Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2002-06-05
2003-02-18
Phan, Trong (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000, C365S173000
Reexamination Certificate
active
06522577
ABSTRACT:
TECHNICAL FIELD
The present invention is directed to magnetic memory devices, specifically, magnetic random access memory devices. More particularly, the present invention is directed to a method and system for facilitating erasing and writing to magnetic memory devices.
BACKGROUND OF THE INVENTION
Magnetic or magnetoresistive random access memory (“MRAM”) devices offer advantages over conventional transistor-based random access memory (“RAM”) devices and rewriteable nonvolatile read only memory devices. MRAM devices exploit the inherent nonvolatility of magnetic storage, used in early RAM devices, and long used in sequential memory devices in disk and tape storage. Unlike dynamic random access memory (“DRAM”) devices which consume appreciable quantities of power in having to be continually refreshed to preserve the integrity of their memory contents, MRAM cells do not need to be refreshed. In fact, unlike transistor-based RAM devices, once a cell of an MRAM device is polarized to its desired state, the cell retains its polarity without having to be supplied with power. Furthermore, unlike nonvolatile flash electronically erasable programmable read only (“flash EEPROM”) memories, the contents of which can become corrupted with heavy use, MRAM devices are highly reliable. Moreover, while flash EEPROM devices can only be rewritten by erasing them and rewriting them in their entirety, cells in MRAM devices can be selectively written and rewritten without erasing the contents stored in the entire device.
Unlike previous uses of magnetic storage, such as disk and tape storage or bubble memory, MRAM devices provide direct, random access to their contents. Accordingly, MRAM devices provide the advantages of conventional RAM devices with the reliable nonvolatility of magnetic storage.
MRAM devices exploit the inherent interrelationship between the flow of electric current and corresponding magnetic fields. As is known in the art, a current flowing through a longitudinal conductor creates a magnetic field which encircles latitudinally about the axis of the longitudinal conductor. Specifically, MRAM devices exploit this interrelationship by using electric currents to generate magnetic fields which, in turn, are applied in close proximity to storage elements comprised of magnetically susceptible materials. Electric currents directed in a first direction results in a magnetic field having a corresponding first polarity. Exposed to the field of that corresponding polarity, if the field has sufficient magnitude, the magnetically susceptible element becomes magnetized in that same polarity. The magnetic field generated by the magnetized element then is capable of reacting to other applied magnetic fields, such as those caused by other currents flowing through the conductor. As a result, if an electric current of the same polarity was applied to the same conductor which first caused the element to become magnetized, the magnetic field of that magnetized element would not resist that current. On the other hand, if an electric current of opposite polarity was applied to the conductor, inducing a magnetic field of opposite polarity to encircle the conductor, those magnetic fields would conflict, and affect the resistivity of the conductor to the flow of current. Measuring the discrepancies in current caused by the differing resistance encountered as a result of the influence of these previously magnetized elements allows the stored polarity of these elements to be read.
It will be appreciated that, while a current of opposite polarity applied to the conductor will be opposed by the current induced by the magnetic element, that current of opposite polarity will not necessarily repolarize the magnetic field of that element. Magnetic materials exhibit a hysteresis effect in that a stronger current must be applied to repolarize them than might be required to polarize them initially. This principle is relied upon by MRAM devices: currents of lower magnitude can be used to detect the magnetic field created in the magnetic elements and thereby allow the bit written to that magnetic element to be read, while currents of greater magnitude generate magnetic fields which can be used to overcome hysteresis and write or rewrite the bit written to that magnetic element. However, as is understood in the art, an acknowledged problem in MRAM devices is that relatively high currents are required to induce a magnetic field of sufficient magnitude to reliably write and rewrite MRAM memory cells.
As shown in
FIG. 1
, an MRAM device comprises a Cartesian array
100
of MRAM memory cells
104
. Each MRAM memory cell
104
comprises an element of magnetically susceptible material
108
disposed at an intersection of a row line
112
or
116
and a column line
120
,
124
, or
128
. Electrical current is selectively applied to the row lines
112
and
116
and column lines
120
,
124
, or
128
to effect writing and reading of data to and from each of the memory cells. As is known in the art, writing these cells is accomplished by selectively and simultaneously directing the current in the row lines
112
and
116
and column lines
120
,
124
, and
128
so as to subject a particular element
108
to a desired combination of magnetic fields generated by the current flowing through the conductive lines.
FIGS. 2A and 2B
show how a magnetic element can become polarized and, therefore, written.
FIG. 2A
shows an MRAM cell
200
which, physically, is comprised of magnetic element
204
disposed at the intersection of the row line
208
and the column line
212
. An electrical row current of a first polarity
216
is applied to the row line
208
and thereby induces a magnetic flux field
220
of a first polarity to which the magnetic element
204
is exposed. At the same time, an electrical column current
224
is applied to the column line
212
and thereby induces a magnetic field
228
to which the magnetic element
204
is exposed. The combination of these complementary magnetic fields
220
and
228
cause the magnetic element to become polarized to radiate a composite magnetic field in a predetermined direction to represent a stored data bit. Once the magnetic element
204
has become polarized, the magnetic element
204
generates a magnetic field which, as previously described, will interact with the magnetic field generated by currents of a different polarity flowing through the row line
208
and column line
212
. It will be appreciated that, as in any Cartesian grid, selection of a single row and a single column singularly identify a single point on the grid. Correspondingly, applying the row current
216
to the row line
208
and the column current
224
to the column line
212
allow the individual MRAM cell
200
at the intersection of the row line
208
and the column line
212
to be programmed.
FIG. 2B
, for the sake of completeness, shows the opposite case in which an MRAM cell
250
is programmed to store a magnetic field of the opposite polarity. If, in the example shown in
FIG. 2A
, the field stored in the magnetic element
204
of the MRAM cell
200
is considered to represent a logical zero,
FIG. 2B
shows the MRAM cell
250
being programmed to read as a logical one. The magnetically susceptible element
254
disposed at the intersection of the row line
258
and the column line
262
exposed to an electrical row current of a first polarity
266
applied to the row line
258
and induces a magnetic field
270
of a first polarity. At the same time, a column current
274
is applied to the column line
262
and induces a magnetic field
278
to which the magnetic element
254
is exposed. The composite magnetic field of magnetic fields
270
and
278
causes the magnetic element
254
to become polarized to radiate a magnetic fields of opposite polarity.
Once programmed, magnetic elements
204
and
254
in
FIGS. 2A and 2B
, respectively, will retain their magnetic fields in the absence of power. Accordingly, MRAM array
100
(
FIG. 1
) will retain the data stored therein where it can be re
Earl Ren D.
McKee Jeffrey A.
Dosey & Whitney LLP
Micro)n Technology, Inc.
Phan Trong
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