# US 5,353,268 A · Thermomagnetic recording system employing a medium having high storage density and direct-overwrite capability as a result of along-track isocoercivity

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- Number: US 5,353,268 A
- Document ID: US5353268A
- Status: Granted
- Kind: A
- Application number: 08/150,771
- Filed: 1993-11-12
- Granted: 1994-10-04
- Earliest claimed: 1990-08-10
- Assignees: Minnesota Mining and Manufacturing Company
- Inventors: Michael B. Hintz
- Web page: https://patentgrep.com/patent/US5353268A

## CPC

- G11B 13/045: {combined recording by magnetic and optic means}
- G11B 5/012: Recording on, or reproducing or erasing from, magnetic disks (G11B17/00, G11B19/00 take precedence)
- G11B 5/78: Tape carriers
- G11B 5/82: Disk carriers
- G11B 2005/0002: {Special dispositions or recording techniques}
- G11B 2005/0005: {Arrangements, methods or circuits}
- G11B 2005/0021: {Thermally assisted recording using an auxiliary energy source for heating the recording layer locally to assist the magnetization reversal}
- G11B 2005/0029: {using magnetisation components of the recording layer disposed mainly perpendicularly to the record carrier surface}
- G11B: INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER (recording measured values in a way that does not require playback through a transducer G01D9/00; recording or playback apparatus using mechanically marked tape, e.g. punched paper tape, or using unit records, e.g. punched or magnetically marked cards G06K; transferring data from one type of record carrier to another G06K1/18; circuits for coupling output of reproducer to radio receiver H04B1/20; gramophone pick-ups or like acoustic electromechanical transducers or circuits therefor H04R)

## Abstract

A thermomagnetic recording system achieves high areal bit density by combining the high linear bit density of magnetic recording and the high track density typical of magneto-optic recording. The system employs a medium which comprises a ferrimagnetic recording material having a compensation temperature approximately equal to ambient operating temperature, yet small coercivity and high saturation magnetization at elevated temperatures. A source of heat (preferably a laser) controls the magnetic properties of the material. Data is recorded and played back with high performance magnetic recording heads, producing direct overwrite capability, at temperatures less than the Curie point of the material.

## Claims

### Claim 1

A thermomagnetic recording medium for use with a heat source which raises temperature of a heated region of the medium from an ambient temperature to an operating temperature, a magnetic field source which imposes a magnetic field upon an area of the medium including a portion of the heated region, and a magnetic field detector which senses remanent magnetic moment of the medium over a substantially rectangular sensing region having longest dimension perpendicular to the direction of a data track on the medium, the medium comprising:
a substrate;
a barrier layer deposited on the substrate;
a magnetizable recording layer deposited on the barrier layer, wherein the recording layer has a net magnetic moment and coercivity dependent upon temperature, such that the net magnetic moment is substantially zero at the ambient temperature and substantially different from zero at the operating temperature, the coercivity decreases with temperature to zero at a Curie temperature which is substantially above the operating temperature, and the portion of the heated region which is exposed to the imposed magnetic field exhibits substantially constant coercivity along the track direction; and
a protective layer deposited on the recording layer.

### Claim 2 (depends on 1)

The medium of claim 1, wherein the recording layer comprises an alloy of at least one rare earth element and at least one transition metal.

### Claim 3 (depends on 2)

The medium of claim 2, wherein the rare earth element of the recording layer comprises at least one of terbium, gadolinium, and dysprosium.

### Claim 4 (depends on 2)

The medium of claim 2, wherein the transition metal of the recording layer comprises at least one of cobalt, iron, manganese, and nickel.

### Claim 5 (depends on 2)

The medium of claim 2, wherein the recording layer comprises terbium, dysprosium, iron, and cobalt.

### Claim 6 (depends on 5)

The thermomagnetic recording medium of claim 5, wherein the recording layer comprises terbium, dysprosium, iron, and cobalt in proportions given by Tb.sub.w Dy.sub.y Fe.sub.y Co.sub.z, wherein 0<w<25, 0<x<25, 0<y<50, 10<z<70, and w+x+y+z=0100.

### Claim 7 (depends on 2)

The medium of claim 2, wherein the recording layer further comprises at least one element chosen from the group consisting of chromium, yttrium and tantalum.

### Claim 8 (depends on 1)

The thermomagnetic recording medium of claim 1, wherein the recording layer comprises a plurality of compositionally distinct sublayers magnetically coupled to one another by exchange interactions.

### Claim 9 (depends on 8)

The thermomagnetic recording medium of claim 8, wherein at least one sublayer comprises an alloy chosen from the group consisting of (a) at least one rare earth element, (b) at least one transition metal, and (c) an alloy comprising at least one rare earth element and at least one transition metal.

### Claim 10 (depends on 1)

The medium of claim 1, wherein the recording layer exhibits perpendicular magnetization.

### Claim 11 (depends on 1)

The medium of claim 1, wherein the recording layer exhibits parallel magnetization.

### Claim 12 (depends on 1)

The medium of claim 1, wherein the recording layer exhibits anisotropic thermal conductivity parallel to the plane of the recording layer.

### Claim 13 (depends on 12)

The medium of claim 12, wherein the substrate comprises grooved tracks having a thermally conductive material deposited in the tracks.

### Claim 14 (depends on 1)

The medium of claim 1, further comprising a lubricant layer deposited on the barrier layer.

### Claim 15 (depends on 1)

The medium of claim 1, wherein the substrate is flexible.

### Claim 16

A thermomagnetic recording system, comprising:
a thermomagnetic recording medium having data tracks provided thereon;
a heat source which raises temperature of a heated region of the medium from an ambient temperature to an operating temperature; and
a magnetic assembly comprising a magnetic field source, which imposes a magnetic field upon an area of the medium including a portion of the heated region, and a magnetic field detector which senses a remanent magnetic moment of the medium over a substantially rectangular sensing region having longest dimension normal to the length of data tracks on the medium;
wherein the medium comprises:
a substrate;
a barrier layer deposited on the substrate;
a magnetizable recording layer deposited on the barrier layer, wherein the recording layer has a net magnetic moment and coercivity dependent upon temperature, such that the net magnetic moment is substantially zero at the ambient temperature and substantially different from zero in the heated region which is at an operating temperature, the coercivity decreases with temperature to zero at a Curie temperature which is substantially above the operating temperature, and the portion of the heated region which is exposed to the imposed magnetic field exhibits substantially constant coercivity along the length of the tracks; and
a protective layer deposited on the recording layer.

### Claim 17 (depends on 16)

The system of claim 16, wherein the heat source is a laser.

### Claim 18 (depends on 17)

The system of claim 17, wherein the heat source and the magnetic assembly are on a same side with respect to a surface of the thermomagnetic recording medium.

### Claim 19 (depends on 17)

The system of claim 17, wherein the magnetic field source comprises a magnetic transducer, having a gap, and a waveguide which directs laser light to the thermomagnetic recording medium through the gap.

### Claim 20 (depends on 19)

The system of claim 19, wherein the waveguide comprises a fiber optic assembly.

### Claim 21 (depends on 17)

The system of claim 17, wherein the laser is a laser diode incorporated into the magnetic assembly.

### Claim 22 (depends on 16)

The system of claim 16, wherein the magnetic field detector comprises a magnetic transducer sensitive to magnetization of the thermomagnetic recording medium which has a direction normal to the plane of the medium.

### Claim 23 (depends on 16)

The system of claim 16, wherein the magnetic field detector comprises a magnetic transducer sensitive to magnetization of the thermomagnetic recording medium which has a direction parallel to the plane of the medium.

### Claim 24 (depends on 16)

The system of claim 16, wherein the magnetic field detector comprises a first magnetic transducer for detecting remanent magnetic properties of the magnetic recording medium and a magnetic field source comprising a second magnetic transducer for changing remanent magnetic properties of the magnetic recording medium.