Magnetoresistive effect element and magnetic memory

A magnetoresistive effect element includes a nonmagnetic layer having mutually facing first and second surfaces. A reference layer is provided on the first surface and has a fixed magnetization direction. A magnetization variable layer is provided on the second surface, has variable magnetization direction, and has a planer shape including a rectangular part, a first projected part, and a second projected part. The rectangular part has mutually facing first and second longer sides and mutually facing first and second shorter sides. The first projected part projects from the first longer side at a position shifted from the center toward the first shorter side. The second projected part projects from the second longer side at a position shifted from the center toward the second shorter side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-207499, filed Jul. 15, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetoresistive effect element, and to a magnetic memory. For example, the present invention relates to the shape of a ferromagnetic tunnel junction element.

2. Description of the Related Art

There has been a magnetic memory using a ferromagnetic tunnel junction (MTJ) element as a memory cell. The MTJ element is mainly composed of ferromagnetic layer/insulating layer/ferromagnetic layer, which are successively stacked. A junction resistance to current tunneling the insulating layer to flow is minimum when the magnetization directions of two ferromagnetic layers are parallel to each other and is maximum when the magnetization directions of two ferromagnetic layers are anti-parallel to each other. This is called tunnel magnetoresistive effect.

One ferromagnetic layer is called a reference layer whose magnetization direction is fixed. The other ferromagnetic layer is called a recording layer whose magnetization direction changes. Parallel or anti-parallel state of the magnetization direction of the reference and storage layers is made to correspond to binary information, and thereby, information is stored. Write current flowing a write line that is provided around a memory cell generates a magnetic field to switch the magnetization of the recording layer to write information.

An advance in the scale-down of memory cell in order to improve the integration of magnetic memories reduces a ferromagnetic material forming the memory cell. Smaller ferromagnetic material generally has larger coercivity. Larger coercivity requires greater switching magnetic filed for switching the magnetization direction of a recording layer. Thus, a write current required for generating a switching magnetic filed becomes large; as a result, power consumption is increased. Therefore, in order to realize lower power consumption, it is important to reduce the coercivity of the small magnetic material.

Meanwhile, a parameter called a thermal fluctuation constant exists as an index of memory cell stably holding information for long term. The thermal fluctuation appears to be proportional to volume and coercivity in general. Therefore, high thermal stability is required to hold information for long term; nevertheless, reduced power consumption degrades the thermal stability.

Moreover, an edge domain gives an influence to a switching magnetic field. The edge domain is a special magnetic domain and appears around an edge of a rectangular micro ferromagnetic material whose width of the shorter axis is, for example, less than sub-micron from several microns. The edge domain is disclosed in the document, J. App. Phys. 81, 5471 (1997)(J. App. Phys. 81, 5471 (1997)), for example.

The edge domain is generated because the magnetization is given along the shorter side of rectangle and form a spirally rotating pattern in order to reduce anti-magnetic energy in the shorter side. The magnetization is generated in the direction different from the middle portion in both edge portions of the magnetic material although it is generated in the direction according to magnetic anisotropy in the middle portion of the magnetic material. During spin switching in the rectangular ferromagnetic material, the edge domain is known to grow its size. Thus, the edge domain serves to increase a switching magnetic filed.

The edge domain is sensitive to the shape of the ferromagnetic material. For this reason, the use of an elliptic ferromagnetic material is proposed in U.S. Pat. No. 5,757,695.

Moreover, U.S. Pat. No. 5,748,524 and JPN. PAT. APPLN. KOKAI Publication No. 2000-100153 disclose that the edge domain is fixed to prevent a complicate change of magnetic structure from occurring in spin switching as much as possible. The technique can control magnetization behavior in spin switching but cannot reduce a switching magnetic filed. Moreover, another structure must be added to fix the edge domain; therefore, this is not suitable for achieving high density.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a magnetoresistive effect element comprising: a nonmagnetic layer having a first surface and a second surface which face each other; a reference layer provided on the first surface and having a fixed magnetization direction; and a magnetization variable layer provided on the second surface, having variable magnetization direction, and having a planer shape including a rectangular part, a first projected part, and a second projected part, the rectangular part having a first longer side and a second longer side which face each other and a first shorter side and a second shorter side which face each other, the first projected part projecting from the first longer side at a position shifted from the center toward the first shorter side, the second projected part projecting from the second longer side at a position shifted from the center toward the second shorter side.

According to a second aspect of the present invention, there is provided a magnetoresistive effect element comprising: a first nonmagnetic layer having a first surface and a second surface which face each other; a first reference layer provided on the first surface and having a fixed magnetization direction; a second nonmagnetic layer having a third surface and a fourth surface which face each other; a second reference layer provided on the third surface and having a fixed magnetization direction; and a magnetization variable layer interposed between the second and fourth surfaces, having variable magnetization direction, and having a planer shape including a rectangular part, a first projected part, and a second projected part, the rectangular part having a first longer side and a second longer side which face each other and a first shorter side and a second shorter side which face each other, the first projected part projecting from the first longer side at a position shifted from the center toward the first shorter side, the second projected part projecting from the second longer side at a position shifted from the center toward the second shorter side.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention will be described below with reference to the accompanying drawings. In the following description, the same reference numerals are used to designate components having the identical function and configuration, and the overlapping explanation is made if necessary.

One embodiment of the present invention will be explained using one example of computer simulation results.FIG. 1is a top plan view showing a MTJ element (magnetoresistive effect element, magnetic recording element) according to one embodiment of the present invention. As shown inFIG. 2, an MTJ element1has at least ferromagnetic layer11, insulating layer12and ferromagnetic layer13, which are successively stacked.

The ferromagnetic layers11and/or13may have a stacked structure comprising several sub-layers. The magnetization direction of one ferromagnetic layer (e.g., ferromagnetic layer11) is fixed. This is achieved by providing an anti-ferromagnetic layer14below the ferromagnetic layer11. The ferromagnetic layer11is hereinafter referred to as a reference layer.

On the other hand, the fixing mechanism is not given with respect to the magnetization direction of the ferromagnetic layer13. Thus, the magnetization direction of the ferromagnetic layer13is variable. The ferromagnetic layer13is hereinafter referred to as a magnetization variable layer (recording layer). Ferromagnetic layers11,13and insulating layer12have a planar shape shown inFIG. 1.

As seen fromFIG. 2, the MTJ element1may have a so-called double tunnel barrier layer structure. In this case, the MTJ element1has at least ferromagnetic layer11, insulating layer12, ferromagnetic layer13, insulating layer15and ferromagnetic layer16, which are successively stacked. The ferromagnetic layer16has the fixed magnetization direction, and functions as a reference layer. This is achieved by providing an anti-ferromagnetic layer17above the ferromagnetic layer16. The magnetization direction of the ferromagnetic layer11is parallel with that of the ferromagnetic layer16. Of course, the ferromagnetic layer16may have a stacked structure comprising several sub-layers.

It is not essential that these layers11to17forming the MTJ element1have all the same shape. In other words, at least the ferromagnetic layer13having a variable magnetization has the planar shape shown inFIG. 1.

As illustrated inFIG. 1, the planar shape of the MTJ element1is composed of a rectangular part1aand two projected parts1b1and1b2.

The rectangular part has a rectangular shape as the most typical example; in this case, it may have a substantially rectangular shape. In other words, a square shape is given having the following two longer and shorter sides. Specifically, two longer sides (first and second longer sides L1, L2) mutually face each other. Two shorter sides (first and second shorter sides S1, S2) mutually face each other and shorter than the longer side. Two longer sides and two shorter sides have no need to be parallel with each other so long as they are arranged along the similar direction.

Individual sides of the rectangular part1ahave no need to be fully straight line, and part thereof may comprise a curved line. In other words, a certain side may include a curved portion as long as the side is substantially directed along one direction as a whole. The corner portion of the rectangular part1amay be any of right angle, acute angle and obtuse angle, and further, may be rounded.

Two projected parts (first and second projected parts1b1and1b2) project along a direction crossing the longitudinal direction of the rectangular part1a. One projected part (e.g., projected part1b1) is connected to one longer side (e.g., L1). The other projected part (e.g., projected part1b2) is connected to the other longer side (e.g., L2). These projected parts1b1and1b2are substantially trapezoid. The width of each trapezoid becomes gradually wide from the side far from the rectangular part1atoward there.

Moreover, one projected part (e.g., projected part1b1) is positioned off the center of the longitudinal direction of the rectangular part1atoward one shorter side (e.g., shorter side S1). The other projected part (e.g., projected part b2) is positioned off the center of the longitudinal direction of the rectangular part1atoward the other shorter side (e.g., shorter side S2). Two projected parts1b1and1b2are positioned between the corner of the rectangular part and the center thereof, and not positioned at the corner.

Some corner of the rectangular shape may be rounded on purpose. The rounded corner is one of two corners formed by one longer side and two shorter sides, that is, a corner on the side where the projected part connected with the longer side shifts from the center point of the rectangular part1a. In other words, the rounded corner is two corners, that is, a corner C1formed by the longer side L1and the shorter side S1and a corner C2formed by the longer side L2and the shorter side S2. When remaining two corners, that is, a corner C3formed by the longer side L1and the shorter side S2and a corner C4formed by the longer side L2and the shorter side S1are actually formed to be intentionally or unintentionally rounded, the curvature of radius of the corners C1and C2is larger than that of the corners C3and C4.

The MTJ element1has the planer shape described above; therefore, the planar shape ofFIG. 1is point symmetry with respect to the center point, and non-line symmetry with respect to the longitudinal straight line passing through the center point.

The dimension of each part of the MTJ element1having the foregoing shape is as follows. The maximum width is preferably smaller than about 1 μm. The maximum width means the center width of the MTJ element1, that is, a length of two shorter sides of the rectangular part1a. The length (in the longitudinal direction) is preferably 1 to 10 times as much as the maximum width. The thickness of individual ferromagnetic layers11,13and16forming the MTJ element1is less than 20 nm, preferably, 10 nm. In order to achieve high integration of the MTJ element1, the element (device) size is preferably smaller.

Magnetic materials such as Fe, Co, Ni, these stacked films and alloys may be used as a material for ferromagnetic layers11,13and16. Ferromagnetic layers11,13and16may be a stacked film including a layer consisting of nonmagnetic metal such as Cu, Au, Ru and Al. In the following simulation, Ni8Fe is used as a ferromagnetic material.

Note thatFIG. 1schematically shows the magnetization direction of each position (each magnetic domain) shown by an arrow under no external magnetic field applied (hereinafter, referred to as zero magnetic filed state). InFIG. 1, each large arrow shows a rough magnetization direction on each position where the arrow is shown and its surroundings. As seen fromFIG. 1, the magnetization of each magnetic domain points to the same direction (right direction) in the most part of the rectangular part1aincluding the middle portion of the MTJ element1. In projected parts1b1and1b2, the magnetization of each magnetic domain points to the same direction as the rectangular part1a. Around the shorter sides S1and S2of the rectangular part1a, the magnetization points to the same direction (from upper left toward lower right) along each edge of the corners C1and C2having a large curvature of radius.

The magnetic filed pattern is formed in the zero magnetic field state as described above. For this reason, a change of the magnetic filed pattern is different between when a magnetic filed is applied to the MTJ element1in one direction only and when the magnetic field is applied to the MTJ element1in two directions.FIG. 4shows a magnetization state when magnetic field is applied to the MTJ element1ofFIG. 1in one direction only. More specifically,FIG. 4shows a state that magnetization is applied in the direction from the right toward the left inFIG. 4along the easy magnetization axis (hereinafter, referred simply to as easy axis) of the MJT element1. In other words, this state corresponds to a semi-select memory cell state in a magnetic memory comprising MTJ elements arrayed like a matrix, as described later. The easy axis (direction of the easy magnetization) is defined as the direction in which the magnetic moment of the ferromagnet is the easiest to point.

As seen fromFIG. 4, a ¾ circular magnetic field pattern is formed at individual upper left and lower right positions in the MTJ element1around each base of projected parts1b1and1b2. This is a development of a so-called C-type magnetic domain. Two C-type magnetic domains circulating in the direction reverse to each other are formed. In the middle portion of the MTJ element1, that is, portion between projected parts1b1and1b2, magnetization points from the bottom to the top inFIG. 4. Thus, the magnetization of the MTJ element1is hard to be switched. In other words, the magnetic field of the MTJ element1is hard to be switched in the semi-select state.

On the other hand, in the selected MTJ element, a magnetic field is applied before it is applied along the easy axis direction. Specifically, a magnetic field is applied in the direction along the hard magnetization axis (hereinafter, referred simply to as hard axis) of the MTJ element1, that is, the direction from top to bottom inFIG. 4. Thus, magnetization points from the top to the bottom inFIG. 4in the potion between projected parts1b1and1b2by the magnetic field applied to the hard axis direction. As a result, the magnetization of the MTJ element1is easier to be switched by the magnetic field along the easy axis direction thenFIG. 1. Therefore, the magnetization of the selected MTJ element1is easily switched. The hard axis (direction of the hard magnetization) is defined as the direction in which the magnetic moment of the ferromagnet is the hardest to point.

The following is an explanation about simulation results relevant to the MTJ element having the shape shown inFIG. 1.FIG. 5is a graph showing a magnetization curve obtained by simulation relevant to the MTJ element having the shape shown inFIG. 1.

The dimension of the MTJ element used for this simulation is as follows. The length is 0.72 μm. The width is 0.24 μm near the shorter side of the rectangular part. The center width of the MTJ element1, that is, the maximum width is 0.36 μm. The thickness of individual ferromagnetic layers11,13and16is 3 nm. This embodiment is not limited to the foregoing dimensions and various different values may be properly set so long as the MTJ element has the shape having the foregoing features.

InFIG. 5, the broken line shows a change of magnetization of the MTJ element1(i.e., recording layer13) when changing a magnetic filed applied in the easy axis (longitudinal direction of the MTJ element1ofFIG. 1) with no magnetic field applied in the hard axis (i.e., direction perpendicular to the longitudinal direction ofFIG. 1) of the MTJ element1. In other words, the broken line shows the relationship between magnetization of the recording layer13of a semi-selected MTJ element and the magnetic field of the easy axis in a magnetic memory having MTJ elements1ofFIG. 1arrayed like a matrix.

On the other hand, the solid line inFIG. 5shows a change of magnetization of the MTJ element1(i.e., recording layer13) when changing a magnetic filed applied in the easy axis direction with a magnetic field of 40 Oe applied in the hard axis of the MTJ element1. In other words, the solid line shows the relationship between magnetization of the recording layer13of a selected MTJ element and the magnetic field of the easy axis in a magnetic memory having MTJ elements1ofFIG. 1arrayed like a matrix.

As seen fromFIG. 5, a change of magnetization in accordance with a change of the magnetic field is different between the semi-selected MTJ element and selected MTJ element.

As seen fromFIG. 5, the coercivity is about 150 Oe in the easy axis of the semi-selected MTJ element. In addition, the magnetization magnitude gradually reduces with an increase of the magnetic field in the semi-selected MTJ element jumps up when reaching a certain value, and thereafter, is switched after further reduces.

On the other hand, in the selected MTJ element, jump-up, which would occur in the semi-selected MTJ element, does not occur. Moreover, the magnetization slightly increases from zero, and then, is switched when reaching about 40 Oe. In other words, magnetization switching is sharply made in the selected MTJ element. Therefore, no intermediate magnetization state other than “1” an “0” is given. This phenomenon implies that micro magnetic domains are not generated in the complicated form in a magnetization switching process.

According to the shape of the MTJ element of this embodiment, spin switching progress differently between the semi-selected state and the selected state. Moreover, the difference of switching magnetic filed is large between the semi-selected state and the selected state. These features mean that the semi-selected MTJ element1has a low possibility of being unexpectedly switched; in other words, the MTJ element1is hard to be disturbed.

In general, a gap or disturbed portion exists in the magnetization direction of the ferromagnetic material. When a ratio of residual magnetization to saturation magnetization becomes less than 1, tunnel magnetic resistance in the MTJ element using the ferromagnetic material is lower than one using the ferromagnetic material with no gap or disturbed portion. On the contrary, according to this embodiment, ferromagnetic layers11,13and16have each the same shape including insulating layers12and15; therefore, this means that these layers11,13and16have the same magnetic domain structure. As a result, there is no decrease of the tunnel magnetic resistance in the magnetization direction even when the ratio is less than 1.

FIG. 6shows an asteroid curve (solid line) obtained by simulation relevant to the MTJ element1having the shape ofFIG. 1and using the foregoing dimension and material. InFIG. 6, there is shown an asteroid curve (broken line) calculated using a MTJ element having a so-called track shape (L-letter shape), for comparison.FIG. 6is obtained by plotting values standardized with the coercivity of easy axis.

As seen fromFIG. 6, in the MTJ element having the track shape, the asteroid curve does not largely dent toward the origin. Thus, the switching magnetic field (i.e., magnetic field of points positioned outside the asteroid curve) is large, and also, large with respect to the coercivity Hc. For this reason, one magnetic filed (e.g., Hx) which is to be applied in a semi-selected state and large enough to set the MTJ element to a selected state has to be large. The larger the magnetic filed to be applied in a semi-selected state is, the more the magnetization of the recording layer is easy to be switched by only magnetization (e.g., Hx) to set a semi-selected state. In other words, thermal stability is low in the semi-selected state.

On the contrary, in the MTJ element1having the shape of this embodiment, the asteroid curve largely dents toward the origin, as compared with the track shape. Thus, the switching magnetic field is small, and also, small with respect to the coercivity Hc. Therefore, one magnetic filed (e.g., Hx) which is to be applied in a semi-selected state and large enough to set the MTJ element to a selected state can be small. As a result, the magnetization of the recording layer is hard to be switched by only magnetization (e.g., Hx) to set a semi-selected state. In other words, thermal stability is high in the semi-selected state.

In the MTJ element1of this embodiment, a ratio of the residual magnetization of easy axis direction to saturation magnetization is 0.92. This ratio is almost the same as the MTJ element having a rectangular shape. This is because edge domains exist in the MTJ element1of this embodiment.

The MTJ element1of the embodiment may have additionally the following shape features.FIG. 7is a top plan view showing another MTJ element according to one embodiment of the present invention. As illustrated inFIG. 7, a longer side L1has a dent D next to the projected part1b1at the side opposite to the shifted direction of the projected part1b1(i.e., corner C3side). Likewise, a longer side L2has a dent D next to the projected part1b2at the side opposite to the shifted direction of the projected part1b2(i.e., corner C4side).

In the shape shown inFIG. 1, the MTJ element1is point symmetry with respect to the center point, and non-line symmetry with respect to the longitudinal direction passing the center point.

InFIG. 7, there is schematically shown a magnetization direction on at each position in a zero magnetic field using an arrow, likeFIG. 1. As shown inFIG. 1, the MTJ element1has the foregoing shape; therefore, each magnetic domain points to the following direction. Specifically, the magnetization of each magnetic domain points to the same direction in the most part of the rectangular part1aincluding the middle portion of the MTJ element1, like the case ofFIG. 1. In projected parts1b1and1b2, the magnetization of each magnetic domain points to the same direction (right direction) as the rectangular part. In shorter sides S1and S2of the rectangular part1a, each magnetization points to the same direction (from upper left toward lower right) along corners C1and C2having a large curvature of radius.

FIG. 8shows a state of magnetization when a magnetic filed is applied to the MTJ element ofFIG. 7in one direction only, like the case ofFIG. 4. As depicted inFIG. 8, right and left sides of the MTJ element1in the longitudinal direction is formed with two C-type magnetic domains, like the case ofFIG. 4. In these two C-type magnetic domains, their magnetizations point to the direction opposite to each other around the dent D. Therefore, a magnetic filed of the MTJ element1is hard to be switched in a semi-selected state.

The selected MTJ element1takes the same magnetization pattern as the case ofFIG. 1.

FIG. 9is a graph showing a magnetization curve obtained by simulation relevant to the MTJ element having the shape shown inFIG. 7. The dimension of the MTJ element used for this simulation is as follows. The length is 0.76 μm. The width of the edge portion is 0.28 μm. The center width of the MTJ element1, that is, the maximum width is 0.38 μm. The thickness of individual ferromagnetic layers11,13and16is 3 nm.

InFIG. 9, the broken line shows a change of magnetization of the MTJ element1(i.e., recording layer13) when changing a magnetic filed applied in the easy axis of the MTJ element1with no magnetic field applied in the hard axis of the MTJ element1, like the case ofFIG. 5. In other words, the broken line shows the relationship between magnetization of the recording layer13of the semi-selected MTJ element1and the magnetic field of the easy axis.

On the other hand, the solid line inFIG. 5shows a change of magnetization of the MTJ element1(i.e., recording layer13) when changing a magnetic filed applied in the easy axis with a magnetic field of 40 Oe applied in the hard axis of the MTJ element1like the case ofFIG. 5. In other words, the solid line shows the relationship between magnetization of the recording layer13of the selected MTJ element1and the magnetic field of the easy axis.

As seen fromFIG. 9, magnetization switching is sharply made in the selected state MTJ element1, like the case ofFIG. 5. On the other hand, the magnetization is hard to be switched in the semi-selected MTJ element1. Moreover, the difference of switching magnetic field is large between the semi-selected state and the selected state. This means that the MTJ element1has high thermal stability, and is hard to be disturbed, like the case ofFIG. 5.

InFIG. 9, switching magnetic field is smaller than the case ofFIG. 5. Therefore, a write current required for switching the magnetization of the MTJ element having the shape ofFIG. 7is smaller than that ofFIG. 1. As seen fromFIG. 9, the residual magnetization state is high, that is, 0.927.

The following is an explanation about the method of manufacturing the MTJ element according to the embodiment. In general, the MTJ element is formed via the following processes. First a stacked film comprising several films forming the MTJ element is formed. Then, a mask material consisting of resist is coated on the stacked layer, and thereafter, a pattern is transferred into the mask material using light, electron beam or X rays. The mask material is developed, and thereby, a pattern is formed in the mask material. Ion milling or etching is carried out using the mask material as a mask, and thereby, the stacked film is patterned into a shape corresponding to the pattern of the mask material. Thereafter, the mask material is removed.

If a relatively large size, for example, micron-order MTJ element is manufactured, the following process may be carried out. First, a stacked film is formed using sputtering, and thereafter, a hard mask such as silicon oxide and silicon nitride is formed on the stacked film. Then, the hard mask is patterned using reactive ion etching (RIE). Thereafter, the stacked film is subjected to ion milling using the hard mask, and thereby, a MTJ element is formed.

Moreover, if a smaller, for example, sub-micron size MTJ element having a range from 0.1 to 2 or 3 0.1 μm is manufactured, photo-lithography may be used. In this case, a hard mask having a shape pattern of MTJ element is prepared, and the stacked film is patterned using the hard mask.

If a smaller size, for example, MTJ element having 0.5 μm or less is manufactured, electron beam exposure is used. However, in this case, the element itself is very small; for this reason, a shape portion for widening edge domain becomes smaller in the MTJ element of this embodiment. As a result, it is very difficult to manufacture the foregoing MTJ element. In order to manufacture the MTJ element1of this embodiment, proximity effect correction by electron beam may be used.

Usually, the proximity effect correction involves correcting proximity effect in a graphic pattern, which is caused by back scattering of electron beam from a substrate, to form a pattern loyal to a desired pattern. For example, if a rectangular pattern is formed, accumulated electron charge near the corner is not high enough; for this reason, the corner of the rectangle may be rounded. In order to clear the corner, correction point beam may be applied outside the pattern in the vicinity of the vertex if a 0.5 μm or less device (element) to be manufactured, in particular. By doing so, the electron charge is increased, and thereby, a pattern loyal to the desired pattern is obtained.

The proximity effect correction can be used to form a shape of the MTJ element having an edge portion whose width is widened. For example, if shapes shown inFIG. 1andFIG. 7are to be formed, a rectangle is used as a basic pattern, and correction point beam is applied to two corners facing each other. By doing so, a shape with a widened width at an edge is formed. In this case, the shape of the rectangle is controlled in addition to correction on the shape of the corner in the following manner. Specifically, it is controlled by increasing the applied charge, or properly controlling an applied position of the correction point beam, or using both of the former and the latter. By doing so, the shape of the MTJ element1according to this embodiment is realized. The foregoing correction point beam is applied to several points, and thereby, it is possible to form a projected part, and to give a roundness of the corner of a rectangular part.

The MTJ element1according to this embodiment is applicable to a magnetic random access memory (MRAM). In general, a random access memory has a need to have a small size and large capacity. Thus, it is preferable that line width and area of each memory cell are small. The MTJ element1serves to realize a high-speed switching memory cell having small switching magnetic field, small write current required for writing data and low power consumption. Therefore, the MTJ element1is preferably applicable to the memory cells of the MRAM.

The case where the MTJ element of this embodiment is used as a memory cell of MRAM will be explained below with reference toFIG. 10toFIG. 12.

FIG. 10schematically shows a planer layout of a cell array of a MRAM. As shown inFIG. 10, several bit lines BL for writing and reading and word lines WL (write word line WWL) extend to the direction different from each other. Typically, the word line WL extends along the x direction and the bit line extends along the y direction perpendicular to the x direction. The intersection of the bit line BL and the word line WL is provided with a memory cell MC including the MTJ element1.

The longitudinal direction of the MTJ element1extends along the word line WL. Each bit lines BL is electrically connected with one terminal of several MTJ elements1in the same row (or column). Each word line WL is arranged to closely face the other end of several MTJ elements1in the same row (or column). For example, the memory cell MC has a so-called cross point structure or so-called 1Tr+1MTJ structure, as described later.

A write current flowing through the word line WL toward the left along the x direction applies a magnetic field Hy to the upper direction along the y direction to memory cells through which the foregoing word line WL passes. The write current flowing through the bit line BL toward the upper direction along the y direction applies a magnetic field Hx to the left direction along the x direction to memory cells through which the foregoing bit line BL passes.

As described above, application of two magnetic fields to the MTJ element1of the memory cell MC which is provided at the intersection point of the bit line BL and the word line WL switches the magnetization direction of the recording layer13of the MTJ element1. Thus, information is written to a target memory cell MC.

Information is read in by applying a voltage to recording layer13and reference layer11of the selected memory cell MC, and reading a resistance value from a current flowing through it. Information may be read by flowing a constant current through the MTJ element1of the selected memory cell to read a voltage between recording layer13and reference layer11.

FIG. 11shows an application of the MTJ element1of the embodiment to a memory cell having a so-called cross point structure. As depicted inFIG. 11, the memory cell MC comprises the MTJ element1only. One end of the MTJ element1is electrically connected to a bit line BL via an electrode UE. The other end of the MTJ element1is electrically connected to a word line WL via an electrode BE.

The bit line BL and word line WL are connected to a driver circuit and sink circuit for carrying a current to a direction corresponding to written data. These sink circuit and driver circuit are connected to a decoder circuit. The decoder circuit controls sink and driver circuits so that a magnetic filed is applied to the MTJ element in accordance with an address signal supplied externally and a current is carried through bit line BL and word line WL. The bit line BL is further connected with a read circuit such as sense amplifier.

FIG. 12shows an application of the MTJ element1of the embodiment to a memory cell having a so-called 1Tr+1MTJ structure. As seen fromFIG. 12, a memory cell MC is composed of MTJ element1(shown inFIG. 2) and MOS (Metal Oxide Semiconductor) transistor TR used as a read switching element.

One end of the MTJ element1is electrically connected to a bit line BL via an upper electrode UE. The other end of the MTJ element1is electrically connected to one end of the switching element TR via bottom electrode BE, conductive layer and contact plug CP. The other end of the switching element is grounded.

A diode may be used as the switching element TR in place of the MOS transistor.

The word line (write word line) WL is used to carry a current in an write operation, and positioned below the conductive layer IC and apart from it. The word line WL is connected to driver circuit and sink circuit. These sink circuit and driver circuit are connected to a decoder. The bit line BL is connected to driver circuit, sink circuit and read circuit such as sense amplifier. The switching element TR is supplied with a signal for selecting a predetermined memory cell MC in a read operation via a read word line (not shown).

According to one embodiment of the present invention, the MTJ element1is composed of substantially rectangular part and two projected parts, which face each other and positioned off the center of the rectangular part along the longitudinal direction thereof. The foregoing shape can preferably controls the magnetic structure, in particular, edge domain of the ferromagnetic layer (i.e., ferromagnetic layers11,13,16) of the MTJ element1. When the MTJ element1is used as a memory cell of an MRAM, the magnetic structure of the ferromagnetic layer is different between a semi-selected state and a selected state. Thus, large write margin is secured as compared with the conventional case, and the MTJ element1having thermal stability is realized.

Moreover, the thermal stability is high; therefore, even if the MTJ element1is made into a small size, a write error is hard to occur resulting from thermal fluctuation. Reducing MTJ element1can improve the integration level of a memory cell when the MTJ element1is applied as a memory cell of a MRAM. Small sized MTJ element1with small switching magnetic field can reduce power consumption in the MRAM.

This embodiment does not employ the configuration of controlling edge domains by applying a bias magnetic filed to the MTJ element1. Therefore, no additional structure for applying the bias magnetic filed is required.