GB1559119A - Magnetic recording medium with highly anisotropic particles - Google Patents

Description

(54) MAGNETIC RECORDING MEDIUM WITH HIGHLY ANISOTROPIC PARTICLES (71) We, MINNESOTA MINING AND MANUFACTURING COMPANY, a Corporation organized and existing under the laws of the State of Delaware, United States of America, of 3M Center, Saint Paul, Minnesota 55101, United States of America, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates in general to magnetic recording media and more specifically to recording media having highly anisotropic magnetic recording characteristics.

Although performance of reproduction of known recording magnetic media has reached a high level of quality, many uses now exist for magnetic media wherein the usual high quality parameters of such media are less important. For example, in some instances it may be more desirable to provide a magnetic medium that is magnetizable or erasable by particular magnetic fields than to provide a medium with high reproduction qualities. One highly useful application of a medium magnetizable or erasable by only certain fields is in the area of machine readable magnetically encoded documents such as credit cards.

As society becomes more and more dependent upon the use of magnetically encoded documents as currency substitutes, the need for such documents to be counterfeit resistant and fraud resistant ever increases.

The present invention provides a magnetic recording medium having a uniformly aligned magnetic particle population that is uni-axially highly anisotropic with an easy axis intrinsic coercive force of less than 1900 oersteds. The expression "uniaxially highly anisotropic" is defined as meaning that when a magnetic particle population is initially magnetized with a field of at least 2500 oersteds applied along the easy axis of the population, the level of residual magnetic moment of the population will be greater than 40% of the initial residual magnetic moment after the application along a hard axis of the population of an a.c.

erase field of peak value equal to two times the intrinsic easy axis coercive force of the population.

The recording medium of the present invention is preferably formed by suspending highly anisotropic particles in an organic binder in a mutually aligned fashion to produce a single easy axis of magnetization. An intense magnetic response is observed along such easy axis of magnetization and a relatively square hysteresis loop is associated therewith. In contrast, the magnetic response observed due to a magnetic field of 1,000-3,000 oersteds applied along an axis normal to the easy axis of the medium li.e. along a hard axis of magnetization) is negligible. Thus, the medium appears to be virtually nonmagnetic along the hard axes of the magnetic particles. As a result, the recording medium of the present invention may be readily selectively magnetized and demagnetized along the easy axes of the particles, but yet may be exposed to intense magnetic fields in a direction along their hard axes without significantly being affected.

The highly anisotropic magnetic recording particles employed in the recording medium of the present invention preferably are hexagonal ferrite particles that are generally euhedral platelets. Examples of materials suitable for such particles are lead, barium and strontium ferrites and iron cobalt phosphide. Media containing particles formed from materials such as other transition metal phosphides, yttrium cobalt, rare earth cobalt, rare earth cobalt compounds, manganese bismuth, Alnico compounds, iron-cobalt alloys, chromium dioxide or iron borides which exhibit highly anisotropic behaviour of the type desired, e.g. in the form of single domain size particles of sufficient crystal perfection, also come within the scope of the claims.

Hereinafter the invention is described by way of example and with reference to the accompanying drawings wherein: Fig. 1 is three graphs of the residual magnetic moment vs. hard axis erase field characteristics for highly anisotropic particle populations that may be employed in forming the present invention; Fig. 2 is three graphs of the residual magnetic moment vs. hard axis erase field characteristics for conventional recording particle populations; Fig. 3 is a plan view of a magnetically encoded document having a layer of the recording medium of the present invention: Fig. 4 is an end view of the encoded document of Fig. 3; Fig. 5 is a view in perspective of a standard magnetic read/record head; and Fig. 6 is a schematic view of an apparatus for reading the encoded data of the document of Fig. 3.

Referring now to Figs. 1 and 2, Fig. 1 illustrates graphs of residual magnetic moment vs. hard axis erase field characteristics of three representative samples of particles that may be used in forming magnetic media of the present invention, and Fig. 2 illustrates graphs of the same parameters of three known magnetic media not included in the present invention. In Fig. 1, graphs A, B and C respectively indicate the performance of an iron cobalt phosphide sample and two barium ferrite samples. The samples each comprised four stacked layers of 1/4 in. circular disks of magnetic recording tape with parallel aligned easy axes of magnetization. The tapes were prepared by coating dispersions of the various tvpes of magnetic particles in standard binder systems onto 1-1 1/2 mil.

polyester film using well known alignment methods to produce an easy axis parallel to the longitudinal dimension of the samples such as, for example, by applying a strong magnetic field along the longitudinal dimension of the samples during the coating operation. The hard axes erase fields were applied parallel to the plane of the samples, to eliminate possible errors due to shape effects. In Fig. 2, graphs D, E and F respectively indicate the performance of equant magnetite, chromium oxide, and cobalt doped iron ferrous oxide samples. The ordinates of the graphs of Figs. 1 and 2 represent the levels of residual magnetic moment of the samples and have been normalized for purposes of clarity.

None of the coercive forces of the samples of the graphs of Figs. 1 and 2 are the same and, therefore, the coordinates of twice the coercive force of each sample are indicated by circlet on the graphs.

To determine the coordinates for plotting the graphs of Figs. 1 and 2, a magnetic field of approximately 3000 oersteds was applied along the easy axis of each sample to induce an initial magnetic moment therein approaching saturation. Subsequently, the field was removed and the resulting level of residual magnetic moment of each sample was measured with a vibrating sample magnetometer. Such levels of residual magnetic moment were used as base levels. Next, individual readings of decreases in the base levels of the residual magnetic moment of each sample in response to hard axes erase fields were obtained by applying an a.c. erase field perpendicular to the easy axis of each sample and measuring the residual magnetic moment remaining after such field application. Initially.

the a.c. field was applied at a low level 3nd then progressively increased and a measurement was made of the residual magnetic moment remaining after the application of each level of erase field.

After a hard axis erase field equal to two times the coercive force was applied perpendicular to the easy axis of the highly anisotropic samples, each of samples A, B and C had a level of residual magnetic moment remaining that was considerablv higher than 40% of their base levels of residual magnetic moment (they ranged from 58% to 96% of such base levels).

The nonhighly anisotropic samples D, E and F in contrast did not have over 30% of their base levels of residual magnetic moment after application of an a.c. erase field equal to two times the coercive force (they ranged from 18% to 27%). Gamma iron oxide, which is the most common recording material available, is not covered in the graphs, but was also found to fall within such 18 - 27% range. Although the samples were magnetized with a field of approximately 3000 oersteds to ensure that they were sufficiently magnetized to provide meaningful results, a lower field could have been used so long as the samples were magnetized to at least 5052 of their saturated magnetic moment. For example in the embodiments an intrinsic coercive force of no more than 1600 Oersteds may be used.

Highly anisotropic particles make the magnetic recording medium of the present invention particularlv suitable for use in magnetically encoded documents that mav be employed together with a document reading apparatus (as described in detail below) to provide a system that significantly impedes the unauthorized use of such documents. Referring now to Figs. 3 and 4, a magnetically encoded document in the form of a credit card 1 is shown constructed of a non-magnetic layer 2, that may be embossed with appropriate identifying indicia in standard fashion as indicated, and a layer 3 of the magnetic recording medium of the present invention carried by the layer 2. The layer 3 may be a single strip, as shown, or it may be a layer completely covering one side of the nonmagnetic layer 2.

In certain applications, it may be advantageous to sandwich the layer 3 between two nonmagnetic layers 2 in order to protect the layer 3 against physical damage or other hazards that could degrade its recording capability. The layer 3 preferably has a single easy axis of magnetization along the longitudinal dimension of the card 1, and two of the family of hard axes of magnetization are respectively in the plane of the card 1 and perpendicular to the plane of the card 1. The layer 3 may be applied to the layer 2 by several methods such as direct coating, coating on an intermediate substrate and subsequently transferring the coating, or bonding by means of an adhesive.

The magnetizable layer 3 of the card 1 may be encoded with magnetically recorded digital data by moving the card at a constant relative velocity past a standard magnetic read / record head 7, as shown in Fig. 5, driven by electrical signal currents representing digital data. To achieve optimum recording of data on the layer 3, it may be necessary to use larger write currents than those driving standard read/ record heads, since the layer 3 may have a higher than standard intrinsic coercive force. Standard computer tapes generally have an intrinsic coercive force of 280-300 oersteds, whereas the highly anisotropic particles of the layer 3 will usually have an easy axis intrinsic coercive force of 500 oersteds or more, which is in the range of high energy (i.e. high coercive force) recording materials.

The read/record head 7 is a conventional ring-type head used in recording or reproducing magnetic signals from magnetic recording media. The head 7 includes a core 8 that has a small magnetizing gap 9 and an electrical coil 10 that is wound around a portion of the core 8 and is connected to electrical circuitry (not shown) that senses induced magnetization of the core 8. The orthogonal axes (w, 1.

d) are illustrated with the read / record head 7 to indicate the direction of gap width, gap length, and gap depth respectively.

Time varying currents through the coil 10 induce time varying magnetization in the core 8, resulting in a time varying magnetizing field across and adjacent the gap 9 with components substantially along the "1" and axles axe only. The "1" and "d" com- ponents of such field may be used to record magnetization patterns corresponding to electrical signals on a magnetic recording surface translating across the gap 9. In a conventional two frequency coherent phase recording for encoding magnetic strip credit cards, the layer 3 may be magnetized to saturation with reversals of direction of the saturating field of the read / record head 7 corresponding to clock pulses and data bits. Usually the relative motion of translation of a conventional recording medium is along the "1" axis of the head, but the relative translation of the medium may be at an angle "a" (not shown) in the "l-w" plane. Small angles of "a" (less than 500) are preferred, but angles up to 900 are possible.

Magnetic recording documents of the present invention may also be encoded as they are translated by the gap 9 of the read/record head 7 with relative motion of translation along the "1" axis, but with the easy axis of the documents at a small angle "a" in the "l-w" plane. Thus, the direction of the easy axis can be varied if a corresponding variation in the direction of the gap length is made to maintain a substantially parallel relationship between the "l-d" plane and the direction of the easy axis of magnetization of the recording document.

An apparatus 11 for reading the card 1 is illustrated in Fig. 6 and includes a pair of endless belts 12 for translating the card 1 through the apparatus 11, and a hard axis field source in the form of a pair of permanent magnetic 13 positioned on each side of the card 1. However, the magnets 13 may be both positioned on the side of the card 1 on which the layer 3 is affixed. In fact, in certain applications it may be more advantageous to have both the magnets 13 on the same side to apply a hard axis field in the plane of the card 1 and thereby minimize undesirable shape effects asso ciated with magnetization perpendicular to the plane of the layer 3. A predetermined level of magnetic field is provided by the magnets 13 along the hard axis of the card 1 before the card 1 reaches a read-record head 14, similar to the head 7. The strength of the hard axis field provided by the magnets 13 preferably is between the easy axis coercive force of the highly anisotropic layer 3 and the field applied by a record head during normal recording of the layer 3. If the field of the magnets 13 is too low, conventional magnetically encoded cards will not be erased, but if the field is too high, cards of the present invention as well as conventional cards will be erased.

The provision of the hard axis field source is not restricted solely to the use of the permanent magnets 13 but instead may be provided by current carrying coils, electromagnets, or a transversely oriented head. However, the permanent magnets are advantageous to use because they do not require a power source. Because the magnetizable layer 3 of the card 1 is formed of highly anisotropic material, the magnetized regions along the easy axis of the layer 3 will not be sufficiently affected by the hard axis field to erase recorded data thereon, which data is retained and sensed as the card 1 passes by the read/record head 14.

Following below are a number of examples of the type of documents employing magnetic media of the present invention, which examples are described for purposes of illustration only and not of limitation.

EXAMPLE I A magnetically encoded planar document is formed bearing a magnetic strip of recording material of the present invention oriented with its easy axis of magnetization in the plane of the document, but at an oblique angle to the longitudinal axis of the document. The hard axes of magnetization of the strip include any axis in a plane perpendicular to the easy axis. This document is useful with reading systems where the length of the gap of the reading head is parallel to the easy axis of magnetization of the recording material.

EXAMPLE 2 A magnetically encoded planar document is formed bearing a magnetic strip of recording material of the present invention having its easy axis of magnetization oriented at an oblique angle to the plane of the magnetic strip but with no horizontal component perpendicular to the longitudinal axis of the document. The term "horizontal component" is used with reference to the document being oriented wholly within a horizontal plane.

EXAMPLE 3 A magnetically encoded document that is formed as described in Example 2 except that the easy axis of the recording material strip is oriented such that the direction of the easy axis includes a horizontal component perpendicular to the longitudinal axis of the document.

EXAMPLE 4 A magnetically encoded planar document magnetization in any direction parallel to the plane of the document.

WI AT WE CLAIM IS: 1. A magnetic recording medium comprising a uniformly aligned population of uniaxially highly anisotropic particles which population has an easy axis and a plurality of hard axes of magnetization, and an easy axis intrinsic coercive force less than 1900 oersteds, whereby when the population is initially magnetized with a field of at least 2500 oersteds applied along said easy axis, the level of residual magnetic moment of the population will be greater than 40% of the initial residual magnetic moment after the application along one of said hard axes of an a.c. erase field of peak value equal to two times the intrinsic easy axis coercive force of the population.

2. A magnetic recording medium as recited in claim 1, wherein the anisotropy and alignment of said magnetic particles is sufficient to enable substantial erasure of the residual magnetic moment of substantially all of sail particles with a predetermined intensity magnetic field applied along the easy axis of said particles au.- whereas the same intensity magnetic field applied along the hard axis of said particles is insufillicent to produce such erasure.

3. A magnetic recording medium according to claim 1, wherein said magnetic particles are formed of one or more of barium, strontium and lead ferrites.

4. A magnetic recording medium as recited in claim 1, wherein the intrinsic coercive force of said population is no more than 1600 oersteds.

5. A magnetic recording medium as reis formed bearing a magnetic strip of recording material of the present invention having an easy axis of magnetization oriented along an axis perpendicular to the plane of the strip, and hard axes of cited in any preceding claim, wherein said medium is carried in surface-to-surface contact with a substrate layer of nonmagnetizable material to form a machine readable magnetically encoded document.

6. A magnetic recording medium as recited in claim 5, wherein said easy axis of magnetization of said particle population is oriented parallel to the longitudinal axis of said substrate layer.

7. A magnetic recording medium as re

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. invention as well as conventional cards will be erased. The provision of the hard axis field source is not restricted solely to the use of the permanent magnets 13 but instead may be provided by current carrying coils, electromagnets, or a transversely oriented head. However, the permanent magnets are advantageous to use because they do not require a power source. Because the magnetizable layer 3 of the card 1 is formed of highly anisotropic material, the magnetized regions along the easy axis of the layer 3 will not be sufficiently affected by the hard axis field to erase recorded data thereon, which data is retained and sensed as the card 1 passes by the read/record head 14. Following below are a number of examples of the type of documents employing magnetic media of the present invention, which examples are described for purposes of illustration only and not of limitation. EXAMPLE I A magnetically encoded planar document is formed bearing a magnetic strip of recording material of the present invention oriented with its easy axis of magnetization in the plane of the document, but at an oblique angle to the longitudinal axis of the document. The hard axes of magnetization of the strip include any axis in a plane perpendicular to the easy axis. This document is useful with reading systems where the length of the gap of the reading head is parallel to the easy axis of magnetization of the recording material. EXAMPLE 2 A magnetically encoded planar document is formed bearing a magnetic strip of recording material of the present invention having its easy axis of magnetization oriented at an oblique angle to the plane of the magnetic strip but with no horizontal component perpendicular to the longitudinal axis of the document. The term "horizontal component" is used with reference to the document being oriented wholly within a horizontal plane. EXAMPLE 3 A magnetically encoded document that is formed as described in Example 2 except that the easy axis of the recording material strip is oriented such that the direction of the easy axis includes a horizontal component perpendicular to the longitudinal axis of the document. EXAMPLE 4 A magnetically encoded planar document magnetization in any direction parallel to the plane of the document. WI AT WE CLAIM IS:

1. A magnetic recording medium comprising a uniformly aligned population of uniaxially highly anisotropic particles which population has an easy axis and a plurality of hard axes of magnetization, and an easy axis intrinsic coercive force less than 1900 oersteds, whereby when the population is initially magnetized with a field of at least 2500 oersteds applied along said easy axis, the level of residual magnetic moment of the population will be greater than 40% of the initial residual magnetic moment after the application along one of said hard axes of an a.c. erase field of peak value equal to two times the intrinsic easy axis coercive force of the population.

2. A magnetic recording medium as recited in claim 1, wherein the anisotropy and alignment of said magnetic particles is sufficient to enable substantial erasure of the residual magnetic moment of substantially all of sail particles with a predetermined intensity magnetic field applied along the easy axis of said particles au.- whereas the same intensity magnetic field applied along the hard axis of said particles is insufillicent to produce such erasure.

3. A magnetic recording medium according to claim 1, wherein said magnetic particles are formed of one or more of barium, strontium and lead ferrites.

4. A magnetic recording medium as recited in claim 1, wherein the intrinsic coercive force of said population is no more than 1600 oersteds.

5. A magnetic recording medium as reis formed bearing a magnetic strip of recording material of the present invention having an easy axis of magnetization oriented along an axis perpendicular to the plane of the strip, and hard axes of cited in any preceding claim, wherein said medium is carried in surface-to-surface contact with a substrate layer of nonmagnetizable material to form a machine readable magnetically encoded document.

6. A magnetic recording medium as recited in claim 5, wherein said easy axis of magnetization of said particle population is oriented parallel to the longitudinal axis of said substrate layer.

7. A magnetic recording medium as re

cited in claim 5, wherein the easy axis of said magnetization of said particle population is oriented perpendicular to the plane of said substrate layer.

8. A magnetic recording medium as recited in claim 5, wherein said easy axis of magnetization of said particle population is oriented at an oblique angle with the longitudinal axis of said substrate layer.

9. A magnetic recording medium as recited in claim 8, wherein the easy axis of said particle population is also oriented parallel to the plane of the document.

10. A magnetic recording medium, herein described with reference to Figures 1, 4 and 6 of the accompanying drawings.