Static information storage and retrieval – Systems using particular element – Magnetic thin film
Utility Patent
1999-03-18
2001-01-02
Dinh, Son T. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S173000
Utility Patent
active
06169688
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic storage device capable of providing a large read output signal and of easily achieving high-density integration and a driving method therefor.
Magnetic storage devices for storing information as a digital bit using a magnetic state of a magnetic thin film and its change in resistance are disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication Nos. 5-101641 (prior art 1) and 8-306014 (prior art 2).
Prior art 1 discloses a magnetic storage device having data transfer lines each comprising two ferromagnetic films sandwiching a nonmagnetic conductive film therebetween and having almost the same coercivity, and data selection lines extending perpendicularly to the data transfer lines. The multilayered structure comprising the two ferromagnetic films and the nonmagnetic conductive film forms a magnetic storage portion
16
whose resistance changes in accordance with the magnetic state.
In this magnetic storage device, when the influence of an offset magnetic field is small, the data selection line current necessary for a non-destruction read is equal to or larger than the lower limit of the data selection line current necessary for a write, so a large current must be supplied to the data selection lines in the read.
Supply of a large current readily causes electro-migration due to an increase in the data selection line current, resulting in a decrease in reliability. In addition, heat generated from the data selection line decreases or locally changes the resistance change ratio. Also, since the rate of an increase in temperature changes in accordance with the number of times of read operation, the resistance changes depending on the read operations previously conducted.
More specifically, the signal output of read data changes, or the resistance values of memory cells in the neighborhood change. For this reason, when the degree of integration is increased, array noise increases to make the read difficult.
In prior art 2, the magnetization easy axes of a ferromagnetic film and a soft magnetic film are parallel to a data transfer line. One-bit logic information (two levels) is stored by setting the direction of magnetization of the ferromagnetic film forward or reverse along the magnetization easy axis.
Since this prior art uses a data selection line having a bending pattern, the pattern interval is not constant unlike a straight data selection line pattern. This structure requires a lithography or etching margin, and the degree of integration can hardly be increased. In addition, since the data selection line and data transfer line need be stacked in parallel, the margin to the vertical alignment shift of the data transfer line becomes smaller than that in a structure wherein a data selection line and a data transfer line extend perpendicularly to each other.
Prior art 2 also discloses a memory cell in which the magnetization easy axis of a ferromagnetic film is matched with the direction of a synthesized magnetic field formed by a data selection line and a data transfer line.
In this memory cell structure, when a plurality of memory cells are connected to corresponding one of data transfer lines, and a plurality of data transfer lines are connected to the data selection lines, two different magnetization directions along the magnetization easy axis of a ferromagnetic film
11
are used to store information. To write arbitrary information, not only the current of the data transfer lines but also the current of the data selection line must be changed to two levels in accordance with the write information.
The reason for this will be described below. Assume that binary information of level “0” or “1” given by a magnetic field formed by the data selection line is to be written in a specific memory cell selected by the data selection line.
When binary information is to be written, magnetic fields along the magnetization easy axis, which are formed by the data transfer line in correspondence with magnetization information of two levels, are defined as H
BL0
and H
BL1
in correspondence with the bit information of level “0” and “1”, respectively. A magnetic field along the magnetization easy axis, which is formed by a selected data selection line for a selected memory cell connected to the selected data selection line, is defined as H
WLSEL
. A magnetic field formed by an unselected data selection line for an unselected memory cell connected to the unselected data selection line is defined as H
WLUNSEL
.
The magnitude of a coercivity of the ferromagnetic film along the magnetization easy axis is represented by H
K
, the bias magnetic field is represented by H
0
, and the positive and negative directions of magnetization along the magnetization easy axis are made to correspond to the information of level “0” and “1”, respectively. The condition for writing the information of level “0” in the selected memory cell is as follows.
H
BL0
+H
WLSEL
>H
K
+H
0
(1)
At this time, the information of level “0” is prevented from being erroneously written in a memory cell connected to the same data transfer line as that connected to the memory cell in which the information of level “0” is to be written, and connected to an unselected data selection line under the following condition.
H
BL0
+H
WLUNSEL
<H
K
+H
0
(2)
The solution of simultaneous inequalities (1) and (2) yields
H
WLUNSEL
<H
WLSEL
(3)
The information of level “1” is written in the selected memory cell under the following condition because magnetization must be caused in a direction reverse to that for information of level “0”.
H
BL1
+H
WLUNSEL
<−H
K
+H
0
(4)
At this time, the information of level “1” is prevented from being erroneously written in a memory cell connected to the same data transfer line as that connected to the memory cell in which the information of level “1” is to be written, and connected to an unselected data selection line under the following condition.
H
BL1
+H
WLUNSEL
>−H
K
+H
0
(5)
The solution of simultaneous inequalities (4) and (5) yields
H
WLSEL
<H
WLUNSEL
(6)
Inequalities (3) and (6) cannot be simultaneously satisfied by constant magnetic fields H
WLSEL
and H
WLUNSEL
. That is, in the memory cell structure of prior art 2, the current flowed to the data selection line must be changed in accordance with the condition for writing the information of level “1” and “0”.
More specifically, when a plurality of memory cells are connected to one data selection line, and random data are to be written in the memory cells, the magnitude of the current of the data selection line must be changed to at least two levels in correspondence with the write information of “0” and “1” and supplied with a time lag to write the information along the opposite directions of magnetization.
To write the information of level “0” and information of level “1”, the current direction of the data selection line is inverted to write the information of level “0” and “1”. Three-level control is required to drive the data selection line, resulting in complexity.
In addition, high-speed operation is difficult because a sufficient current value which changes to two levels in the write need be ensured and stabilized.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a magnetic storage device using a magnetoresistance effect. According to the present invention, there is provided a magnetic storage device comprising a plurality of data selection lines, a first power supply node for supplying currents of two types having the same polarity and different magnitudes to the plurality of data selection lines, a plurality of data transfer lines arranged to intersect the plurality of data selection lines, and a plurality of memory cells each comprising a first magnetic material element having a magnetization easy axis arranged to substantially be parallel to the longitudinal direction of the data selection lines, a second magnetic material h
Dinh Son T.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Phung Anh
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