Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
1999-10-22
2001-05-01
Nelms, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000, C365S157000
Reexamination Certificate
active
06226197
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic thin film memory, a method of writing information in this magnetic thin film memory, and a method of reading the information written in this magnetic thin film memory.
2. Related Background Art
Recently, the development of devices including a thin film magnetic head to which the magneto-resistance effect is applied has been promoted. Among them, a magnetic thin film memory using a magnetoresistive thin film is noted, which can be substituted for a DRAM or an EEPROM used at present. Since the magnitude of resistance of a magnetoresistive thin film can largely be changed by the state of magnetization of the magnetism, a nonvolatile solid memory can be realized by combining the magnetoresistive thin film with a semiconductor device such as a transistor.
As a conventional magnetic thin film memory, for example, a memory is proposed in Japanese Patent Application Laid-Open No. 6-84347, which is formed by connecting a magnetoresistive thin film to the source electrode of a field effect transistor (hereafter, referred to as “FET”). The configuration of this memory is shown in FIG.
1
. In
FIG. 1
, the numeral
1
denotes a magnetoresistive thin film, and in order to show the positions in the circuit, the magnetoresistive thin films are denoted by the subnames like
1
aa
,
1
ab
,
1
ac
,
1
ba
,
1
bb
and
1
bc
, but hereafter, in the case when the positions in the circuit are not especially specified, it is simply referred to as the magnetoresistive thin film
1
. Furthermore, the other numerals are also referred to similarly. The numeral
2
denotes an FET;
3
, a first bit wire;
5
, a second bit wire;
4
, a word wire; and
6
, a resistor. The word wire
4
is provided in the lateral direction of FIG.
1
and is connected to the gate electrode of the FET
2
, and the first bit wire is provided in the longitudinal direction of the figure and is connected to the drain electrode of the FET
2
. Furthermore, the source electrode of the FET
2
is connected to the second bit wire
5
, and further, the magnetoresistive thin film
1
is connected to the second bit wire
5
. The other end of the magnetoresistive thin film
1
which is not connected to the second bit wire
5
is connected to a grounding source.
FIG.
2
A and
FIG. 2B
show cross sectional views taken along the line A—A′ of the magnetoresistive thin film
1
ac
shown in FIG.
1
. In the figures, arrows show the direction of the magnetic field, and the mark “∘” in the circle (i.e., the mark “⊚”) shows the state where the current flows from the back to the front of the figure, and the mark “x” in the circle (i.e., the mark “{circle around (x)}”) shows the state where the current flows from the front to the back of the figure. Furthermore, FIG.
3
A and
FIG. 3B
show the configuration of the magnetoresistive thin film
1
.
The magnetoresistive thin film
1
, as shown in FIG.
3
A and
FIG. 3B
, comprises a giant magnetoresistive thin film in which a magnetic layer a with a large coercive force and a magnetic layer b with a small coercive force are stacked several times with interposition of a nonmagnetic layer c therebetween. The resistance value of the magnetoresistive thin film
1
is characterized in that it is small when the direction of magnetization of the magnetic layer a and the direction of magnetization of the magnetic layer b are the same, and that it is large when the direction of magnetization of the magnetic layer a and the direction of magnetization of the magnetic layer b are opposite.
In
FIG. 1
, when the information of “1” is written in the magnetoresistive thin film
1
ac
, a potential of +V
3
is applied to the first bit wire
3
c
. In this case, when a voltage of V
4
is applied to the word wire
4
a
, the FET
2
ac
is turned on, and a comparatively large current I
1
flows into the magnetoresistive thin film
1
ac
and the second bit wire
5
ac
. By this current I
1
, a magnetic field H
1
is applied to the magnetoresistive thin film
1
ac
, and the magnetic layer b with a small coercive force which is shown in FIG.
3
A and which relates to the writing of the magnetoresistive thin film
1
is magnetized to the left which is the direction of the magnetic field H
1
.
On the other hand, when the information of “0” is written in the magnetoresistive thin film
1
ac
, a potential of −V
3
is applied to the first bit wire
3
c
. In this case, when a voltage of V
4
is applied to the word wire
4
a
, the FET
2
ac
is turned on, and a comparatively large current I
0
flows into the magnetoresistive thin film
1
ac
and the second bit wire
5
ac
in the opposite direction (from the front to the back in the figure) of the above I
1
. By this current I
0
, a magnetic field Ho is applied to the magnetoresistive thin film
1
ac
, and the magnetic layer b with a small coercive force which relates to the writing of the magnetoresistive thin film
1
is magnetized to the right which is the direction of the magnetic field H
0
as shown in FIG.
3
B.
Since the FET
2
ac
is set to be turned on only when a proper voltage is applied to the word wire
4
a
, the current does not flow into other magnetoresistive thin films
1
connected to the first bit wire
3
c
.
Furthermore, since the current does not flow through the first bit wires
3
other than the first bit wire
3
c
, the current also does not flow into other magnetoresistive thin films
1
connected to the word wire
4
a
. Since the magnetic layer a with a large coercive force is initialized so that the direction of magnetization may face to the right at all times, the magnetoresistive thin film
1
is set to have a large resistance when the information of “1” is written in it and to have a small resistance when the information of “0” is written in it.
On the other hand, when reading the information written in the magnetoresistive thin film
1
ac
, a current I
3
is supplied into the first bit wire
3
c
, and a voltage V is applied to the word wire
4
a
so as to turn on the FET
2
ac
. Consequently, since the current I
3
flows only into the magnetoresistive thin film
1
ac
from the top to the bottom in the
FIG. 1
, the voltage V&agr;&bgr; between the positions &agr; and &bgr; at this moment is measured. In the cases of the same and opposite directions of magnetization of the magnetic layer a and the magnetic layer b constituting the magnetoresistive thin film
1
ac
, the resistance values of the magnetoresistive thin film
1
ac
are different, and therefore, the values of the voltage V&agr;&bgr; are also different. Accordingly, it is possible to judge whether the information read from the magnetoresistive thin film
1
ac
is “0” or “1” by the magnitude of the voltage value of the voltage V&agr;&bgr;.
FIG. 4
is a circuit diagram of the configuration of the conventional magnetic thin film memory shown in FIG.
1
. The reference characters M
101
to M
104
denote MOS (metal/oxide/semiconductor) FET's; R
101
to R
104
, magnetoresistive thin films; W
101
, a word wire; B
101
to B
104
, bit wires; and G, a grounding wire.
FIG. 5
shows a circuit diagram in which circuits shown in
FIG. 4
are arranged in the form of a matrix. One end of each of the magnetoresistive thin films R
101
to R
109
is connected to either the source electrode or the drain electrode of each of the MOSEFT M
101
to MOSFET M
109
, and the other end of each of the magnetoresistive thin films R
101
to R
109
is connected to the grounding source. The drain electrodes of the MOSFET M
101
to MOSFET M
109
are connected to the bit wires B
101
to B
103
, and the word wires W
101
to W
103
are arranged to the gate electrodes of the MOSFET M
101
to MOSFET M
109
. Furthermore, the reference characters J
101
to J
103
denote writing wires. Accordingly, in the circuit in
FIG. 5
, when the MOSFET M
101
and the magnetoresistive thin film R
101
are magnetic thin film memory elements, wires necessary for constituting the magnetic thin film memory elements are 4
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nelms David
Yoha Connie C.
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