Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2003-02-06
2004-07-27
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S295000, C257S294000
Reexamination Certificate
active
06768152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetoresistive effect element for generating so-called MR (magnetoresistive) effect in which a resistance value changes with application of a magnetic field from the outside and a magnetic memory device fabricated as a memory device capable of storing information by the use of a magnetoresistive effect element.
2. Description of the Related Art
In recent years, as information communication devices, in particular, personal small information communication devices such as portable terminal devices (e.g. personal digital assistants) are widely spreading, it is requested that devices such as memories and logic devices comprising these information communication devices or portable terminal devices should become higher in performance, such as they should become higher in integration degree, they can operate at higher speed and they can consume lesser electric power. Particularly, technologies that can make nonvolatile memories become higher in density and larger in storage capacity are becoming more and more important as complementary technologies for replacing hard disk devices and optical disk devices with nonvolatile memories because it is essentially difficult to miniaturize hard disk devices and optical disk devices because they cannot remove their movable portions (e.g. head seek mechanism and disk rotation mechanism).
Flash memories using semiconductors and an FeRAM (ferro electric random-access memory) using a ferro dielectric material are widely known as nonvolatile memories. However, flash memories are slow in information write speed as compared with a DRAM (dynamic random-access memory) and a SRAM (static random-access memory). Further, it has been pointed out that the FeRAM cannot be rewritten so many times. Accordingly, a magnetic memory device called an MRAM (magnetic random-access memory) utilizing a magnetoresistive effect has been proposed and receives a remarkable attention as a nonvolatile memory that can overcome these defects (e.g. “Naji et. al ISSCC2001”).
The MRAM is able to record information by the use of a giant magnetoresistive effect (giant magnetoresistive: GMR) type storage element or a tunnel magnetoresistive effect (tunnel magnetoresistive: TMR) type storage element (these elements will be generally referred to as a “magnetoresistive effect element”). The magnetoresistive effect element includes a multilayer film structure including two ferromagnetic material layers and a nonmagnetic material layer made of an insulating material layer or a conductor sandwiched between the two ferromagnetic material layers. One ferromagnetic material layer is used as a free layer (free layer) whose magnetization direction can be inverted and the other ferromagnetic material layer is used as a fixed layer (pinned layer) whose magnetization direction is fixed (pinned) This magnetoresistive effect element is able to record information by utilizing the fact that a resistance value is changed in response to the magnetization direction of the free layer to discriminate “0” and “1” of information.
In the MRAM, these magnetoresistive effect elements are arrayed in an XY matrix fashion. The MRAM includes word lines and bit lines crossing these element groups in the horizontal and vertical directions. Then, the magnetization direction of the free layer in the magnetoresistive effect element located at the crossing area is controlled by using a synthetic current magnetic field generated when a current flows through both of the word lines and the bits lines, whereby information can be written in the magnetoresistive effect element. At that time, the magnetization direction of the free layer in each magnetoresistive effect element is not changed by magnetic fields solely generated from the word lines or the bit lines but the above magnetization direction is changed by a synthesized magnetic field of both of the word lines and the bit lines. Accordingly, even when the magnetoresistive effect elements are arrayed in a matrix fashion in the MRAM, information can be selectively written in a desired magnetoresistive effect element.
On the other hand, when information is read out from each magnetoresistive effect element, the magnetoresistive effect element is selected by using a device such as a transistor and the magnetization direction of the free layer in the magnetoresistive effect element is obtained as a voltage signal through MR effect to thereby read out information from the magnetoresistive effect element. This point will be described below more in detail. In general, electron spins are polarized in the ferromagnetic material layer such as the free layer or the fixed layer and up-spins and down-spins become either majority spins having large state density or minority spins having small state density. When the magnetization directions of the free layer and the fixed layer are parallel to each other, if the up-spins in the free layer are majority spins, then up-spins are majority spins in the fixed layer. When on the other hand the magnetization directions of the free layer and the fixed layer are anti-parallel to each other, if the up-spins are majority spins in the free layer, then up-spins in the fixed layer become minority spins. When electrons pass the nonmagnetic material layer between the free layer and the fixed layer, spins are preserved and a probability at which a certain spin will pass the nonmagnetic material layer is proportional to a product of state densities of spins of the two ferromagnetic material layers which sandwich the nonmagnetic material layer. Therefore, when the magnetization directions of the free layer and the fixed layer are parallel to each other, majority spins having large state densities become able to pass the nonmagnetic material layer. When the magnetization directions of the free layer and the fixed layer are anti-parallel to each other, majority spins having large state density become unable to pass the nonmagnetic material layer. For this reason, when the magnetization directions of the free layer and the fixed layer are anti-parallel to each other, a resistance increases as compared with the case in which the magnetization directions of the free layer and the fixed layer are parallel to each other. Therefore, if a voltage between the free layer and the fixed layer is detected through the word lines and the bit lines, then it becomes possible to read out information from the free layer.
As described above, since the MRAM using the magnetoresistive effect element utilizes the magnetization direction of the free layer in the magnetoresistive effect element to judge information, the MRAM is able to record information in a nonvolatile fashion with excellent response characteristics. Further, since the structure of the storage element (memory cell) that can hold information is simple, the magnetoresistive effect element becomes suitable for microminiaturization and increasing integration degree.
However, in the magnetoresistive effect element for use with the above-mentioned related-art MRAM, since the structure of the magnetoresistive effect element is simple, the magnetoresistive effect element is suitable for microminiaturization and increasing the integration degree. However, if the magnetoresistive effect element is microminiaturized much more and is increased in integration degree much more, then disorder of magnetization occurs at the end portion of the magnetoresistive effect element, which causes the following problems to arise.
These problems will be described in detail. Specifically, since the magnetoresistive effect elements are arrayed in the MRAM in a matrix fashion, if the magnetoresistive effect element is microminiaturized much more and is increased in integration degree much more, then each magnetoresistive effect element is influenced by a leakage magnetic field from the adjacent magnetoresistive effect element. There is a risk that coercive force in the free layer of each magnetoresistive effect element will change. The change of such
Depke Robert J.
Holland & Knight LLP
Nelms David
Nguyen Thinh T
LandOfFree
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