Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1999-09-10
2002-05-14
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S690000, C428S690000, C428S690000, C428S690000, C428S692100, C428S900000
Reexamination Certificate
active
06387476
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P11-200840 filed Jul. 14, 1999 which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic functional element adapted to make its magnetic state variable and also to a magnetic recording medium comprising a plurality of such magnetic functional elements for recording information.
2. Prior Art
Devices made of a magnetic material are technologically attractive and appealing in two aspects if compared with devices made of a semiconductor and widely used in recent years.
Firstly, an electrically conductive metal material can be used to produce such devices. Therefore, devices made of a magnetic materials show a high carrier density and a low resistance if compared with devices made of a semiconductor material and hence are expected to be good for an enhanced degree of miniaturization and integration.
Secondly, the bistability of magnetic materials in terms of direction of magnetization reveals their high potential of being used for non-volatile memories. In other words, it is expected to realize solid state non-volatile memories that can keep the information they store if the power supply is suspended by utilizing the bistability of magnetic materials.
Solid-state non-volatile memories are expected to fund applications in various technological fields as highly energy-saving memories. More specifically, solid-state non-volatile memories consume little power when left inactive so that they are expected to take a key role in small and portable electronic information processing apparatus because small electronic apparatus comprising such memories require only a small battery capacity and hence will be very lightweight. Additionally, solid-state non-volatile memories fund a strong demand in the rising satellite media business because they can support the operations of satellites when they are shadowed by the earth and their solar light power generation systems have to remain dormant.
In short, devices made of a magnetic material provides the advantages of (1) having a non-volatile memory effect, (2) being free from degradation due to repeated recording/reproducing operations, (3) being adapted to high speed writing operations, (4) being adapted to down-sizing and high density arrangement and (5) being capable of withstanding radiation. These advantages will be discussed below.
(1) Having a Non-volatile Memory Effect
Due to the bistability of magnetic materials in terms of direction of magnetization, the information recorded by utilizing the direction of magnetization is retained without being lost if the drive force fades away as in the case of magnetic recording media including magnetic tapes and magnetic disks.
(2) Being Free from Degradation Due to Repeated Recording/Reproduction Operations
For instance, memories made of a ferroelectric material that is bistable (F-RAMs: ferroelectric random access memories) like a magnetic material have been proposed as solid-state non-volatile memories. In the case of F-RAMs, information is rewritten there by inverting the spontaneous dielectric polarization and thereby changing the memory state. However, as the memory state is changed in an F-RAM, ions are moved in the crystal lattice of the device to eventually develop crystal defects there if the rewriting operation is repeated for a number of times exceeding a hundred million. Thus, F-RAMs show a service life the is inevitably limited by the fatigue of the material. To the contrary, in devices realized by utilizing the bistability of a magnetic material, the inversion of magnetization is not accompanied by any migration of ions so that their service life is not limited by the fatigue of the material and information can be rewritten almost limitlessly.
(3) Being Adapted to High Speed Writing Operations
The rate of inversion of magnetization of a magnetic material is very high, although it does not exceed 1 ns. Therefore, devices adapted to high speed writing operations can be realized by exploiting the high switching rate.
(4) Being Adapted to Down-sizing and High Density Arrangement
The magnetic state of a magnetic alloy can be made to vary remarkably by appropriately selecting the composition and the texture thereof to provide an enhanced degree of freedom for designing a device made of such a magnetic material. Additionally, a device can be made of an electrically conductive magnetic alloy. A device made of an electrically conductive magnetic alloy can be made to show an improved current density in the device if compared with a device made of a semiconductor material for the purpose of down-sizing and high density arrangement.
(5) Being Capable of Withstanding Radiation
Known D-RAMs (dynamic random access memories) adapted to change the memory state for rewriting information by charging an electric load give rise to an electric discharge when exposed to ionizing radiation that penetrates the device and change the memory state. To the contrary, the direction of magnetization of a magnetic material is not disturbed if exposed to ionizing radiation. Therefore, devices made of a magnetic material are highly capable of withstanding radiation. Thus, devices made of a magnetic material can effectively be used in applications that require an enhanced ability of withstanding radiation such as communication satellites. As a matter of fact, magnetic bubble memories made of a magnetic material are widely used in satellites.
As described above, devices made of a magnetic material provide various advantages and there have been proposed various solid-state magnetic memories (M-RAMs: magnetic random access memories) that are designed to fully exploit these advantages. Generally, a magnetic thin film having a uniaxial magnetic anisotropy is used as memory carrier in an M-RAM and information is recorded in the memory by inverting the direction of magnetization of the magnetic thin film. In other words, an M-RAM is a magnetic memory device utilizing the arrangement of a magnetic material for storing information. Thus, unlike a magnetic tape or a magnetic disk, it can store information without requiring an operation of moving the memory carrier relative to a magnetic head.
However, known M-RAMs are provided with conductors arranged close to the memory carrier in order to invert the direction of magnetization of the carrier. Then, the operation of inverting the direction of magnetization of the carrier is controlled by applying a current pulse to the conductor and utilizing the magnetic field generated by the current pulse. However, the operation of inverting the direction of magnetization of the carrier by utilizing the magnetic field generated by a current pulse is accompanied by two major problems.
Firstly, cross talks can arise as a result of an operation of inverting the direction of magnetization by means of a magnetic field. Since a magnetic field can exert force over a long distance, it can innegligibly affect regions neighboring the memory carrier for inverting the direction of magnetization to consequently give rise to cross talks. If such memory carriers are arranged highly densely in a device, it will no longer be possible to stably carry out the operation of inverting the direction of magnetization and the reliability of the device. While there have been proposed memory carriers provided with a structure for shielding the carriers from magnetic fields [see, inter alia, Z. G. Wang, et al., IEEE Trans Magn., Mag 33, 4498 (1997)], such an arrangement makes the device structurally complex.
Secondly, because a magnetic field generated by applying a current pulse to conductors, the coercive force of the memory carrier can be reduced as fine conductors are used for the purpose of miniaturization. This problem will be discussed hereinafter.
The current density i [A/m
2
] of a conductor has a limit that is defined by the material of the conductor. As the device is miniaturized and
Bessho Kazuhiro
Iwasaki Yoh
Kiliman Leszek
Sonnenschein Nath & Rosenthal
Sony Corporation
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