Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-08-27
2004-01-13
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S421000
Reexamination Certificate
active
06677631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic memory elements, and more specifically, to magnetic tunnel junction structures having reduced demagnetization coupling between pinned and free ferromagnetic layers for use in a magnetic random access memory (MRAM) device.
2. Brief Description of the Related Art
Various types of digital memory are used extensively in digital systems such as microprocessor-based systems, digital processing systems, and the like. Recently, magnetic random access memory (MRAM) devices have been investigated for possible use in non-volatile random access memory. The resistance of such a device changes based on the relative magnetized state of a sense (free) ferromagnetic layer to a pinned ferromagnetic layer. The magnetic moment of the pinned layer remains fixed while the magnetic moment of the free layer can change depending on an applied magnetic field. The relative magnetic direction of the free layer to the pinned layer are referred to as “parallel” and “antiparallel”.
Typically, a magnetic memory element, such as a magnetic tunnel junction (MTJ) memory element, has a structure that includes the free and pinned ferromagnetic layers separated by a non-magnetic tunnel junction barrier layer. These magnetic memory elements are formed using thin-film materials.
In response to parallel and antiparallel magnetic states, the magnetic memory element represents two different resistances to a current provided across the memory element in a direction perpendicular to the plane of the ferromagnetic layers. The resistance has minimum and maximum values corresponding to when the magnetization vectors of the free and pinned layers are parallel and antiparallel, respectively. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs across the barrier junction between the two separated sets of ferromagnetic layers.
Magnetic memory elements structurally include very thin layers, some of which are tens of angstroms thick. Due to the very thin layers and small size of the element, the magnetic field response of the free layer is affected by magnetic coupling between the free and pinned layers. Consequently, the magnetic vector of the free layer, for example, may preferentially orient in the antiparallel direction. This may destabilize the memory element and also make it more difficult to switch the magnetic vector of the free layer to the parallel direction.
A representative prior art MTJ structure is shown in FIG.
1
. The structure includes a free ferromagnetic layer
10
separated from a pinned ferromagnetic layer
12
by a tunnel barrier layer
6
. The free and pinned ferromagnetic layers may each be formed as a plurality of stacked individual layers. A seed layer
8
is typically provided below the free ferromagnetic layer
10
. Pinned ferromagnetic layer
12
is pinned by an antiferromagnetic pinning layer
14
. A cap layer
16
is also typically provided.
A disadvantage of the prior art MTJ structure shown in
FIG. 1
is that demagnetizing coupling occurs between pinned layer
12
and free layer
10
, as indicated by the curved arrows. As a result, in the absence of an applied external field, the magnetism of free layer
10
will tend to want to orient under the coupling influence of pinned layer
12
in the anti parallel direction. Consequently, the free layer
10
has different magnetic field strength switching thresholds when going from an anti parallel state to a parallel state and vice versa. This produces a magnetic field offset required to switch the free layer
10
from one orientation to the other.
FIG. 2
illustrates another prior art MTJ structure in which a set of pinned ferromagnetic layers
32
,
38
are produced by a “synthetic” antiferromagnet in which the two ferromagnetic layers
32
and
38
are separated by an anti-parallel coupling layer
36
made of ruthenium, for example. The coupling layer
36
enhances magnetic coupling between pinned layer
32
and ferromagnetic layer
38
which reduces the undesired magnetic coupling between pinned layer
32
and free layer
30
. While helping to demagnetize the free layer
30
from the effects of pinned layer
32
, additional layers are required in the memory device as a special anneal process is required.
An MTJ structure having reduced demagnetization coupling between pinned and free layers, without the need for synthetic antiferromagnets, is desirable.
BRIEF SUMMARY OF THE INVENTION
The present invention utilizes an additional ferromagnetic film on top of an antiferromagnetic pinning film to reduce the demagnetization coupling between free and pinned ferromagnetic layers in MRAM devices. A magnetic tunnel junction memory element according to the present invention includes a pinned ferromagnetic layer, a free ferromagnetic layer, and a barrier layer separating the pinned stack from the free stack. The pinned ferromagnetic layer has a ping antiferromagnetic layer adjacent to it. An offset ferromagnetic layer is further provided on a side of the pinning antiferromagnetic layer opposite the pinned ferromagnetic layer. The offset ferromagnetic layer reduces demagnetization coupling between the free ferromagnetic layer and the pinned ferromagnetic layer.
REFERENCES:
patent: 5764567 (1998-06-01), Parkin
patent: 6466419 (2002-10-01), Mao
patent: 2002/0030489 (2002-03-01), Lenssen et al.
patent: 2002/0141120 (2002-10-01), Gill
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