Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Magnetic field
Reexamination Certificate
2002-03-15
2004-03-30
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Magnetic field
C257S295000
Reexamination Certificate
active
06713830
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetoresistive element applied to a nonvolatile memory or the like.
2. Related Background Art
Recently, magnetic memory elements for storing information by using a magneto-resistance effect receive attention as high-density, high-response, nonvolatile solid-state storage elements. It has been examined to constitute a RAM (Random Access Memory) by using the magnetic memory element. The magnetic memory element can store information by the magnetization direction of a magnetic layer, and can constitute a nonvolatile memory for semipermanently holding information. Magnetic memory elements are expected to be used as various recording elements such as information storage elements for a portable terminal and card. Especially a magnetic memory element using a spin tunneling magnetoresistance (TMR) effect can utilize a high-output characteristic obtained by the TMR effect. This magnetic memory element also allows high-speed read, and its practical use is expected.
In the magnetic memory element, the minimum unit for storing information is called a magnetic memory cell. The magnetic memory cell generally has a memory layer and reference layer. The reference layer is a magnetic material layer whose magnetization direction is fixed or pinned in a specific direction. The memory layer is a layer for storing information, and is generally a magnetic material layer capable of changing its magnetization direction by externally applying a magnetic field. The logic state of the magnetic memory cell is determined by whether the magnetization direction in the memory layer is parallel to that in the reference layer. If these magnetization directions are parallel to each other because of the MR (MagnetoResistance) effect, the resistance of the magnetic memory cell decreases; if these directions are not parallel, the resistance of the magnetic memory cell increases. The logic state of the magnetic memory cell is determined by measuring its resistivity.
Information is written in the magnetic memory cell by changing the magnetization direction within the memory layer by a magnetic field generated by flowing a current through a conductor. Written information is read out using an absolute detection method of detecting the absolute value of a resistance.
Another memory cell has a memory layer and detection layer. This memory cell employs a differential detection method for read because the magnetization direction of the detection layer is changed and the magnetization direction of the memory layer is detected from a change in resistance.
The magnetic memory cell must shrink in feature size for high integration degrees. Generally in a longitudinal magnetization layer, the spin curls at the film edge due to a demagnetizing field within the film surface along with the miniaturization. The magnetic memory cell cannot stably store magnetic information. To prevent this problem, the present inventor has disclosed in U.S. Pat. No. 6,219,725 an MR element using a magnetic film (perpendicular magnetization film) magnetized perpendicularly to the film surface. The perpendicular magnetization film is free from any curling even upon miniaturization, and is suitable for miniaturization.
A magnetic memory cell using an MR element includes two magnetic layers stacked via a thin nonmagnetic layer (tunnel insulating layer). A magnetic field leaked from one magnetic layer within the magnetic memory cell influences the other magnetic layer. The magnetic field is kept applied even in the absence of an external magnetic field.
FIGS. 20A and 20B
show examples of the magnetization direction of a TMR element having a perpendicular magnetization film. A magnetic film
100
having a low coercive force and a magnetic film
200
having a higher coercive force are stacked via a tunnel insulating film
300
. In both the examples shown in
FIGS. 20A and 20B
, the magnetic film
200
is magnetized downward. The magnetic film
100
is magnetized downward in
FIG. 20A
, and upward in FIG.
20
B. Hence, the resistance value of the magnetic memory cell is larger in
FIG. 20B
than in FIG.
20
A.
This state may be considered as a structure using the absolute value detection method in which the magnetic layer
200
is a reference layer (pinned layer), the magnetic layer
10
is a memory layer, “0” is recorded as shown in
FIG. 20A
, and “1” is recorded as shown in FIG.
20
B. Alternatively, this state may be considered as a structure using the differential detection method in which the magnetic layer
200
is a memory layer, the magnetic layer
10
is a detection layer, and the magnetization is switched from the state shown in
FIG. 20A
to the state shown in
FIG. 20B
by an external magnetic field in detection.
FIG. 21A
shows the MH curve of this element (graph showing the relationship between the magnetization and the application magnetic field) on the assumption that no magnetic field is leaked from the other magnetic film with a squareness ratio of 1. A magnetic field small enough to keep the magnetization direction unchanged is applied to the magnetic layer
200
. Therefore, a curve corresponding to the magnetization direction of the magnetic layer
100
appears. In the absence of a magnetic field leaked from the other magnetic film, i.e., an offset magnetic field, information can be recorded on the memory layer only by applying a magnetic field H
1
or H
2
equal to a coercive force Hc. Alternatively, the magnetization of the detection layer can be switched. The magnetic field H
1
switches the first magnetic film from the upward direction to the downward direction. The magnetic field H
2
switches the first magnetic film from the downward direction to the upward direction.
In practice, the other magnetic layer, in this case, the magnetic film
200
applies a downward magnetic field to the magnetic film
100
. The MR curve shifts by the offset magnetic field Ho, as shown in FIG.
21
B. In this case, the recording magnetic field is H
2
=Hc+Ho and H
1
=Hc−Ho. The magnetic field necessary to change the state of
FIG. 21B
to that of
FIG. 21A
decreases by Ho. To the contrary, the magnetic field necessary to change the state of
FIG. 21A
to that of
FIG. 21B
increases by Ho. This means that a current value flowing through a write line increases. Current consumption may increase, or when the current exceeds the allowable current density of write line wiring, write may fail. In this case, the magnitude of a switching magnetic field changes depending on information recorded on a memory cell. If memory cell information which requires the switching magnetic field H
2
is rewritten in recording information in memory cells arrayed in a matrix via two perpendicular write lines, adjacent memory cell information which requires the switching magnetic field H
1
is also rewritten. Such erroneous recording operation may occur at a high possibility. If the offset magnetic field Ho becomes larger than the coercive force Hc, as shown in
FIG. 21C
, only one resistance value can be taken in zero magnetic field. This makes absolute detection difficult.
When the squareness ratio is not 1, a magnetization M in zero magnetic field becomes smaller than a maximum magnetization value Mmax of an antiparallel magnetization state. The resistance value also changes depending on the magnetization magnitude of the low-coercive-force layer. In this case, a readout resistance value difference R
2
−R
1
decreases, degrading the detection sensitivity. This phenomenon occurs even in an offset magnetic field Ho smaller than the coercive force Hc. Note that R
1
represents the minimum resistance value in the absence of an external magnetic field; and R
2
, the maximum resistance value in the absence of an external magnetic field.
FIG. 22A
shows the resistance value in the presence of the offset magnetic field Ho, and
FIG. 22B
shows the resistance value in the absence of the offset magnetic field Ho.
For a squareness ratio of not 1, even application
Ikeda Takashi
Nishimura Naoki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ho Tu-Tu
Nelms David
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