JPS61280022A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To prevent the fraudulent use and leakage of magnetic recording information by a forged card by coating the fine particles having 10Angstrom -1mum particle diameter of a ferromagnetic alloy having 50-200 deg.C Curie point and >=50(Oe) coercive force on the surface magnetic layer. CONSTITUTION:A Ni-Fe alloy is heated and vaporized in the atmosphere of gaseous argon to form particles of single magnetic domain structure having 10Angstrom -1mum particle diameter, the particles are kneaded with an adhesive and the kneaded material is coated on a substrate magnetic film 2 through an isolat ing layer 3 to form a ferromagnetic surface magnetic layer 4. The Curie point of the surface magnetic layer is regulated to 50-200 deg.C and the coercive force is controlled to >=50(Oe). A surface protective layer 5 is then provided, signals are recorded on the substrate magnetic layer 2 and the function of preventing forgery is provided to the surface magnetic layer 4.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of Industrial Application The present invention is directed to magnetic cards widely used in cash cards, identification cards, etc., which are particularly resistant to counterfeiting and prevent the leakage of magnetically recorded information. Pertains to magnetic cards with functions.

(2) Conventional technology Magnetic cards have been widely used as cash cards and identification cards. Recently, it has begun to be applied to transportation tickets and commuter passes. However, magnetic recording and reproducing devices that perform recording and reproduction on this type of magnetic card do not have a function to accurately check the appearance, shape, etc. of the card itself. Therefore, even a counterfeit card is used to be treated as a legitimate card if the information recorded on the card is correct.

For this reason, as a surface magnetic layer of a magnetic recording medium, a soft magnetic alloy powder of several 6E or less with a Curie point of 300C or more is coated on the underlying magnetic layer to form a composite recording medium and magnetically prevent leakage of magnetic information. Magnetic recording media were also proposed, but
In this case, there was a drawback that magnetically recorded information in the underlying magnetic layer could be easily read by applying a weak magnetic field.

(3) Problems to be Solved by the Invention The present invention proposes a new magnetic recording medium that is more useful in preventing theft and leakage of magnetically recorded information by such counterfeit cards.

(4) Means for Solving the Problems In order to eliminate the above-mentioned drawbacks, the present invention adopts a thermal method as a method for demagnetizing the magnetically recorded information in the nine surface magnetic layers. For this purpose, ferromagnetic alloy fine particles having magnetic properties such as a large coercive force He and a small Curie point Tc are coated on the surface of the underlying magnetic layer.

Generally, magnetic alloys have a low Curie point Tcf.

If the magnetocrystalline anisotropy is small, the coercive force He will decrease. On the other hand, the fine particles of the 9 ferromagnetic alloy with a grain size of about 10K to several μm are divided into multi-domain structure particles, stable single-domain structure particles, and then SuperParamagnetigm.
Fine particles can be classified into six regions. Among these, stable single-domain structured particles are in a region where the height of decrease in static magnetic energy is smaller than the height of increase in domain wall energy.

The limit value Re of the particle size is given by Rc=9σ(//4πIs(C, Kittel: Phys, Rev
, 70 (1946) 965].

Here, σ and I8 indicate domain wall energy and saturation magnetization value, respectively. The critical magnetic field necessary for inversion of this single magnetic domain structure grain is given by H6=Ku/I. Here, Ku is uniaxial anisotropy energy. The critical magnetic field H8 is the rotational magnetization angle θ of the fine particles.

(the angle between the axis of easy magnetization and the external magnetic field).

In order to increase the coercive force Hat'', irreversible rotational magnetization of θ<45° may be used.

FIG. 1 schematically shows the relationship between the size of nine particles and the coercive force.

As is clear from the figure, as the particle size is reduced, the coercive force H8 increases rapidly from the middle region, peaks, and then rapidly decreases.

Ferromagnetic particles having a particle size in the region where Hc increases are coated as a surface magnetic layer. Generally, the particle size is in the range from 50A to 1 μm.

In accordance with the above-mentioned principles, the present invention provides a low key temperature point, T
c. High coercive force act-Four ferromagnetic alloy fine particles were coated as a surface magnetic alloy layer to form a material suitable for the above purpose.

(5) Example 33at%Ni-F with low temperature of about 100C
When e-alloy is prepared by the usual melting method, Hc at room temperature
could not fulfill the above-mentioned objective function at less than a few Oe. Therefore, this 33 at % Ni--Fe alloy was heated and evaporated in an argon gas atmosphere.

A large amount of single magnetic domain structure particles with a particle size of 0.5 to 0.4 μm were produced. Some of the particles are several to several tens of particles connected in a chain. The granular powder prepared as described above was kneaded with an adhesive and applied onto the underlying magnetic film with a separation layer interposed therebetween in a structure as shown in FIG. 2. In the figure, 1 is a non-magnetic base layer made of plastic material. 2 is the ferromagnetic underlayer coated on it, Hc=2000㎢e, '
r6=300C. A similarly formed ferromagnetic surface magnetic layer 4 is coated thereon with an isolation layer 3 interposed therebetween. The coercive force Hc'' of this surface magnetic layer was 1000e f. Note that 5 is a surface protective layer.The magnetic recording medium thus obtained records signals in the base magnetic layer 2 and has a counterfeit prevention function in the surface magnetic layer 4. .

(6) Effects of the invention The surface magnetic layer and the underlying magnetic layer of the composite magnetic alloy film thus prepared can be independently magnetized and demagnetized, and function as a recording/reproducing medium. It was confirmed that the

The underlying magnetic layer is Hc=20000e, 'rc=sooC
Adashi.

33 at % Hc number Oe of N1-Fa alloy film, T
Since c is sufficiently high compared to 100C, the underlying magnetic layer is hardly affected by recorded information even when heated to around 10DC. As explained above, the composite magnetic recording medium according to the present invention has a low curvature and a high coercive force.

Since the surface layer and the base layer have a dual structure depending on the temperature,
By using this magnetic recording medium, the magnetic recording medium is heated to a predetermined temperature during reproduction, and by comparing the reproduced information before heating and the reproduced information after heating, it can be determined whether the magnetic medium is genuine or not. can be identified, and unauthorized use due to forgery can be prevented.

Remaining days below

[Brief explanation of the drawing]

FIG. 1 is a curve diagram showing the relationship between particle size and coercive force, and FIG. 2 is a cross-sectional view in the thickness direction of a composite magnetic recording medium according to the present invention. In the figure: 1: Base layer 2: Underlayer 3: Isolation layer 4: Surface magnetic layer 5: Surface protective layer

Claims (1)

[Claims] 1. In a composite magnetic recording medium in which the Curie temperature and coercive force of the underlying magnetic layer are larger than the Curie temperature and coercive force of the surface magnetic layer, the Curie point of the surface magnetic layer is 50.
A composite magnetic recording medium characterized in that fine particles of a ferromagnetic alloy having a magnetic property of a coercive force of 50 Oe or more at a temperature of 10 A to 1 μm in diameter at a temperature of 10 A to 1 μm are coated.