Static information storage and retrieval – Read/write circuit – Differential sensing
Reexamination Certificate
1999-07-12
2001-07-24
Le, Vu A. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Differential sensing
C365S055000, C365S171000
Reexamination Certificate
active
06266289
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates primarily to magnetic write and read of data and can be used in arrangements with reverse digital high-density memory devices which are intended for operations with large data arrays and in other electronic equipment.
One of the fundamental units in the modem computer technology and in other electronic systems is a memory device, the efficiency of which is characterized by the following basic parameters: stored data capacity and write density, access time (duration of the whole cycle of random address data retrieval), non-volatility of storage (possibility to store data in the event of power loss), lifetime under use and cost for mass production of 1 MB of the memory.
Recently, optical disks surpassed all other memory devices in capacity (from 128 to 2000 MB), but unfortunately, they are either not adapted to the rewrite operations or their write and read time is very long (for example, the access time amounts to 30-50 ms). The hard magnetic disks can store large arrays of information (up to 10 GB) and possess a rewrite capability. The access time is reduced to 8-15 ms, and yet it is still very long. Both types of memory devices (optical disks and hard magnetic disks) are non-volatile.
A dynamic random-access memory (DRAM) employs a charged capacitor as the data storing element (memory cell). Such devices are made in a form of integrated circuits, which results in a very short access time of about 10-50 ns and a write density of 3-12 MB/cm
2
. However, DRAMs are volatile and require a periodic refreshing.
A static memory device employs a solid-state gate in the transistor assembly storing element (memory cell). Such devices are intricate for fabrication and can be used only as a cache memory, i.e., as a buffer for exchange between the slow and fast memory devices. The disadvantages of static memories are limited non-volatility, comparatively low density (1-2 MB/cm
2
) and rather high cost.
Recently, efforts have been made to create a memory device that is free of the above disadvantages, but concurrently provides a high density of writing (about 100 MB/cm
2
), a short access time (about 10 ns), is non-volatile has an unlimited number of rewrite and read cycles, and a low cost of fabrication. The so-called ferroelectric and magnetic memory devices meet these requirements most closely.
It is known that a numerous class of polycrystals, designated as ferroelectrics, possesses the property of being able to maintain the state of a given electrical polarization for a long time. This property allows them to be used in memory cells instead of the dielectric between the capacitor plates in the dynamic memory with the result that the dynamic memory takes on the property of nonvolatility.
Unfortunately, the ferroelectric memory devices have a limited lifetime. Even after a limited number of polarization reversals, the ferroelectric begins to age due to accumulating a spurious electric charge in the crystal. The absolute magnitude of the electrical polarization in the memory cell decreases, and the cell becomes less operable and eventually loses its memory properties (U.S. Pat. No. 5,768,182, Int.Cl. H01L 31/062 (365/145), 1998).
The same disadvantage is peculiar to the memory devices based on other high-quality dielectrics, as the mere fact of availability of electric charges inevitably leads to gradual ageing of the dielectric memory through the leakage of the spurious charge (U.S. Pat. No. 5,796,670, Int. Cl. G11C 13/00 (365-228), 1998).
There is a method and a device with a so-called “magnetic” random access memory (MRAM), where the write and read operations rest on the property of a conductor to change the electrical resistance in the event of applying a magnetic field (effect of anisotropic resistance (AMR) or giant magnetoresistance (GMR)).
The device comprises either individual circuits for writing and reading data or (in the event of using conductors with the GMR) multilayer structures. Each memory cell comprises separate elements for writing, storing and reading the data. Such a memory cell is capable of bearing a practically unlimited number of rewriting cycles, due to stability of the so-called “exchanging” forces of interactivity of atomic electrons and by the absence in the nature of magnetic charges that could lead to depolarization of the magnetic dipole moments of data carriers (U.S. Pat. No. 5,587,943, Int.Cl. G11C 11/15 (365/158, 1996).
A disadvantage of this method is that the effect of the AMR manifests itself only against the background of very high ohmic resistance of conductors. When a single circuit is used both for writing and for reading the information, a strong current is passed through the conductor, this results in manifestation of an electromigration effect (transfer of a portion of the substance of the current busbar together with the electrical current), which leads to the loss of normal operation of the memory cell.
Additional disadvantages of a device realizing the method are rather low write density and difficulty of its fabrication, while the usage of conductors with the GMR results in strong dependence of the device operation on the temperature, i.e., lower temperatures are preferable.
A memory device (magnetic transistor) is known, whose operation is based on using spin dependent effects of transferring the charges in a magnetic field (electron tunneling effect). The charge tunneling in the magnetic transistor between two layers of a metal ferromagnetic, separated by a layer of the dielectric, is controlled by the magnetic field (U.S. Pat. No. 5,650,958, Int. Cl. G11B 5/127 (365/173), 1997).
A disadvantage of this design is the dependence of the memory device operation on the temperature, i.e., the warm-up of the device during long-term operations causes instability of hysteresis curves. In addition, the technology of the device fabrication is complicated; it is required to apply about ten layers of various materials, including dielectrics. There is a method and a memory device to realize it, which is based on usage of magnetic particles (domains) being longitudinally or transversely arranged on the carrier and possessing magnetic dipole moments.
For writing and reading the information the memory device uses relative movement of a carrier with magnetic particles and a magnetic head, which generates during writing and records during reading a magnetic field which is homogeneous in direction and varying in strength. The writing density in this case is at the level of 10
8
-10
9
bit/cm
2
(M. B.Gitlits “Magnetic Recording of Signals”, Moscow, “Radio & Svyaz”, 1990, p.232).
Among the disadvantages of the above design are limitations of the writing density resulting from the rigid coupling of magnetic particles (domains) during multiplexing, a risk of losing information under influence of external fields, as well as a limited lifetime of carriers and a limited reliability of memory devices due to the necessity to use units which execute high speed mechanical motion for write and read operations.
There is a method of magnetic-toroid writing and reading of information based on the interaction of magnetized particles of the carrier, which are concentrically closed into toroid-like patterns (aggregates of the carrier's magnetic particles) with a controlling vortex magnetic field: The magnetic field changes the orientation of the moments of magnetic particles during writing of information and registers the parameters of electrical field excited by moving aggregates of the carrier's magnetic particles during reading of information (V. M. Dubovik, A. M. Martsenyuk, N. M. Martsenyuk “Magnetic reversal of Aggregates of Magnetic Particles by a Vortex Magnetic Field and Usage of a Toroid Feature for Data Writing.” Reprint of Joint Institute for Nuclear Research, R .17-92-541, 1992). Two magnetization states of such aggregates, referred to as logical “0” and logical “1” in the digital code, differ by opposite (clockwise or counter-clockwise) directions of the magnetization vortex and, accordingly
Dubovik Vladimir Mikhailovich
Kislyakov Yury Vyacheslavovich
Martsenyuk Mikhail Andreevich
Ossipov Pavel Albertovich
Senchenko Viktor Alekseevich
Amphora
Gray Cary Ware & Freidenrich
Le Vu A.
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