Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2002-06-04
2004-04-06
Lam, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S145000, C365S173000
Reexamination Certificate
active
06717843
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates firstly to a magnetoresistive read/write memory according to the preamble of patent claim
1
. Furthermore, the invention relates to a method of writing to such a memory according to the preamble of patent claim
7
and to a method of reading from such a memory according to the preamble of patent claim
11
.
DESCRIPTION OF THE RELATED PRIOR ART
Magnetoresistive read/write memories, which are referred to as MRAM memories (magnetoresistive random access memory), are memories in which data can be stored and also read again at one address. The memories generally have one or more storage cells, the storage effect residing in the magnetically variable electrical resistance of the storage cell or the storage cells.
Hitherto the development concerned using single-value storage cells, as they are known, with a binary storage content for the magnetoresistive memories. These storage cells comprise two intersecting electric conductors, which are preferably aligned at right angles to each other. At the intersection between these conductors, there is a layer system comprising two magnetic layers, one of the layers being formed from soft magnetic material and the other layer from hard magnetic material. Between the magnetic layers there is a tunnel oxide. Such. a storage cell is described in the figure description in connection with FIG.
1
. Depending on the state of magnetization, the magnetization directions in the magnetic layers can be aligned in parallel or antiparallel. Depending on the magnetization direction in the magnetic layers, the storage cell has a different electric resistance. In this case, a parallel magnetization direction leads to a lower electrical resistance of the storage cell, a defined item of information is written to the memory cell. Depending on whether the storage cell experiences a change in its information content in the process, a current or voltage pulse, or else the absence of such a pulse, can be detected on the corresponding read line. In this case, each read operation results in a loss of information from the cell. In order to compensate for this loss, the information read out is written back to the storage cell again after the read operation has been completed.
U.S. Pat. No. 5,734,605 discloses a single-value MRAM storage cell in which two or more magnetic layers are arranged between two electric conductors. Because of ferromagnetic coupling between the magnetic layers, their magnetization directions are always oriented in parallel with each other in the absence of a magnetic field. The information stored in the storage cell is encoded in the direction of the common orientation of the magnetic layers. In order to read the information, a possible change in the electrical resistance of the layers, based on the large magnetoresistive effect, is detected by a magnetic field which is sufficient to fold over the magnetization direction of a first magnetic layer but is not sufficient to fold over the magnetization direction of a second magnetic layer being applied.
With regard to the problem of the increased area requirement of single-value storage cells in order to achieve powerful memory systems, in recent times multivalue storage concepts have been developed, but have hitherto been proposed only for other types of memories than MRAMs. Such a memory type is, for example, an EEPROM (electrically erasable PROM), which may be written to and erased electrically. In this case, specific subregions of a basically continuous value range are set equal to discrete states. Assignments of this type cannot be performed in the MRAM storage cells, since no storage of continuously variable information is possible. This is because, on the basis of their principle, MRAM storage cells fundamentally constitute a system with a finite number of discrete states. The approaches to multivalency with regard to the aforementioned EEPROMs cannot therefore be transferred to the magnetoresistive memories.
Jeong et al. (Jeong, W C, Lee, B I, Joo, S K (1998) “A New Multilayered Structure for Multilevel Magnetoresistive Random Access Memory (MRAM) cell”, IEEE Transactions on Magnetics, US, IEEE Inc., New York, Vol. 34, No. 4, Part 01, pp. 1069-1071) have shown, by using a layer sequence of magnetic nickel-iron, nickel-iron/cobalt and cobalt layers, in each case separated from one another by a copper intermediate layer and formed on a silicon copper double layer, that because of different coercivity field strengths of the three magnetic layers, the magnetization directions of the magnetic layers can be folded over individually by means of applying a tuneable magnetic field. This phenomenon was measured by using a respectively abrupt change in the resistance of the layer sequence, detected by means of a four-point measurement, resulting from the large magnetoresistive effect. Furthermore, Jeong et al. propose to set the individual magnetization directions of the magnetic layersin a sequence beginning with the magnetic layer with the highest magnetic hardness down as far as that magnetic layer with the lowest electric hardness.
Also known with regard to multivalue structures is U.S. Pat. No. 5,585,986, which discloses a digital magneto-resistive sensor for reading from a magnetic data storage system. This sensor has a multilayer structure, whose electrical resistance is measured. The multilayer structure comprises a number of ferromagnetic layers, the magnetic moments and therefore the magnetization directions of the individual layers being influenced by an external magnetic field, which is generated by the corresponding data carrier. magnetic field strengths. In this way, the resistance of the sensor changes in graduated steps when the external magnetic field is varied, which produces a digital output signal.
BRIEF SUMMARY OF THE INVENTION
On the basis of the aforementioned prior art, the intention is to provide a magnetoresistive read/write memory and a method of writing to and reading from such a memory in which the disadvantages and problems described in relation to the prior art are avoided. In particular, with regard to a possible saving of area, an improved MRAM memory and a suitable method of writing to and reading from such an MRAM memory are to be provided.
According to the first aspect of the invention, this object is achieved by a magnetoresistive read/write memory having one or more storage cells, each storage cell having two intersecting electric conductors and a layer system comprising magnetic layers located at the intersection of the electric conductors. According to the invention, the memory is characterized in that the layer system is designed as a multilayer system with two or more magnetic layers, in that at least two, but a maximum of all the magnetic layers have a magnetization direction that can be set independently of one another, and in that the magnetization direction in the individual layers is changed or can be changed on the basis of the electric current flowing through the electric conductors.
In this way, a multivalue memory having one or more multivalue storage cells and therefore with a correspondingly high storage capacity is created. Furthermore, this memory or the at least one storage cell can be written to and/or read from in a simple way.
A fundamental feature of the memory according to the invention or of the storage cell or storage cells forming the memory consists in using an n-layer multilayer system with n greater or equal to two magnetic layers at the intersection of the two electric conductors, it being possible for the magnetization direction in m layers, with n greater than or equal to m and M greater than or equal to two, to be set independently of the magnetization direction in the other layers. While in the case of the single-value memories the magnetization direction is set in only one magnetic layer, in the case of the multivalue memories according to the invention, this can now be carried out in any desired number of magnetic layers. In this case, the number of magnetic layers can va
Schwarzl Siegfried
Thewes Roland
Weber Werner
Briggs and Morgan, PA
Infineon - Technologies AG
Lam David
Stone Jeffrey R.
LandOfFree
Polyvalent, magnetoresistive write/read memory and method... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polyvalent, magnetoresistive write/read memory and method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polyvalent, magnetoresistive write/read memory and method... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3255300