Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2002-07-25
2004-07-20
Lam, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000, C365S173000
Reexamination Certificate
active
06765819
ABSTRACT:
FIELD OF THE INVENTION
The present disclosure relates to memory devices. More particularly, the disclosure relates to magnetic memory devices having improved switching characteristics.
BACKGROUND OF THE INVENTION
Magnetic memory devices, such as magnetic random access memory (MRAM) devices, are non-volatile semiconductor devices that can be used to store data. An example of one such device
100
is illustrated in FIG.
1
. As shown in this figure, the magnetic memory device
100
comprises a plurality of memory cells
102
that are arranged in a two-dimensional array (only a portion of the array shown in FIG.
1
). The magnetic memory device
100
also comprises a plurality of column conductors
104
and row conductors
106
that are electrically coupled to the memory cells
102
. Specifically, each memory cell
102
is connected to a column conductor
104
and a row conductor
106
at a cross-point of the conductors. Additionally, the magnetic memory device
100
includes column and row control circuits
108
and
110
which control switching for the various column and row conductors
104
and
106
, respectively.
FIG. 2
provides a detailed view of a single memory cell
102
and its connection to its associated column and row conductor
104
and
106
. As is evident from
FIG. 2
, each memory cell
102
typically comprises two magnetic layers
200
and
202
that are separated by a thin insulating layer
204
. One of the magnetic layers (e.g., the bottom magnetic layer
202
) has a fixed or “pinned” magnetic orientation, while the other magnetic layer (e.g., the top magnetic layer
200
) has a “free” magnetic orientation that can be toggled from an orientation in which it is aligned with the orientation of the fixed magnetic layer to an orientation in which it opposes the orientation of the fixed magnetic layer. The former state is called the “parallel” state and the latter state is called the “anti-parallel” state. Typically, the orientation of magnetization in the free layer (also referred to as the data layer or the storage layer) is aligned along its “easy” axis.
The two different memory cell states can be used to record data due to their disparate effect on resistance of the memory cell
102
. Specifically, the memory cell
102
has a relatively small resistance when in the parallel state, but has a relatively high resistance when in the anti-parallel state. The parallel state can be designated as representing a logic value “1” while the anti-parallel state can be designated as representing a logic value “0” or vice versa. In such a scheme, the magnetic memory device
100
can be written to by changing the magnetic orientation of the free layer of selected memory cells
102
.
The control circuits
108
and
110
(
FIG. 1
) are used to facilitate selection of any given memory cell
102
during reading and writing. Normally, these circuits
108
,
110
include a plurality of switches, for instance transistors, that are used to apply voltage and current to the selected conductors.
A memory cell
102
is usually written to a desired logic state by applying external magnetic fields that rotate the orientation of magnetization of its free layer. Typically, the external magnetic fields are applied by delivering current through both conductors
104
,
106
associated with the selected memory cell
102
through use of the control circuits
108
,
110
. In particular, with reference to
FIG. 2
, the write current flowing through the column conductor
104
, I
YW
, generates a write magnetic field along the X direction, H
XW
, and the write current flowing through the row conductor
106
, I
XW
, generates a write magnetic field along the Y direction, H
YW
. These external magnetic fields, H
XW
and H
YW
, in combination flip the orientation of magnetization of the free layer along its easy axis to either the parallel or anti-parallel orientation.
With this writing scheme, only the selected memory cell
102
receives both H
XW
and H
YW
. The other memory cells
102
coupled to the particular conductors
104
,
106
used to flip the selected memory cell only receive a single magnetic field (either H
XW
or H
YW
). In that they only receive one of the two magnetic fields, these other memory cells
102
are said to be “half-selected.” The magnitudes of the applied magnetic fields are preferably chosen to be high enough so that the selected memory cell
102
switches its logic state, but low enough so that the half-selected memory cells do not switch their states.
The above-described switching characteristics are illustrated in
FIG. 3
, which provides an example switching curve
300
for a memory cell. In the graph, the X axis represents magnetic field in the X direction, H
X
, and the Y axis represents the magnetic field in the Y direction, H
Y
. Switching occurs when the memory cell receives a field that extends to or beyond the switching curve
300
. Therefore, the switching curve represents the threshold magnetic field that is required to switch a memory cell. When a given X direction write field, H
XW
, and a given Y direction write field H
YX
are applied to the selected memory cell, a resultant field, H
R
, is produced that reaches or exceeds the switching curve
300
.
As indicated in
FIG. 3
, the switching curve crosses the X axis at the magnetic coercivity point, H
c
, and runs asymptotically along the Y axis in both the positive and negative directions. In prior solutions, this configuration was considered ideal in that half-selected memory cells that only receive H
YW
can never switch and half-selected memory cells that only receive H
XW
will not switch unless H
XW
is very large. This latter phenomenon provides a margin, known as the half-select margin, between the applied X direction field (H
XW
) and that required to switch the memory cell when no Y direction field is applied.
Unfortunately, manufacturing variation among memory cells can skew the switching curve and therefore increase the likelihood of half-select switching, i.e. unintended switching of a half-selected memory cell. For example, manufacturing variation in the various dimensions or shapes of the memory cells may increase the likelihood of half-select switching. In addition, variation in the thickness or the crystalline anisotropy of the memory cell free layers may increase the likelihood of half-select switching. Such manufacturing variation decreases the yield in manufacturing processes for magnetic memory devices and reduces their reliability.
FIG. 4
illustrates an example of a switching curve
400
that may result from undesired manufacturing variation. As indicated in this figure, the magnitude of the X direction magnetic field required to switch the selected memory cell is very close to that which would be required to switch a half-selected memory cell that only receives the X direction magnetic field. Hence, the half-select margin for such a memory cell is small, thereby making half-select switching more likely.
From the above, it can be appreciated that it would be desirable to have magnetic memory devices that having improved switching characteristics such that half-select switching can be reduced.
SUMMARY OF THE INVENTION
The present disclosure relates to a magnetic memory device. In one embodiment, the device comprises a memory cell having an easy axis aligned along a first direction, the memory cell being configured so as to be most easily switched from one logic state to another when only receiving a magnetic field along the first direction, and a magnetic biasing element associated with the memory cell, the magnetic biasing element having a magnetic orientation aligned along a second direction different from the first direction.
With such a magnetic memory device, writing can be accomplished by applying a magnetic bias field to the memory cell, applying a first write magnetic field substantially equal in magnitude and opposite in direction to the magnetic bias field to negate the magnetic bias field, and applying a second write magnetic field in a direction different from the magnetic bia
Anthony Thomas C.
Bhattacharyya Manoj
Johnson Steven C.
Perner Frederick A.
Hewlett-Packard Development Company LP.
Lam David
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