Static information storage and retrieval – Read/write circuit – Including magnetic element
Reexamination Certificate
2002-06-26
2004-01-27
Lebentritt, Michael S. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Including magnetic element
C365S173000, C365S158000
Reexamination Certificate
active
06683815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to magnetic memory devices and, more particularly, to a circuit and method for providing tunable writing current magnitudes to the magnetic memory devices within a magnetic memory circuit.
2. Description of the Related Art
The following descriptions and examples are given as background information only.
Recently, advancements in the use of magneto-resistive materials have progressed the development of magnetic random access memory (MRAM) devices to function as viable non-volatile memory circuits. In general, MRAM devices exploit the electromagnetic properties of magneto-resistive materials to set and maintain information stored within the individual memory cells of the circuit. In particular, MRAM circuits utilize magnetic direction to store information within a memory cell, and differential resistance measurements to read information from the memory cell. More specifically, information is stored within an MRAM cell as a magnetic bit, the state of which is indicated by the magnetic moment direction within one layer of the memory cell relative to another layer of the memory cell. For example, information may be stored within an upper layer as either a parallel or antiparallel magnetic moment direction relative to a magnetic moment direction of a lower magnetic layer. Note that the term antiparallel is used herein to describe a magnetic moment direction oriented 180° from the magnetic moment direction of the lower magnetic layer. In addition, a differential resistance can be measured between layers of the memory cell. Such a differential resistance indicates a difference in the magnetic moment directions between layers of the memory cell, and thus, can be used to read the magnetic state of the bit stored within the memory cell.
In order for a magnetic element to operate as a memory cell, it must be adapted to maintain two substantially different resistance states (i.e., representing two different magnetic states). It may be preferred, however, that a magnetic memory cell be adapted to maintain two substantially different resistance states at rest (i.e., when substantially no external magnetic field is applied to the cell). Such a magnetic memory cell may need substantially less operating current than a memory cell not adapted to maintain different magnetic states at rest.
More specifically, it is desirable that a magnetic memory cell is adapted to maintain either a parallel or an antiparallel magnetic state at rest. Such a case may be equivalent to having a nearly centered resistance (R) versus magnetic field (or, applied current, I) response, such as the R-I curve illustrated in FIG.
2
A. In other words, optimum MRAM operation usually exhibits a nearly centered resistance versus applied current response, and thus, allows substantially equal thresholds of writing current to switch the magnetic state of the memory cell from a low resistance state to a high resistance state, and vice versa. As such,
FIG. 2A
depicts an ideal response in which the magnitude of current (|I
1
|) needed to switch the magnetization from a low to high resistance state is substantially equal to the magnitude of current (|I
2
|) needed to switch the magnetization from a high to low resistance state. In addition, a magnetic memory cell demonstrating a nearly centered R-I curve can maintain two substantially different magnetic states (reference numerals
32
a
and
32
b
) at rest.
However, the R-I curve may be significantly affected by variations within and/or between individual memory cells of an MRAM circuit. As such, it is not typical that all memory cells within the MRAM circuit exhibit such a nearly centered R-I curve. Instead, a portion of the memory cells within the circuit may demonstrate an offset in the R-I curve. In some cases, the offset may be such that two substantially different magnetic states can be maintained at rest. Such an offset, however, may introduce unequal thresholds of writing current between the two magnetic states, thereby requiring substantially more current to switch one magnetic state versus the other. In other cases, however, the offset may be such that only one magnetic state can be maintained at rest. An offset of such a degree typically results in false write operations, thereby indicating failure of the memory device to store accurate information.
In some cases, the R-I curve may be significantly affected by variations within individual memory cells that cause ferromagnetic coupling between layers of the cells. During fabrication of MRAM cells, for example, individual cell layers may be fabricated having surfaces that are not completely flat but instead exhibit surface and/or interface roughness. Such variation in surface topology causes the formation of magnetic poles along an interface between two or more layers of the MRAM cell. In this manner, interface roughness causes unintentional magnetic coupling of the magnetic poles formed along the interface. Such unintentional magnetic coupling tends to introduce an offset into the R-I curve by forcing the magnetic moments of the two or more cell layers to point along a single direction. Therefore, such unintentional magnetic coupling may be referred to as ferromagnetic or positive coupling. Note, however, that ferromagnetic coupling typically produces an offset in the same direction as the direction of current (I) flow along the width of the memory cell that causes a high resistance state in the memory cell. For example, ferromagnetic coupling may introduce a positive offset into the R-I curve if the direction of current flow along the width of the memory cell is in a positive direction.
In other cases, an offset may be introduced into the R-I curve on account of antiferromagnetic coupling. Antiferromagnetic coupling generally refers to the unintentional magnetic coupling between an upper magnetic layer, which stores memory information, and the ends of one or more lower magnetic layers, which have magnetic moments fixed in a particular direction. As such, an offset may be introduced into the R-I curve when the combined magnetic moments of the one or more lower magnetic layers are non-zero. In this case, such unintentional magnetic coupling introduces an offset into the R-I curve by forcing the magnetic moments of the upper and lower magnetic layers to point along substantially opposite directions. This offset, termed negative offset, is generally along the same direction as the direction of current (I) flow along the width of a memory cell that causes a low resistance state in the memory cell. Thus, such unintentional magnetic coupling is generally referred to as antiferromagnetic or negative coupling.
Any offset (positive or negative), however, may produce unequal thresholds of writing current, such that substantially more current is needed to write one magnetic state versus another magnetic state within an individual memory cell. Such unequal thresholds of writing current may cause a false write operation to occur in one or more memory cells of the MRAM circuit, thereby causing the memory device to store inaccurate information.
As stated above, it is desirable for a magnetic memory cell to maintain two substantially different magnetic states at rest. In some embodiments, however, unintentional ferromagnetic coupling may introduce a positive offset so severe that only one magnetic state, such as the low resistance state, can be maintained when no external magnetic field is applied. In
FIG. 2B
, for example, the positive offset introduced into the R-I curve may be so large that only a low resistance state (reference numeral
34
) is maintained when no external magnetic field is applied (i.e., when the applied current, I, is substantially zero). In such a case, a biasing magnetic field (and thus, a biasing current) may be needed to maintain a high resistance state in the presence of unintentional ferromagnetic magnetic coupling.
In other embodiments, unintentional antiferromagnetic coupling may introduce a negative of
Chen Eugene Y.
Ounadjela Kamel A.
Pancholy Ashish
Conley & Rose, P.C.
Daffer Kevin L.
Lebentritt Michael S.
Nguyen Tuan T.
Silicon Magnetic Systems
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