Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2000-09-13
2002-02-26
Nelms, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S173000, C365S066000
Reexamination Certificate
active
06351410
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims the benefit of a Japanese Patent Application No. 11-264430 filed Sep. 17, 1999, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to magnetic memories, and more particularly to a ferromagnetic random access memory and a memory cell array using the same.
The random access memory is essential for use as a main storage of an information processing apparatus such as a computer. Conventionally, the random access memory is formed by a semiconductor memory device such as a DRAM, but it is possible to form the random access memory by a magnetic random access memory using magnetoresistance. The magnetic random access memory has a simple structure in which a nonmagnetic layer made of an insulator or a conductor is sandwiched between a pair of ferromagnetic layers, and this simple structure enables miniaturization to suit integration. Generally, such a magnetic random access memory is nonvolatile and has a good response characteristic. Accordingly, the magnetic random access memory is regarded as being suited for use as a memory of an ultra high-speed computer which will be developed in the future. For example, the magnetic random access memory is described in Parkin et al., “Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory (invited)”, Journal of Applied Physics, Vol. 85, No. 8, pp.5828-5833, Apr. 15, 1999.
2. Description of the Related Art
FIG. 1
is a diagram showing the structure of a conventional magnetic random access memory using a ferromagnetic tunnel junction proposed in Parkin et al.
In a magnetic random access memory (MRAM)
10
shown in
FIG. 1
, a pinning layer
11
which is made of an antiferromagnetic material is formed on a word line (pattern) WL which extends in a row direction, and a pinned layer
12
which is made of a ferromagnetic material is formed on the pinning layer
11
. In the ferromagnetic pinned layer
12
, a magnetization direction is fixed, that is, pinned, in a direction of an arrow, by the antiferromagnetic pinning layer
11
provided thereunder. Furthermore, a nonmagnetic tunnel insulator layer
13
and a free layer
14
which is made of a ferromagnetic material are successively formed on the ferromagnetic pinned layer
12
, and a bit line (pattern) BL which extends in a column direction is formed on the ferromagnetic free layer
14
. The ferromagnetic free layer
14
is magnetized in a direction indicated by an arrow or, in an opposite direction, by a combined magnetic field which is formed by a write current which flows through the word line WL and the bit line BL. In other words, in the MRAM
10
, information is written in the form of the magnetization of the ferromagnetic free layer
14
.
On the other hand, a magnetoresistance of a ferromagnetic tunnel junction formed by the ferromagnetic pinned layer
12
, the ferromagnetic free layer
14
and the nonmagnetic tunnel insulator layer
13
interposed therebetween is used when reading the information written in the MRAM
10
.
More particularly, spin polarization is generated within conduction electrons within a ferromagnetic layer such as the ferromagnetic free layer
14
and the ferromagnetic pinned layer
12
, and the number of up-spin electrons and the number of down-spin electrons differ. In a case where the magnetization directions of the ferromagnetic free layer
14
and the ferromagnetic pinned layer
12
are parallel, the up-spin electrons or the down-pin electrons within the ferromagnetic free layer
14
can tunnel through the nonmagnetic tunnel insulator layer
13
at a vacancy level of the electrons existing within the ferromagnetic pinned layer
12
and having a corresponding spin state, and the ferromagnetic tunnel junction has a low resistance. On the other hand, in a case where the magnetization directions of the ferromagnetic free layer
14
and the ferromagnetic pinned layer
12
are antiparallel, a vacancy level corresponding to the up-spin electrons or the down-spin electrons within the ferromagnetic free layer
14
does not exist within the ferromagnetic pinned layer
12
, and for this reason, the tunneling of the electrons does not occur within the nonmagnetic tunnel insulator layer
13
. In other words, when the ferromagnetic free layer
14
and the ferromagnetic pinned layer
12
are magnetically antiparallel, the ferromagnetic tunnel junction has a large resistance.
Accordingly, in the MRAM
10
shown in
FIG. 1
, it is possible to read the information written in the ferromagnetic free layer
14
by detecting a voltage across the word line WL and the bit line BL. The information which is written within the ferromagnetic free layer
14
in the form of the magnetization is held even if a power supply is turned OFF, and as a result, the MRAM
10
forms a nonvolatile memory. In addition, the magnetization of the ferromagnetic free layer
14
will not be reversed even if the resistance is detected, and a nondestructive read can be made from the MRAM
10
.
On the other hand, when the MRAM
10
shown in
FIG. 1
is further miniaturized, a ratio of the surface area with respect to the volume of the magnetic material increases, and due to the effects of a closure magnetic field which is generated by the magnetization of the ferromagnetic free layer
14
or the ferromagnetic pinned layer
12
as shown in
FIG. 2A
, magnetic domains are generated in the ferromagnetic free layer
14
or the ferromagnetic pinned layer
12
as shown in FIG.
2
B. When such magnetic domains are generated, an apparent magnetization as a whole is lost, and the ferromagnetic tunnel junction cannot operate.
In order to avoid this problem, it is necessary to use a material having a large coercivity for the ferromagnetic pinned layer
12
or the ferromagnetic free layer
14
. However, a large current is required to write the information if such a material having the large coercivity is used for the ferromagnetic pinned layer
12
or the ferromagnetic free layer
14
. For example, in order to generate a magnetic field on the order of approximately 10 Oe which is required to reverse the magnetization by a current supplied to the word line WL which is formed at a position 100 nm from the MRAM
10
shown in
FIG. 1
, for example, it is necessary to use a current on the order of several mA. However, when such a large current is supplied to the word line WL which is formed by a 0.1 &mgr;m rule, a current density becomes on the order of 10
7
A/cm
2
.
On the other hand, a conventional MRAM shown in
FIG. 3
which is suited for miniaturization is proposed in a U.S. Pat. No. 5,477,482.
A spin valve MRAM
20
shown in
FIG. 3
has a stacked structure similar to that of the MRAM
10
shown in FIG.
1
. The spin valve MRAM
20
shown in
FIG. 3
includes a disk-shaped antiferrromagnetic pinning layer
21
formed on a word line (pattern) WL, a disk-shaped ferromagnetic pinned layer
22
formed on the pinning layer
21
, a ring-shaped ferromagnetic free layer
24
formed above the ferromagnetic pinned layer
22
, and a non-magnetic conductor layer
23
interposed between the ferromagnetic pinned layer
22
and the ferromagnetic free layer
24
. A bit line (pattern) BL is formed on the ferromagnetic free layer
24
, in a direction intersecting the word line (pattern) WL. In this spin valve MRAM, a magnetoresistance observed between the word line (pattern) WL and the bit line (pattern) BL changes depending on the magnetization direction of the ferromagnetic free layer
24
, as a result of the scattering which is dependent upon the direction of the spin of the electrons generated at an interface of the ferromagnetic pinned layer
22
and the nonmagnetic conductor layer
23
and at an interface of the nonmagnetic conductor layer
23
and the ferromagnetic free layer
24
.
According to the spin valve MRAM
20
having the structure shown in
FIG. 3
, the ferromagnetic pinned layer
22
and the ferromagnetic free layer
24
bot
Horiguchi Naoto
Nakao Hiroshi
Yamashita Yoshimi
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujitsu Limited
Le Thong
Nelms David
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