As a magnetic memory device using a magnetoresistive effect element of the above-mentioned kind, there has been known a magnetic random access memory (hereinafter also referred to as “the MRAM”). The MRAM stores information using a combination (parallel or antiparallel) of the respective magnetization directions of two ferromagnetic materials included in the magnetoresistive effect element. On the other hand, reading of the stored information is carried out by detecting a change in the resistance value of the magnetoresistive effect element (i.e. a change in electric current or voltage), which changes depending on whether the magnetization directions of the two ferromagnetic materials are parallel or antiparallel to each other.
MRAMs currently operative make use of a giant magnetoresistive (GMR) effect. An MRAM using GMR elements capable of providing the GMR effect is disclosed in U.S. Pat. No. 5,343,422. In this case, the GMR effect is intended to mean a phenomenon that the resistance value becomes minimum when the magnetization directions of two magnetic layers parallel to each other along the easy axis of magnetization are parallel to each other, whereas when the magnetization directions of the two magnetic layers are antiparallel to each other, the resistance value becomes maximum. As MRAMs using the GMR element, there are a coercive force difference type (pseudo spin valve type) MRAM and a switching bias type (spin valve type) MRAM. The coercive force difference type MRAM uses a GMR element having two ferromagnetic layers and a non-magnetic layer sandwiched therebetween, and makes use of the coercive force difference between the two ferromagnetic materials to thereby have information written therein and read out therefrom. Now, assuming that a GMR element is composed e.g. of “nickel-iron alloy (NiFe)/copper (Cu)/cobalt (Co)”, the rate of change in resistance of the element assumes a small value of approximately 6 to 8%. On the other hand, the switching bias type MRAM uses a GMR element having a fixed layer whose magnetization direction is fixed by exchange-coupling with an antiferromagnetic layer, a sensitive magnetic layer whose magnetization direction is changed by an external magnetic field, and a non-magnetic layer sandwiched therebetween, and makes use of the difference between the respective magnetization directions of the fixed layer and the sensitive magnetic layer to thereby have information written therein and read out therefrom. For example, assuming that a GMR element is composed e.g. of “platinum manganese (PtMn)/cobalt iron (CoFe)/copper (Cu)/CoFe”, the rate of change in resistance of the GMR element assumes a value of approximately 10%. This value is larger than the rate of change in resistance of the coercive force difference type MRAM, but insufficient to further enhance the recording speed and the access speed of the GMR element.
To solve the above problems, there has been proposed an MRAM which uses magnetoresistive effect elements (hereinafter also referred to as “the TMR elements”) 120 constructed as shown in FIG. 11 for utilizing a tunnel magnetoresistive effect (hereinafter also referred to as “the TMR effect”), as storage cells. As shown in FIG. 12, this MRAM is comprised of a plurality of bit lines 105 arranged in parallel to each other, a plurality of write word lines 106 arranged in parallel to each other, and at the same time orthogonal to the bit lines 105, a plurality of read word lines 112 arranged along the write word lines 106, and a plurality of TMR elements 120 arranged at orthogonally crossing portions (intersections) where the bit lines 105 and the write word lines 106 orthogonally intersect with each other, in a manner sandwiched therebetween. In this case, as shown in FIG. 11, each TMR element 120 is comprised of a first magnetic layer 102, a tunnel barrier layer 103, and a sensitive magnetic layer 104 as a second magnetic layer, which are sequentially deposited in the mentioned order.
In this case, the TMR effect is intended to mean an effect that a tunnel current flowing through the tunnel barrier layer 103 varies with a relative angle between the respective magnetization directions of the first magnetic layer 102 and the sensitive magnetic layer 104 as two ferromagnetic layers sandwiching the tunnel barrier layer 103 as a very thin insulating layer (non-magnetic conductive layer). More specifically, the resistance value becomes minimum when the magnetization directions of the first magnetic layer 102 and the sensitive magnetic layer 104 are parallel to each other, whereas when they are antiparallel to each other, the resistance value becomes maximum. Further, in the MRAM making use of the TMR effect, when the TMR element 120 is composed e.g. of “CoFe/aluminum oxide/CoFe”, the rate of change in resistance of the MRAM assumes a high value of approximately 40%, and the resistance value thereof is large, and hence it is easy to match the MRAM with a semiconductor device, such as a MOSFET or the like, when they are combined with each other. This makes it possible to easily obtain a higher output compared with the MRAM including the GMR element, and enhancement of the storage capacity and the access speed is expected. The MRAM making use of the TMR effect stores information by changing the magnetization direction of the sensitive magnetic layer 104 of the TMR element 120 to a predetermined direction, using current magnetic fields generated by electric current caused to flow through the bit lines 105 and the write word lines 106. On the other hand, to read out information stored in the MRAM, electric current in a direction perpendicular to the tunnel barrier layer 103 is caused to flow through the TMR element 120 via the bit lines 105 and the read word lines 112, to thereby detect a change in the resistance of the TMR element 120. It should be noted that the MRAM utilizing the TMR effect is disclosed in U.S. Pat. No. 5,629,922, or in Japanese Laid-Open Patent Publication (Kokai) No. H09-91949.