JPWO2006062150A1 - Magnetic random access memory - Google Patents

Abstract

The toggle write magnetic random access memory includes a free magnetic layer, a pinned magnetic layer, and a nonmagnetic layer sandwiched between the free magnetic layer and the pinned magnetic layer. The free magnetic layer has 2n layers (n is an integer of 2 or more) magnetic films antiferromagnetically coupled to each other.

Description

  The present invention relates to a magnetic random access memory (MRAM), and more particularly to a “toggle write system” magnetic random access memory.

  Magnetic random access memory (MRAM) is a promising nonvolatile memory from the viewpoint of high integration and high-speed operation. In the MRAM, magnetoresistive elements exhibiting magnetoresistive effects such as an AMR (Anisotropic MagnetoResistance) effect, a GMR (Giant MagnetoResistance) effect, and a TMR (Tunnel MagnetoResistance) effect are used.

  FIG. 1 is a plan view showing a configuration of an MRAM disclosed in US Pat. No. 5,640,343 (first conventional example). The MRAM 100 includes a write word line 101 and a write bit line 102 formed so as to be orthogonal to each other. A memory cell 103 is arranged at the intersection of the write word line 101 and the write bit line 102, and the memory cell 103 includes a magnetoresistive element.

  FIG. 2 is a conceptual diagram showing the structure of a conventional magnetoresistive element. The magnetoresistive element 110 includes a lower electrode layer 111, an antiferromagnetic layer 112, a pinned magnetic layer (pinned layer) 113, a barrier layer 114, a free magnetic layer (free layer) 115, and an upper electrode layer 116. The barrier layer 114 is a nonmagnetic layer including an insulating film or a metal film, and is sandwiched between the fixed magnetic layer 113 and the free magnetic layer 115. Each of the fixed magnetic layer 113 and the free magnetic layer 115 includes a ferromagnetic layer having spontaneous magnetization. The direction of spontaneous magnetization of the pinned magnetic layer 113 is fixed in a predetermined direction. On the other hand, the direction of spontaneous magnetization of the free magnetic layer 115 can be reversed, and is allowed to be parallel or antiparallel to the direction of spontaneous magnetization of the pinned magnetic layer 113.

The resistance value (R + ΔR) of the magnetoresistive element 110 when the direction of the spontaneous magnetization of the fixed magnetic layer 113 and the free magnetic layer 115 is “antiparallel” is the resistance when they are “parallel” due to the magnetoresistance effect. It is known to be larger than the value (R). The MRAM 100 uses the magnetoresistive element 110 as the memory cell 103, and stores data in a nonvolatile manner by utilizing the change in the resistance value. Data rewriting of the memory cell 103 is performed by reversing the direction of spontaneous magnetization of the free magnetic layer 115. Specifically, the memory cell 103 is arranged so that the easy axis of the free magnetic layer 115 is parallel to the write word line 101 or the write bit line 102. Write currents I _WL and I _BL are supplied to the write word line 101 and the write bit line 102, respectively. When the write currents I _WL and I _BL satisfy a predetermined condition, the direction of the spontaneous magnetization of the free magnetic layer 115 is reversed by the external magnetic field generated by the write current.

FIG. 3A is a diagram showing the predetermined condition (threshold value). The curve shown in FIG. 3A is called an asteroid curve, and shows the minimum write currents I _WL and I _BL necessary for reversing the spontaneous magnetization of the free magnetic layer 115. That is, when the write currents I _WL and I _BL corresponding to the Reversal region outside the asteroid curve are supplied, the data is rewritten. On the other hand, when the supplied write currents I _WL and I _BL fall in the Retention region inside the asteroid curve, data is not rewritten. The external magnetic fields generated by the threshold write currents I _WL and I _BL are H _X and H _Y , and the uniaxial anisotropic magnetic field is H _K. At this time, the following relationship is established for the magnetic fields h _X (= H _X / H _K ), h _Y (= H _Y / H _K ).

FIG. 3B shows an asteroid curve distribution for a plurality of memory cells. There are variations in the characteristics of the magnetoresistive element 110 of each memory cell. Therefore, the asteroid curve group (curve group) for a plurality of memory cells is distributed between the curve Cmax and the curve Cmin as shown in FIG. 3B. In order to perform writing, the write currents I _WL and I _BL need to exist at least in the reversal region outside the curve Cmax. Here, the write currents I _WL and I _BL also affect memory cells other than the target memory cell (hereinafter referred to as “disturb”). It is necessary to prevent writing to a non-target memory cell by a magnetic field generated by one of the write currents I _WL and I _BL . Therefore, the current I _WL flowing through the write word line 101 needs to be smaller than I _{X (min)} , and the current I _BL flowing through the write bit line 102 needs to be smaller than I _{Y (min)} . Therefore, the write currents I _WL and I _BL must correspond to the hatched area (write margin) in FIG. 3B. As the variation in the characteristics of the magnetoresistive element 110 increases, the write margin decreases.

  As a technique for suppressing such deterioration of memory characteristics and erroneous writing (disturbance), a “toggle writing method” has been proposed. For example, US Pat. No. 6,545,906 (second conventional example) discloses a toggle writing MRAM. In the toggle writing type MRAM, “antiferromagnetic coupling force” is used. Some materials have a force (antiferromagnetic coupling force) that causes adjacent magnetizations to work in opposite directions, which is called an antiferromagnetic material. It is known that a similar antiferromagnetic coupling force can be exerted by sandwiching an extremely thin conductive film between ferromagnetic films. In toggle write MRAM, N layers (N is an integer of 2 or more) magnetic films that are antiferromagnetically coupled to each other are used. In the second conventional example, N = 2 and N = 3 are shown.

  4A and 4B are conceptual diagrams showing the structure of the magnetoresistive element 120 used in the toggle write MRAM. The magnetoresistive element 120 includes a lower electrode layer 121, an antiferromagnetic layer 122, a pinned magnetic layer (pinned layer) 123, a barrier layer 124, a free magnetic layer (free layer) 125, and an upper electrode layer 126. Here, the free magnetic layer 125 includes a first magnetic film 131 and a second magnetic film 132 that are coupled “antiferromagnetically” (N = 2). Specifically, a thin nonmagnetic film 133 is sandwiched between the first magnetic film 131 and the second magnetic film 132. Due to this antiferromagnetic coupling, the directions of spontaneous magnetization of the first magnetic film 131 and the second magnetic film 132 become antiparallel in a stable state, as indicated by arrows in FIGS. 4A and 4B. The direction of spontaneous magnetization of the first magnetic film 131 and the second magnetic film 132 in the free magnetic layer (free layer) 125 can be reversed. When one spontaneous magnetization is reversed, the other spontaneous magnetization is also reversed so as to keep the antiparallel state.

  The first magnetic film 131 in the free magnetic layer 125 is stacked on the pinned magnetic layer 123 via the barrier layer 124. 4A shows a “first state” in which the direction of spontaneous magnetization of the first magnetic film 131 and the direction of spontaneous magnetization of the pinned magnetic layer 123 are “antiparallel”, and FIG. 4B shows the spontaneous state of the first magnetic film 131. A “second state” is shown in which the direction of magnetization and the direction of spontaneous magnetization of the pinned magnetic layer 123 are “parallel”. Due to the magnetoresistive effect, the resistance value (R + ΔR) of the magnetoresistive element 120 in the first state becomes larger than the resistance value (R) in the second state. The toggle-write type MRAM uses the magnetoresistive element 120 as the memory cell 103, and stores data in a nonvolatile manner by utilizing a change in resistance value. For example, the first state illustrated in FIG. 4A is associated with data “1”, and the second state illustrated in FIG. 4B is associated with data “0”.

  FIG. 5 is a plan view showing the direction of spontaneous magnetization in a toggle write type MRAM. In FIG. 5, a write word line 101 and a write bit line 102 are formed along the S direction and the T direction, which are orthogonal to each other. The memory cell (the magnetoresistive element 120) is disposed between the write word line 101 and the write bit line 102. Here, the “easy magnetization direction” in the free magnetic layer 125 of the magnetoresistive element 120 is defined as the X direction, and the “hard magnetization direction” is defined as the Y direction. In the toggle write MRAM, as shown in FIG. 5, memory cells are arranged such that the easy axis direction (X direction) forms an angle of 45 degrees with the S direction or the T direction. In a stable state, the spontaneous magnetization of the first magnetic film 131 and the spontaneous magnetization of the second magnetic film 132 are antiparallel to each other and form an angle of 45 degrees with the S direction or the T direction.

  In such an MRAM, data is written by reversing the direction of spontaneous magnetization in the first magnetic film 131 and the second magnetic film 132. According to the toggle writing method, the data (stored data) stored in the target memory cell is read before the data is written. Only when the stored data and the write data are different, the write operation is executed.

6A and 6B are timing charts showing the write operation in the toggle write MRAM. First, at time t1, supplied write current _{I WL} is the write word line 101, at time t2, the write current _{I BL} is supplied to the write bit line 102. Then, at time t3, stops the supply of the write current _{I WL,} at the time t4, the supply of the write current _{I BL} is stopped. By performing such current control, the directions of spontaneous magnetization in the first magnetic film 131 and the second magnetic film 132 are reversed (see the second conventional example). That is, according to the toggle writing method, the magnetization state of the free magnetic layer 125 changes like a toggle switch between the “first state” and the “second state” at every write operation.

FIG. 7 is a graph showing threshold characteristics in a toggle writing MRAM. In FIG. 7, the vertical axis and the horizontal axis indicate the write currents I _WL and I _BL , respectively. Write currents I _WL and I _BL corresponding to an area indicated by “TOGGLE” in the drawing are supplied to the write word line 101 and the write bit line 102 corresponding to the “selected cell” into which data is written. As a result, a toggle operation is performed in the selected cell. Here, as shown in FIG. 7, this threshold characteristic has no X-intercept and Y-intercept. Therefore, only the magnetic field generated by one of the write currents is applied to the “half-selected cell” in which one of the write word line 101 and the write bit line 102 is common to the selected cell. Therefore, no toggle operation occurs in the half-selected cell. As described above, according to the MRAM of the toggle writing system, erroneous writing is greatly reduced as compared with the conventional MRAM shown in FIG. 3B. In addition, since it is not necessary to strictly control the value of the write current, the write margin is greatly improved.

The operation area of the toggle write MRAM is defined as follows. FIG. 8A is a graph showing in detail the threshold characteristics of this MRAM. In FIG. 8A, the vertical axis and the horizontal axis indicate magnetic fields H _WL and H _BL generated by the write currents I _WL and I _BL , respectively. FIG. 8B is a graph showing the magnetoresistive characteristics of the free magnetic layer 125 in the toggle write MRAM. In FIG. 8B, the horizontal axis represents the magnetic field H _X in the direction of the easy axis (X axis), and the vertical axis represents the resistance value.

A magnetic field in the X-axis (easy magnetization axis) direction that is the minimum magnetic field necessary for the toggle operation is defined as “Spin Flop Field H _sf ”. That is, the magnitude of the flop magnetic field H _sf is defined by the distance from the origin to the point a in FIG. 8A. When the free magnetic layer 125 is composed of two equivalent magnetic films, the flop magnetic field H _sf is expressed by the following formula using “uniaxial anisotropic magnetic field H _K ” and “antiferromagnetic coupling magnetic field H _I ”. It is represented by

Further, when the magnetic field during the write operation becomes larger than a certain value, the spontaneous magnetization of each magnetic film included in the free magnetic layer 125 is completely directed in the same direction. At this time, the operation becomes unstable. The critical magnetic field that does not become unstable is shown by curve C in FIG. 8A. The magnetic field at the limit and in the X-axis (easy magnetization axis) direction is defined as “saturation field H _sat ”. That is, the magnitude of the saturation magnetic field H _sat is defined by the distance from the origin to the curve C in FIG. 8A.

Thus, the upper limit and the lower limit of the toggle operation region are defined by the flop magnetic field H _sf and the saturation magnetic field H _sat , respectively. The magnetic field H _{X in the} easy axis direction applied during the write operation needs to fall within the range from the flop magnetic field H _sf to the saturation magnetic field H _sat as shown in FIG. 8B. In toggle write MRAM, a technology that can further expand the toggle operation area is desired. Therefore, a technique that can increase the ratio between the saturation magnetic field H _sat and the flop magnetic field H _sf is desired.

In addition, a technique capable of reducing power consumption is desired in a toggle writing type MRAM. This is because the write current by the toggle write MRAM tends to be larger than the write current by the general MRAM shown in FIG. As an example, the flop magnetic field H _sf for the free magnetic layer 125 composed of the two magnetic films shown in FIG. 4A and the write magnetic field for the single free magnetic layer 115 shown in FIG. 2 are compared. . It is assumed that the uniaxial anisotropic magnetic field H _K is the same in both free magnetic layers. Write field to the free magnetic layer 115 shown in FIG. 2 is about H _K. On the other hand, when the antiferromagnetic coupling magnetic field H _I is given by H _I = 4H _K , the flop magnetic field H _sf is given by the following formula according to the above formula (2).

As described above, according to the toggle writing method, the writing magnetic field, that is, the writing current, is required to be about ^51/2 times that of the general asteroid characteristic. Therefore, a technique capable of reducing the write current in the toggle write MRAM is desired. For this purpose, it is desirable that the flop magnetic field H _sf is small. In order to ensure the thermal stability can not be excessively reduced uniaxial anisotropic magnetic field H _K.

  The following is known as a general MRAM technique that is not the toggle writing method.

  An object of the technique disclosed in Japanese Patent Laid-Open No. 2002-151758 (third conventional example) is to provide a ferromagnetic tunnel magnetoresistive element that is stable against thermal fluctuation. In the free layer of this ferromagnetic tunnel magnetoresistive element, at least five ferromagnetic layers and intermediate layers are laminated. The magnetizations of two ferromagnetic layers adjacent to each other through the intermediate layer are arranged antiferromagnetically.

  A magnetic memory disclosed in Japanese Patent Laid-Open No. 2003-298023 (fourth conventional example) includes two magnetoresistive elements and a common wiring interposed therebetween. The first magnetoresistive element has a first pinned layer and a first free layer. The first pinned layer includes an even number of ferromagnetic layers stacked via a nonmagnetic layer. The first free layer includes a single ferromagnetic layer or a plurality of ferromagnetic layers stacked via a nonmagnetic layer. The second magnetoresistive element has a second pinned layer and a second free layer. The second pinned layer includes a single ferromagnetic layer or three or more ferromagnetic layers stacked via a nonmagnetic layer. The second free layer includes a single ferromagnetic layer or a plurality of ferromagnetic layers stacked via a nonmagnetic layer.

  Japanese Patent Laying-Open No. 2003-331574 (fifth conventional example) discloses an MRAM writing method. In this writing method, a step of applying a first magnetic field parallel to the hard axis to a magnetoresistive effect element having an easy axis and a hard axis, and then a second magnetic field weaker than the first magnetic field and parallel to the hard axis are facilitated. And simultaneously applying a third magnetic field parallel to the axis to the magnetoresistive element.

  According to the magnetoresistive effect element disclosed in Japanese Patent Laid-Open No. 2004-87870 (sixth conventional example), the pinned layer has a function as a magnetic field applying member for applying a static magnetic field to the free layer. . In order to apply the static magnetic field, the strength of the leakage magnetic field from the pinned layer is set to be a predetermined value or more.

  Japanese Patent Laid-Open No. 5-266651 (Seventh Conventional Example) discloses a magnetic thin film memory element. This magnetic thin film memory element stores information according to the magnetization direction of the magnetic thin film. This magnetic thin film has a laminated structure. Specifically, in this magnetic thin film, a magnetic layer a having a large coercive force and a magnetic layer b having a small coercive force are a / c / b / c / a / c / b / c via a nonmagnetic layer c. ... and so on.

An object of the present invention is to provide a toggle writing MRAM capable of suppressing disturbance and capable of expanding an operation area.
Another object of the present invention is to provide a toggle write type MRAM capable of suppressing disturbance and capable of reducing a write current.

  The magnetic random access memory according to the present invention is a “toggle write system”. The magnetic random access memory includes a free magnetic layer, a pinned magnetic layer, and a nonmagnetic layer sandwiched between the free magnetic layer and the pinned magnetic layer.

  The free magnetic layer has 2n layers (n is an integer of 2 or more) magnetic films antiferromagnetically coupled to each other. The free magnetic layer further includes (2n-1) nonmagnetic films, and the 2n magnetic films and the (2n-1) nonmagnetic films are alternately stacked. The non-magnetic films of the (2n-1) layer are almost equal in thickness, and the materials are preferably the same. Further, it is preferable that the thicknesses of the 2n-layer magnetic films are substantially equal and the materials thereof are the same.

  The inventors of the present application discovered and demonstrated that the operation area of the magnetic random access memory is expanded by such a configuration. Furthermore, the present inventors have discovered and demonstrated that such a configuration reduces the write current of the magnetic random access memory.

FIG. 1 is a plan view showing a configuration of a conventional MRAM. FIG. 2 is a conceptual diagram showing the structure of a magnetoresistive element used in a conventional MRAM. FIG. 3A is a graph showing a threshold characteristic (asteroid curve) for a certain memory cell in a conventional MRAM. FIG. 3B is a graph showing the distribution of threshold characteristics for a plurality of memory cells in a conventional MRAM. FIG. 4A is a conceptual diagram showing the structure of a magnetoresistive element used in a conventional toggle write MRAM. FIG. 4B is a conceptual diagram showing the structure of a magnetoresistive element used in a conventional toggle write MRAM. FIG. 5 is a plan view showing the direction of spontaneous magnetization in a conventional toggle write MRAM. 6A and 6B are timing charts showing a write operation in a conventional toggle write MRAM. FIG. 7 is a graph showing threshold characteristics in a conventional toggle write MRAM. FIG. 8A is a graph showing details of threshold characteristics in a conventional toggle write MRAM. FIG. 8B is a graph showing the magnetoresistance characteristics of the free magnetic layer in the conventional toggle write MRAM. FIG. 9 is a plan view showing the configuration of the MRAM according to the present invention. FIG. 10 is a conceptual diagram showing the structure of a magnetoresistive element used in the MRAM according to the present invention. FIG. 11 is a graph showing the magnetization characteristics of the memory cell in the MRAM according to the present invention. FIG. 12A is a graph showing the relationship between the saturation magnetic field and the uniaxial anisotropic magnetic field in the MRAM according to the present invention. FIG. 12B is a graph showing the relationship between the saturation magnetic field and the number N of stacked magnetic films in the MRAM according to the present invention. FIG. 13A is a graph showing a relationship between a flop magnetic field and a uniaxial anisotropic magnetic field in the MRAM according to the present invention. FIG. 13B is a graph showing the relationship between the flop magnetic field and the number N of stacked magnetic films in the MRAM according to the present invention. FIG. 14 is a conceptual diagram showing an example of the structure of a magnetoresistive element used in the MRAM according to the present invention. FIG. 15 is a conceptual diagram showing another example of the structure of the magnetoresistive element used in the MRAM according to the present invention. FIG. 16 is a graph showing the magnetization characteristics of the magnetoresistive element shown in FIGS. 14 and 15. FIG. 17 is a conceptual diagram showing still another example of the structure of the magnetoresistive element used in the MRAM according to the present invention.

  A magnetic random access memory (MRAM) according to the present invention will be described with reference to the accompanying drawings. The MRAM according to the present invention is a “toggle writing type” MRAM.

FIG. 9 is a plan view showing the configuration of the MRAM according to the present invention. The MRAM 1 according to the present invention includes a write word line 2 formed along the S direction and a write bit line 3 formed along the T direction. The S direction and the T direction are substantially perpendicular to each other. That is, the write word line 2 and the write bit line 3 are provided so as to cross each other. During the write operation, a write current I _WL is supplied to the write word line 2, and a write magnetic field H _{WL in the} T direction is generated by the write current I _WL . Also, during a write operation, the write bit line 3 is supplied write current I _BL, the S direction of the write magnetic field H _BL is generated by the write current I _BL.

A memory cell 4 is provided at the intersection of the write word line 2 and the write bit line 3. The memory cell 4 is disposed so as to be sandwiched between the write word line 2 and the write bit line 3, and the write magnetic fields H _WL and H _BL act on the memory cell 4. Further, the memory cell 4 has a magnetoresistive element 10 showing a magnetoresistive effect. The easy axis direction of the free magnetic layer (free layer) included in the magnetoresistive element 10 is defined as the X direction, and the hard axis direction is defined as the Y direction. As shown in FIG. 9, in the present invention, the X direction forms an angle of approximately 45 degrees with respect to the S direction or the T direction. That is, the magnetoresistive element 10 is arranged so that the angle formed between the easy magnetization axis and the write word line 2 or the write bit line 3 is about 45 degrees.

  FIG. 10 is a conceptual diagram showing the structure of the magnetoresistive element 10 according to the present invention. The magnetoresistive element 10 includes a lower electrode layer 11, an antiferromagnetic layer 12, a pinned magnetic layer (pinned layer) 13, a barrier layer 14, a free magnetic layer (free layer) 15, and an upper electrode layer 16. Each of the fixed magnetic layer 13 and the free magnetic layer 15 includes a ferromagnetic layer having spontaneous magnetization. The direction of spontaneous magnetization of the pinned magnetic layer 13 is fixed in a predetermined direction. On the other hand, the direction of spontaneous magnetization of the free magnetic layer 15 can be reversed. The barrier layer 14 is a nonmagnetic layer including an insulating film or a metal film, and is sandwiched between the pinned magnetic layer 13 and the free magnetic layer 15. The barrier layer 14 is, for example, a tunnel insulating film. At this time, a magnetic tunnel junction (MTJ) is formed by the pinned magnetic layer 13, the barrier layer 14, and the free magnetic layer 15.

  According to the present invention, the free magnetic layer 15 includes 2n layers (n is an integer of 2 or more) of magnetic films 20-1 to 20-2n that are antiferromagnetically coupled to each other. That is, the free magnetic layer 15 has four or more even-numbered magnetic films 20, and the plurality of magnetic films 20-1 to 20-2 n are antiferromagnetically coupled to each other. A nonmagnetic film (antiferromagnetic coupling film) 30 for realizing antiferromagnetic coupling is formed between adjacent magnetic films 20. That is, the free magnetic layer 15 also includes (2n-1) non-magnetic films 30-1 to 30- (2n-1). As shown in FIG. 10, the 2n magnetic films 20 and the (2n-1) nonmagnetic films 30 are alternately stacked.

  Preferably, the 2n magnetic films 20 are equivalent to each other. That is, it is preferable that the material and film thickness of the 2n-layer magnetic film 20 are the same. Examples of the material of the magnetic film 20 include at least one selected from the group consisting of Ni, Fe, Co, Mn, and compounds thereof. Further, the film thickness of the magnetic film 20 is exemplified by 1.5 nm to 10 nm. Preferably, the (2n-1) nonmagnetic films 30 are equivalent to each other. That is, it is preferable that the nonmagnetic film 30 of the (2n-1) layer has the same material and film thickness. Examples of the material of the nonmagnetic film 30 include at least one selected from the group consisting of Ru, Os, Re, Ti, Cr, Rh, Cu, Pt, and Pd. Further, the film thickness of the nonmagnetic film 30 is exemplified by 0.4 nm to 3 nm.

  With such a laminated structure, 2n layers of magnetic films 20-1 to 20-2n antiferromagnetically coupled to each other are realized. Thereby, the direction of the spontaneous magnetization in the two adjacent magnetic films 20 is “antiparallel” in the stable state. That is, as shown in FIG. 10, the directions of spontaneous magnetization in the 2n magnetic films 20-1 to 20-2n are alternated.

  The magnetoresistive element 10 has two stable states. In the “first state”, as indicated by an arrow in FIG. 10, the direction of spontaneous magnetization in the magnetic film 20-1 adjacent to the barrier layer 14 is “antiparallel to the direction of spontaneous magnetization in the pinned magnetic layer 13. ". In the “second state”, the direction of spontaneous magnetization in the 2n-layer magnetic film 20 is entirely reversed. That is, the direction of spontaneous magnetization in the magnetic film 20-1 is “parallel” to the direction of spontaneous magnetization in the pinned magnetic layer 13. Due to the magnetoresistive effect, the resistance value (R + ΔR) of the magnetoresistive element 10 in the first state is larger than the resistance value (R) in the second state. The MRAM 1 stores data in a nonvolatile manner by utilizing the change in resistance value. For example, the first state shown in FIG. 10 is associated with data “1”, and the second state is associated with data “0”.

  Reading of data stored in a certain memory cell 4 is performed by detecting the magnitude of the resistance value of the magnetoresistive element 10. Specifically, the magnitude of the resistance value is detected by applying a predetermined voltage between the lower electrode layer 11 and the upper electrode layer 16 and detecting the magnitude of the current flowing through the magnetoresistive element 10. The data stored in the target memory cell 4 is determined based on the detected resistance value.

  Data is written by reversing the direction of spontaneous magnetization in the 2n-layer magnetic film 20 included in the free magnetic layer 15. According to the toggle writing method, the data (stored data) stored in the target memory cell is read before the data is written. Only when the stored data and the write data are different, the write operation is executed.

The write operation is performed in the following order. That is, at time t1, the write current _{I WL} is supplied to the write word line 2, at time t2, the write current _{I BL} is supplied to the write bit line 3. Then, at time t3, stops the supply of the write current _{I WL,} at the time t4, the supply of the write current _{I BL} is stopped. That is, a pulse is applied to the write word line 2 from time t1 to time t3, and a pulse is applied to the write bit line 3 from time t2 to time t4. Here, time t1 <time t2 <time t3 <time t4 (see FIGS. 6A and 6B). By performing such a current control, i.e., by shifting the timing for supplying the write current I _WL and I _BL, the spontaneous magnetization in a plurality of the magnetic film 20 is reversed. According to the toggle writing method, the magnetization state of the free magnetic layer 15 changes like a toggle switch between the “first state” and the “second state” each time the write operation is performed.

  As described above, according to the MRAM 1 according to the present invention, the free magnetic layer 15 has the magnetic film 20 of 2n layers (n is an integer of 2 or more) mutually “antiferromagnetically coupled”. The inventors of the present application have discovered and demonstrated that the operation area of the MRAM 1 is expanded by such a configuration. Furthermore, the present inventors have discovered and demonstrated that such a configuration reduces the write current of the MRAM 1. Hereinafter, the data that serves as the basis will be described in detail.

FIG. 11 shows the magnetization characteristics of the free magnetic layer 15 having the equivalent 2n magnetic film 20. The horizontal axis represents the magnetic field H _X in the easy magnetization axis direction (X direction), and the vertical axis represents the magnetization M _X normalized by the saturation magnetization M _s . Further, the number of stacked magnetic films 20 is N (= 2n). FIG. 11 shows magnetization characteristics for five types of memory cells, N = 2, N = 4, N = 6, N = 8, and N = 10. This magnetization characteristic can be obtained by numerically solving the Landau-Lifshitz equation. As shown in FIG. 11, it was found that “saturation magnetic field H _sat ” increases as the number N of stacked layers increases. In such a multilayer film, it is considered that a plurality of antiferromagnetic coupling magnetic fields act on the magnetic film 20 not at both ends from the upper layer and the lower layer, and as a result, the saturation magnetic field H _sat increased.

  In general, when a magnetic field H is applied in the easy axis direction (X direction), the energy Et of a system composed of “two-layer” equivalent magnetic films is expressed by the following equation.

Here, _Ku is the anisotropic energy, J is the antiferromagnetic coupling energy, M is the magnetization, θ ₁ is the angle between the first magnetic film and the easy axis, and θ ₂ is the second magnetic film and the easy axis. angle between, _{H K} uniaxial anisotropy field _{(= 2Ku / M), H} I indicates anti-ferromagnetic coupling field (= 2J / M), respectively. The energy of a system consisting of “multilayer” equivalent magnetic films is derived by calculating these Jacobians.

FIG. 12A shows the relationship between the saturation magnetic field H _sat and the uniaxial anisotropy magnetic field H _K using the number N of layers as a parameter. In Figure 12A, the vertical and horizontal axes show respectively the saturation magnetic field H _sat and the uniaxial anisotropy field H _K, which is normalized by the antiferromagnetic coupling field H _I. These derivations are performed by calculating the Jacobian of the above formula (4). As shown in FIG. 12A, with respect to certain number of stacked layers N, as uniaxial anisotropic magnetic field H _K / H _I that are standardized small, the saturation magnetic field H _sat / H _I that are standardized increases.

12B _is in the case of _H K = 0, show a dependence on the number of stacked layers N of normalized saturation field _{H sat} / _{H I.} As shown in FIG. 12B, as the number of stacked layers N of the magnetic film 20 is increased, the saturation magnetic field H _sat / H _I that are standardized increases. That is, when the flop magnetic field H _sf is constant, the saturation magnetic field H _sat increases as the number N of stacked layers increases. This means that the operation region (H _sat −H _sf ) is expanded.

FIG. 13A shows the relationship between the flop magnetic field H _sf and the uniaxial anisotropy magnetic field H _K with the number N of layers as a parameter. In FIG. 13A, the vertical and horizontal axes show respectively the flop field normalized H _sf and uniaxial anisotropic magnetic field H _K by the antiferromagnetic coupling field H _I. These derivations are performed by calculating the Jacobian of the above formula (4). As shown in FIG. 13A, the laminated number N an even number (N = 2n), regardless somewhat number, the same dependency, tended to flop magnetic field is normalized H _sf / H _I . Also, if the lamination number N is an odd number (N = 2n-1), the laminated number N as compared with the case of the even number, the flop field H _sf tends to increase was observed for all H _K.

13B _is in the case of _H K = 0, show a dependence on the number of stacked layers N of the flop field _{H sf} / _{H I} that are standardized. As shown in FIG. 13B, when the number N of layers is an even number (N = 2n), the flop magnetic field H _sf is zero. On the other hand, when the number N of layers is an odd number, the flop magnetic field H _sf has a finite value and decreases as the number N of layers increases.

The following can be derived from these results. When the number N of layers is an even number (N = 2n), the flop magnetic field H _sf is constant, and the saturation magnetic field H _sat increases as the number N of layers increases. This means that the operating region is expanded as the number N of stacked layers is increased. On the other hand, when the number of stacked layers N is an odd number, the saturation magnetic field H _sat increases as the number of stacked layers N increases, but the flop magnetic field H _sf also increases compared to the case where N = 2. In other words, the operation region does not expand so much as compared with the case of N = 2. That is, in order to widen the operation margin, the number N of layers needs to be an even number and 4 or more. The increase width of the operation margin is greatest when the number N of stacked layers is changed from 2 to 4. As the number N of layers increases, the increase margin of the operation margin decreases. Actual number of layers N is variation in increment and manufacturing variations of the operation margin (uniaxial anisotropy field H _K, variations in the thickness of the magnetic layer 20, variation in the thickness of the antiferromagnetic coupling film 30, It is determined in consideration of variation in shape).

Table 1 is normalized with the normalized flop magnetic field H _sf / H _I , the normalized saturation magnetic field H _sat / H _I , and the saturation magnetic field H _sat when H _K / H _I = 0.2. Flop magnetic field H _sf (norm) (= H _sf / H _sat = (H _sf / H _I ) / (H _sat / H _I )). Here, by adjusting the film thickness and material of the magnetic film 20 and the antiferromagnetic coupling film 30, the saturation magnetic field H _sat is set equal to the number N of each stack (the antiferromagnetic coupling magnetic field H _I is It is different for each stacking number N). That is, the magnetic field H _sf (norm) means a normalized flop magnetic field H _sf when the saturation magnetic field H _sat is constant. For example, when the number N of layers is 4, the standardized flop magnetic field H _sf (norm) is about half that when the number N of layers is 2. As the number N of layers increases further, the normalized flop magnetic field H _sf (norm) gradually decreases. That is, under the condition that the uniaxial anisotropy magnetic field H _K and the saturation magnetic field H _sat are constant, by setting the number N of layers to 4 or more, the value of the write current is reduced to less than half that in the case of N = 2. You can see that That is, power consumption is reduced.

  Hereinafter, description will be made using a more specific example.

Example 1: Two types of magnetoresistive elements having different numbers of stacked layers N were produced, and flop magnetic field H _sf and saturation magnetic field H _sat were evaluated for them.

  FIG. 14 is a conceptual diagram showing a first magnetoresistive element having the structure A. The magnetoresistive element includes a seed layer 41, an antiferromagnetic layer 42, a pinned magnetic layer 43, a barrier layer 44, a free magnetic layer 45, and a cap layer 46, which are sequentially stacked on a substrate. The seed layer 41 is a Ta film having a thickness of 20 nm. The antiferromagnetic layer 42 is a PtMn film having a thickness of 20 nm. The pinned magnetic layer 43 is composed of a CoFe film having a thickness of 2.5 nm, a Ru film having a thickness of 0.88 nm, and a CoFe film having a thickness of 2.5 nm. The barrier layer 44 is a film obtained by oxidizing an Al film having a thickness of 1 nm. The free magnetic layer 45 has two magnetic films and one nonmagnetic film that are alternately stacked (N = 2). Each magnetic film is composed of a NiFe film having a thickness of 4 nm and a CoFe film having a thickness of 0.5 nm. Each nonmagnetic film (antiferromagnetic coupling film) is a Ru film having a thickness of 2.1 nm. The cap layer 46 is composed of an oxidized Al film having a thickness of 0.7 nm and a Ta film having a thickness of 100 nm.

  FIG. 15 is a conceptual diagram showing a second magnetoresistive element having structure B. FIG. The magnetoresistive element includes a seed layer 51, an antiferromagnetic layer 52, a fixed magnetic layer 53, a barrier layer 54, a free magnetic layer 55, and a cap layer 56, which are sequentially stacked on a substrate. The seed layer 51, the antiferromagnetic layer 52, the pinned magnetic layer 53, the barrier layer 54, and the cap layer 56 have the same film thickness and composition as the seed layer 41, the antiferromagnetic layer 42, the pinned magnetic layer 43, and the barrier, respectively. It is the same as each of the layer 44 and the cap layer 46. The free magnetic layer 55 has four magnetic films and three nonmagnetic films alternately stacked (N = 4). Each magnetic film is composed of a NiFe film having a thickness of 4 nm and a CoFe film having a thickness of 0.5 nm, like the first magnetoresistive element. Each nonmagnetic film (antiferromagnetic coupling film) is a Ru film having a film thickness of 2.1 nm, like the first magnetoresistive element.

FIG. 16 shows the magnetization characteristics of the free magnetic layers 45 and 55 in the structure A (N = 2) and the structure B (N = 4). In FIG. 16, the horizontal axis indicates the magnetic field H _X in the easy magnetization axis direction (X direction), and the vertical axis indicates the magnetization MX normalized by the saturation magnetization Ms. Table 2 shows the flop magnetic field H _sf and the saturation magnetic field H _sat corresponding to the structure A (N = 2) and the structure B (N = 4).

As shown in FIG. 16 and Table 2, the flop magnetic field H _sf for structures A and B was almost the same (62-63 Oe). On the other hand, the saturation magnetic field H _sat greatly changed between the structure A and the structure B. Specifically, in the case of the structure A (N = 2), the saturation magnetic field H _sat was 298 [Oe], whereas in the case of the structure B (N = 4), the saturation magnetic field H _sat was 1180 [Oe]. Met. That is, it has been found that, as the number N of layers increases, the flop magnetic field H _sf does not change much, but the saturation magnetic field H _sat greatly increases. That is, it was found that the operating area was expanded.

Example 2: A third magnetic field having a saturation magnetic field H _sat substantially the same as that of the first magnetoresistive element by adjusting the film thickness of the magnetic film and the nonmagnetic film (antiferromagnetic coupling film) in the free ferromagnetic layer This magnetoresistive element was fabricated. Then, the flop magnetic field H _sf was evaluated for them.

FIG. 17 is a conceptual diagram showing a third magnetoresistive element having the structure C. The magnetoresistive element includes a seed layer 61, an antiferromagnetic layer 62, a fixed magnetic layer 63, a barrier layer 64, a free magnetic layer 65, and a cap layer 66 that are sequentially stacked on a substrate. The seed layer 61, the antiferromagnetic layer 62, the pinned magnetic layer 63, the barrier layer 64, and the cap layer 66 have the same film thickness and composition as the seed layer 41, the antiferromagnetic layer 42, the pinned magnetic layer 43, the barrier, respectively. It is the same as each of the layer 44 and the cap layer 46. The free magnetic layer 65 has four magnetic films and three nonmagnetic films alternately stacked (N = 4). Each magnetic film is composed of a NiFe film having a thickness of 2 nm and a CoFe film having a thickness of 0.3 nm. Each nonmagnetic film (antiferromagnetic coupling film) is a Ru film having a thickness of 3.6 nm. By adjusting in this way the free magnetic layer 65, while H _K / H _I is kept constant, the uniaxial anisotropy field H _K is reduced.

Table 3 shows the flop magnetic field H _sf and the saturation magnetic field H _sat corresponding to the structure A (N = 2) and the structure C (N = 4). In the case of the structure C, the saturation magnetic field H _sat was 350 [Oe], and the flop magnetic field H _sf was 27 [Oe]. That is, in the structures A and C, the saturation magnetic field H _sat is substantially the same. On the other hand, it was found that when the number N of stacked layers increases, the flop magnetic field H _sf decreases when the saturation magnetic field H _sat is constant. That is, by increasing the number N of stacked layers, not only can the operating region be expanded, but also the write current can be reduced. That is, power consumption is reduced.

  As described above, the toggle write type MRAM 1 according to the present invention is configured to have the magnetic film 20 of the 2n layers (n is an integer of 2 or more) in which the free magnetic layers 15 are antiferromagnetically coupled to each other. The As a result, the operation area is expanded. In addition, the write current is reduced and the power consumption is reduced.

  According to the toggle write MRAM according to the present invention, the operation area is expanded. Further, according to the toggle write type MRAM according to the present invention, the write current is reduced.

Claims (10)

Toggle write magnetic random access memory,
A free magnetic layer;
A fixed magnetic layer;
The free magnetic layer and a nonmagnetic layer sandwiched between the pinned magnetic layers,
The said free magnetic layer has a magnetic film of 2n layers (n is an integer greater than or equal to 2) mutually antiferromagnetically coupled. Magnetic random access memory. The magnetic random access memory according to claim 1,
The free magnetic layer further includes (2n-1) nonmagnetic films,
The magnetic random access memory, wherein the 2n magnetic films and the (2n-1) nonmagnetic films are alternately stacked. A magnetic random access memory according to claim 2,
The non-magnetic film thickness of the (2n-1) layer is substantially equal. Magnetic random access memory. A magnetic random access memory according to claim 2 or 3,
The material of the nonmagnetic film of the (2n-1) layer is the same. Magnetic random access memory. A magnetic random access memory according to any one of claims 1 to 4,
The magnetic random access memory, wherein the 2n magnetic films have substantially the same thickness. A magnetic random access memory according to any one of claims 1 to 5,
The material of the magnetic film of the 2n layer is the same. Magnetic random access memory. A word line,
A bit line substantially orthogonal to the word line;
A memory cell provided at an intersection of the word line and the bit line;
The memory cell is
A free magnetic layer;
A fixed magnetic layer;
The free magnetic layer and a nonmagnetic layer sandwiched between the pinned magnetic layers,
The free magnetic layer has 2n layers (n is an integer of 2 or more) magnetic films antiferromagnetically coupled to each other,
The magnetic random access memory, wherein an angle formed by the easy axis of each of the 2n magnetic films and the word line or the bit line is approximately 45 degrees. A magnetic random access memory according to claim 7,
During the write operation of the write data to the memory cell, the stored data stored in the memory cell and the write data are compared, and the write is performed only when the stored data and the write data are different. Magnetic random access memory where the action is performed. A magnetic random access memory according to claim 8,
When the write operation is performed on the memory cell, a pulse is applied to one of the word line and the bit line from time t1 to time t3,
A pulse is applied to the other of the word line and the bit line from time t2 to time t4,
The magnetic random access memory in which the time t1 <the time t2 <the time t3 <the time t4. A free magnetic layer;
A fixed magnetic layer;
The free magnetic layer and a nonmagnetic layer sandwiched between the pinned magnetic layers,
The free magnetic layer is
A magnetic film of 2n layers (n is an integer of 2 or more) that are antiferromagnetically coupled to each other;
A magnetoresistive element comprising: (2n-1) nonmagnetic films alternately stacked with the 2n magnetic films.

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