Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-10-22
2004-08-10
Auduong, Gene N. (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000, C365S171000
Reexamination Certificate
active
06775183
ABSTRACT:
The invention relates to a magnetic memory cell, a magnetic memory comprising an inventive magnetic memory cell, a memory circuit for writing a magnetic memory cell, and a method of writing a magnetic memory cell.
BACKGROUND OF THE ART
Magnetic random access memories (MRAMs) have been proposed due to their non-volatile nature. Unlike dynamic random access memory (DRAM) cells, non-volatile memory cells such as MRAM cells do not require complex circuitry for perpetual electronic refreshing of the stored information.
The first of such MRAMs were based on magnetic multi-layer structures, deposited on a substrate. U.S. Pat. No. 5,343,422, for example, discloses a structure in which two layers of ferromagnetic material are separated by a layer of non-magnetic metallic conducting material. One of the magnetic materials, called the ferromagnetic fixed layer (FMF), has a fixed direction of magnetic moment, e,g., by having a particularly high coercive field or strong unidirectional anisotropy. The other magnetic layer, called the ferromagnetic soft layer (FMS), has a preferred axis for the direction of magnetisation, the so called easy-axis, which is aligned parallel to the magnetic moment of the ferromagnetic fixed layer, The magnetic moment of this ferromagnetic soft layer is free to change direction between parallel and anti-parallel alignment relative to the easy-axis, and as a consequence, also relative to the magnetic moment of the ferromagnetic fixed layer on application of an external magnetic field.
The state of the storage element represents a logical “1” or “0” depending on whether the directions of the magnetic moments of the magnetic layers are in parallel or anti-parallel alignment, respectively. Because the resistance of the storage element is different for different mutual orientations of the magnetic moments, the structure acts as a spin valve. It thus allows the sensing of the state of the storage element by measuring the differential resistance &Dgr;R/R with a current, where &Dgr;R is the difference in resistance of the storage element for two different states of relative orientation of the magnetic moments, and R is the total resistance of the structure in the lower resistance state.
A switching between these orientations can be achieved by passing write currents in the vicinity of the FMS, usually by using write lines which run past the layered structure on either side. These write currents, which do not pass through the layered structure itself, induce a magnetic field at the location of the FMS which alters the orientation of the FMS, if it is stronger than the coercive field H
c
of the FMS.
An alternative is disclosed in U.S. Pat. No. 6,072,718. There, the conducting nonmagnetic spacer layer between the two magnetic layers is replaced by an insulator. The device therefore forms a magnetic tunnel junction (MTJ), where spin polarised electrons tunnel through the insulator.
The cell disclosed in U.S. Pat. No. 6,072,718 is written by simultaneously sending a current through the word and bit line crossing at the location of the cell. Each of these currents causes a magnetic field at the location of the memory cell. As the word lines and the bit lines are perpendicular to each other, the orientations of the magnetic fields caused by the currents at a crossing point of a bit line and a word line are perpendicular, too. One of the two magnetic fields, the so called hard-axis field, extends parallel to the magnetic hard-axis of the ferromagnetic soft layer, while the other one of the magnetic fields, the so called easy-axis field, extends parallel to the magnetic easy-axis of the ferromagnetic soft layer.
In a write process, usually the hard-axis field, which is perpendicular to the magnetic moment of the ferromagnetic soft layer, is applied to the ferromagnetic soft layer in order to move the magnetic moment out of its actual orientation and the easy-axis field is used to set the new orientation of the magnetic moment with respect to the easy-axis of the ferromagnetic soft layer.
During a write process, all memory cells arranged in a first line will experience the same hard-axis field while all memory cells arranged in a second line perpendicular to the first line will experience the same easy-axis field. The strength of both magnetic fields must be chosen such that one of both fields alone is not able to switch a memory cell. Therefore, in an ideal memory array (i.e. all memory cells of the array show the same magnetic response to an applied magnetic field), only the memory cell which is located at the crossing of both lines experiences the hard-axis field as well as the easy-axis field and is therefore written. In contrast to the ferromagnetic soft layer, the ferromagnetic fixed layer has a coercivity that is high enough such that its magnetic moment is left unchanged in this process.
However, in an actual memory cell array, due to many factors related to manufacturing uncertainties and intrinsic magnetic variability, variations in the magnetic response throughout the memory cells in a memory cell array can be very large. Due to these variations, some of the memory cells may be written inadvertently if only one of the magnetic hard-axis field and the magnetic easy-axis field is applied. As a consequence, an array-wide selectivity of the writing process is generally not achieved. The response variations are, e.g., caused by tolerances during the manufacturing process, which for example may lead to differences in the surface roughness of different cells, having a consequent influence on the magnetic response of the cell.
In GB 2 343 308, a magnetic storage device is disclosed, which comprises a first and a second ferromagnetic layer and a tunnel barrier which is disposed between both ferromagnetic layers. The first ferromagnetic layer is a ferromagnetic fixed layer whereas the second ferromagnetic layer is a ferromagnetic soft layer which can change the orientation of its magnetic moment. The device can be written directly by applying a voltage across the cell which causes a tunnelling current to flow through the cell and can switch the orientation of the magnetic moment of the ferromagnetic soft layer with respect to the ferromagnetic fixed layer. The switching is effected by means of an induced exchange interaction between the ferromagnetic fixed layer and the ferromagnetic soft layer related to spin-polarised electrons tunnelling through the tunnelling barrier. Since the addressing of the cells in the write process is direct, array-wide selectivity is achieved.
In GB 2 343 308, it is important for the write process to supply a strong enough tunnelling current to overcome the coercive field of the ferromagnetic soft layer. Therefore, the tunnel barrier has to be very thin. Because the tunnelling current increases exponentially with decreasing thickness of the tunnelling layer, local variations due to the manufacturing process become particularly pronounced for thin barriers. The less uniform the current distribution within the cell, the higher the total current has to be to create a strong enough excitation throughout the entire ferromagnetic soft layer. However, a too strong a current will eventually break the tunnel junction. Therefore, in GB 2 343 308 materials for the tunnelling layer have been proposed with a low energy barrier. Nevertheless, from a manufacturing point of view, there is still a very strong dependence on the quality of the manufacturing process.
It is an objective of the present invention to provide a magnetic memory cell, a method of and a circuit for writing a magnetic memory cell, and a magnetic memory which are improved with respect to the above mentioned drawbacks. It is a further objective of the present invention to provide a method of writing a magnetic memory cell which helps to overcome the above mentioned drawbacks.
SUMMARY OF THE INVENTION
These objectives are achieved by a magnetic memory cell as claimed in claim
1
, a magnetic memory as claimed in claim
17
, a method of writing a magnetic memory as claimed in claim
23
, and a mem
Auduong Gene N.
BTG International Ltd.
Hahn Loeser & Parks LLP
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