forming the first magnetic oxide powder by: dissolving a magnetic oxide material and a glass forming material to obtain a mixture thereof, cooling the mixture rapidly to form an amorphous magnetic oxide/glass mixture, heat-treating the amorphous mixture to crystallize the magnetic oxide, and removing the glass forming material from the crystallized mixture to obtain the first magnetic oxide powder. A method according to one of claims 9-14, wherein the first magnetic ink is formed by mixing the first magnetic oxide with a resin and a dispersant and the second magnetic ink is formed by mixing the second magnetic oxide with a resin and a dispersant. A method for making a printed member comprising: mixing the magnetic powder of one of claims 1-3 with a resin and a dispersant, thereby forming a magnetic ink, and printing the magnetic ink on a paper (10).

Inventors: KOBAYASHI TADAHIKO, MURAKAMI TERUO, NAKAGAWA KATSUTOSHI, OBAMA MASAO, SAWA TAKAO, TAKAHASHI HISASHI, KOBAYASHI, TADAHIKO, MURAKAMI, TERUO, NAKAGAWA, KATSUTOSHI, OBAMA, MASAO, SAWA, TAKAO, TAKAHASHI, HISASHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 2001
Main classification: H01F1
IPC codes: H01F1, B42D15, C09D11, G07D7, C09C3, B41M3, C09C1, G08B13