Magnetic memory device and magnetic substrate

Details Magnetic memory device and magnetic substrate Magnetic memory device and magnetic substrate

2001-11-21

2003-05-20

Mai, Son (Department: 2818)

Static information storage and retrieval

Systems using particular element

Magnetic thin film

C365S158000, C365S171000, C365S230060

Reexamination Certificate

active

06567299

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic memory device, and more particularly to a magnetic memory device having a nonvolatile memory cell array which uses a magnetic tunnel junction as each memory cell.
2. Description of the Background Art
<Tunnel Magnetic Resistance Effect>
A structure in which an insulative material is sandwiched between two ferromagnetic materials is termed “magnetic tunnel junction (MTJ)”.
FIG. 67
shows a concept of an MTJ. In
FIG. 67
, an insulating layer TB is provided, being sandwiched between ferromagnetic layers FM
21
and FM
22
and a voltage is applied to the ferromagnetic layers FM
21
and FM
22
.
In this structure, when a current tunneling the insulating layer TB is measured, it is observed that the measured current value varies depending on the directions of magnetization of the two ferromagnetic layers.
This phenomenon is termed “tunnel magnetic resistance (TMR) effect”. The TMR effect will be discussed, referring to
FIGS. 68
to
70
.
FIG. 68
is a schematic view showing the density of states N(E) of a transition metal. In
FIG. 68
, the horizontal axis indicates the density of states and the vertical axis indicates an energy E, and electrons included in atoms are grouped according to the directions of spin. Specifically, in
FIG. 68
, the density of states of atoms having electrons whose direction of spin is downward is shown on the left hand and that of atoms having electrons whose direction of spin is upward is shown on the right hand.
Further, in
FIG. 68
, since the atoms filled with electrons up to the Fermi level are schematically shown among the 3d orbit and the 4s orbit, the atoms filled with electrons up to the Fermi level are hatched with the Fermi level as the boundary.
The reason why the transition metal becomes a ferromagnetic substance is that the number of electrons whose direction of spin is downward and that of electrons whose direction of spin is upward are different on the 3d orbit among the atoms filled with electrons up to the Fermi level.
In other words, since number of electrons whose direction of spin is downward and that of electrons whose direction of spin is upward are equal on the 4s orbit, the electrons on the 4s orbit do not contribute to generation of magnetism.
FIGS. 69 and 70
are schematic views illustrating the TMR effect. In
FIG. 69
, among the 3d orbit of atoms constituting the ferromagnetic layer FM
21
on the left side of the insulating layer TB, the density of states of atoms having the electrons of downward spin is higher than that of atoms having the electrons of upward spin and therefore the direction of magnetization is downward on the whole.
Similarly, the direction of magnetization of the ferromagnetic layer FM
22
on the right side of the insulating layer TB is downward on the whole.
Tunneling of electrons mainly occurs so that the directions of spin in an initial state and a final state can be conserved. In the case of
FIG. 69
, since both the density of states of downward spin in the initial state (inside the ferromagnetic layer FM
21
) and that in the final state (inside the ferromagnetic layer FM
22
) are large, the tunneling probability is large and a tunneling current is also large. In other words, the tunnel magnetic resistance is small.
On the other hand, in
FIG. 70
, since the density of states of upward spin in the initial state (inside the ferromagnetic layer FM
21
) is large and that in the final state (inside the ferromagnetic layer FM
22
) is small, the tunneling probability is small and the tunneling current is also small. In other words, the tunnel magnetic resistance is large.
The tunnel magnetic resistance rate (TMRR) is expressed by the following equation;
TMRR
=
R
AF
-
R
F
R
A
=
P
1
⁢
P
2
1
-
P
1
⁢
P
2
(
1
)
where the resistance in a case where the directions of magnetization of the two ferromagnetic layers are the same is R
F
and that in a case where those are opposite is R
AF
.
Further, in the above Eq. (1), P
1
and P
2
denotes the spin polarizabilities of the ferromagnetic layers FM
21
and FM
22
, respectively.
Assuming that the density of states of p spin band in the Fermi surface is D&rgr; (E
F
), the spin polarizability is expressed as;
P
=
D
↑
(
E
F
)
-
D
↓
(
E
F
)
D
↑
(
E
F
)
+
D
↓
(
E
F
)
(
2
)
Specifically, the spin polarizability becomes larger as the difference between the density of states of upward spin and that of downward spin is larger. Further, as the spin polarizability approximates 1, the TMRR becomes larger. Furthermore, it is known that the spin polarizability and the magnetization are in proportion to each other. Herein, the spin polarizabilities of various magnetic materials are shown in Table 1:
TABLE 1
Materials
Spin Polarizability
Fe
0.44
Co
0.35
Ni
0.23
Ni
80
Fe
20
0.25, 0.45
FeCo
0.53
NiMnSb
  1, 0.58
PtMnSb
1  
CrO
2
1  
Fe
3
O
4
1  
(La · Sr)MnO
3
1  
A device utilizing the above TMR effect to store data, making correspondence between the directions of magnetization of two ferromagnetic layers and two values, 0 and 1, is an MRAM (Magnetic Random Access Memory).
Accordingly, though it is required to change the direction of magnetization of one of the two ferromagnetic layers in the MTJ, in some cases, the directions of magnetization of both ferromagnetic layers are changed in the structure of
FIG. 67
when the magnetic field is given thereto. Then proposed is a structure as shown in
FIG. 71
, in which an antiferromagnetic layer is formed on the one of the ferromagnetic layers to fix the direction of magnetization of the one of the ferromagnetic layers.
In
FIG. 71
, the insulating layer TB is sandwiched between the ferromagnetic layers FM
21
and FM
22
and an antiferromagnetic layer AF is formed above the ferromagnetic layer FM
21
. Further, a positive electrode of a DC power supply is connected to the antiferromagnetic layer AF and a negative electrode thereof is connected to the ferromagnetic layer FM
22
.
When a ferromagnetic material and an antiferromagnetic material are formed adjacently to each other, a magnetic flux penetrating these materials is closed to fix the direction of magnetization. This structure is termed “spin valve type ferromagnetic tunnel junction element”.
FIG. 72
shows a structure of variation of the spin valve type ferromagnetic tunnel junction element. In
FIG. 72
, the insulating layer TB is sandwiched between the ferromagnetic layers FM
21
and FM
22
, the antiferromagnetic layer AF is formed above the ferromagnetic layer FM
21
and a ferromagnetic layer FM
23
is formed below the ferromagnetic layer FM
22
.
Herein, the antiferromagnetic layer AF is made of, e.g., IrMn containing Ir (iridium) of 20 to 30 atom. %, to fix the direction of magnetization of the ferromagnetic layer FM
21
, and the ferromagnetic layer FM
21
is made of CoFe having large coercivity because it is better that the direction of magnetization should be hard to reverse with respect to the external magnetic field.
Further, as discussed earlier referring to Eq. (1), since the tunnel magnetic resistance rate (TMRR) becomes larger as the spin polarizability is larger, CoFe is used as a material having large spin polarizability.
On the other hand, though the ferromagnetic layer FM
22
is also made of CoFe, it is desirable that the ferromagnetic layer FM
22
should be made of a material having small coercivity so that its direction of magnetization may be controlled by a smaller external magnetic field.
In the structure of
FIG. 72
, Ni
80
Fe
20
(permalloy) having small coercivity and small spin polarizability is used as the ferromagnetic layer FM
23
to allow easy reverse in the direction of magnetization of the ferromagnetic layer FM
22
. The direction of magnetization of the ferromagnetic layer FM
22
can be thereby reversed by a small external magnetic field.
FIG. 73
shows a practical structure of the spin valve type ferromagnetic tunnel junction element of
FIG. 72
, an

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