Abstract:
In a ferromagnetic tunnel junction element, a recording layer is in a circular shape, which can suppress an increase in magnetization switching field due to miniaturization of the element. Further, the recording layer includes a first ferromagnetic layer, a first non-magnetic layer, a second ferromagnetic layer, a second non-magnetic layer, and a third ferromagnetic layer successively stacked. The first and second ferromagnetic layers, and the second and third ferromagnetic layers are coupled antiparallel to each other, so that it is possible to control the magnetization distribution of the recording layer in an approximately single direction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a magnetic storage element, and more particularly to a magnetic storage element that stores data by a magnetoresistive effect.  
         [0003]     2. Description of the Background Art  
         [0004]     The magnetoresistive (MR) effect is a phenomenon in which electric resistance is changed by applying a magnetic field to a magnetic material, which effect is used for a magnetic field sensor, a magnetic head and the like. Recently, a nonvolatile magnetic random access memory (RAM) and a magnetic head using a conventional giant-magnetoresistance (GMR) effect as well as a tunneling magnetoresistance (TMR) effect that assures a still larger rate of change of resistance than the GMR effect have been investigated.  
         [0005]     In the GMR element or the TMR element producing the GMR effect or the TMR effect, a so-called spin valve structure is known, in which a ferromagnetic layer/a non-magnetic layer/a ferromagnetic layer/an antiferromagnetic layer are stacked one on another, with the ferromagnetic layer and the antiferromagnetic layer being exchange-coupled to fix the magnetic moment of the relevant ferromagnetic layer, and spin is readily reversed only in the other ferromagnetic layer by an external magnetic field. In this case, spin can be reversed in one of the ferromagnetic layers with a small magnetic field, so that it is possible to provide a magnetoresistance element of high sensitivity. This magnetoresistance element is used for a high-density magnetic recording and readout head. In the GMR element, a metal film is used for the non-magnetic layer, while in the TMR element, a tunneling insulating film is used for the non-magnetic layer.  
         [0006]     Investigations of application of the GMR element and the TMR element to the MRAM are shown, e.g., in Document 1 (S. Tehrani et al., “High density submicron magnetoresistive random access memory (invited)”, Journal of Applied Physics, vol. 85, No. 8, 15 Apr. 1999, pp. 5822-5827) and Document 2 (Naji et al., “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC 2001 Digest of Technical Papers, p. 122). When using the GMR element and the TMR element in the MRAM, these elements are arranged in a matrix, and a current is flown through a separately provided interconnection to apply the magnetic field. The two magnetic layers constituting each element are controlled parallel or antiparallel to each other, to thereby record data of “1” or “0”. Reading is performed utilizing the GMR effect or the TMR effect, utilizing the change in the element resistance value that depends on the parallel state or the antiparallel state of the magnetic layers.  
         [0007]     The use of TMR elements in MRAM has primarily been investigated, since the TMR effect is more advantageous than the GMR effect from the standpoint of low power consumption. The MRAM utilizing the TMR elements has the MR change rate of not less than 20% at room temperature, and the resistance value in the tunnel junction is also large, so that a greater output voltage can be obtained. Further, spin reversal is unnecessary upon reading, meaning that a less current is required for the reading. With these features, the MRAM utilizing the TMR elements is expected to realize a nonvolatile semiconductor memory device consuming less power and allowing high-speed reading and writing.  
         [0008]     As described above, in MRAM, data “1” or “0” is stored by switching the magnetization of one ferromagnetic layer in the TMR element. This ferromagnetic layer serving as the recording layer has a direction in which magnetization is easy (the low energy state) depending on the crystal structure or the shape. This direction is called an “easy axis”. In the state where the stored information is held, the ferromagnetic layer is magnetized in this direction. In contrast, the direction in which magnetization is difficult is called a “hard axis”.  
         [0009]     The easy axis of the recording layer is normally determined by its shape, and corresponds to the longitudinal direction of the recording layer. As such, the magnetic field required for switching the magnetization of the recording layer, i.e., the switching field, changes depending on the shape of the recording layer. It is known that this switching field is approximately inversely proportional to the width of the recording layer and proportional to the thickness, as shown in Document 3 (E. Y. Chen et al., “Submicron spin valve magnetoresistive random access memory cell”, Journal of Applied Physics, vol. 81, No. 8, 15 Apr. 1997, pp. 3992-3994).  
         [0010]     In MRAM, when cells are downsized for higher integration, the switching field would increase by the demagnetizing field, depending on the width of the recording layer. This means that a large magnetic field is required for writing, which leads to increased power consumption.  
         [0011]     As a method for suppressing an increase of the switching field due to the downsizing as described above, a technique to eliminate the shape anisotropy of the recording layer is described in Document 4 (K. Inomata et al., “Size-independent spin switching field using synthetic antiferromagnets”, Applied Physics Letters, vol. 82, No. 16, 21 Apr. 2003, pp. 2667-2669) and Document 5 (N. Tezuka et al., “Magnetization reversal and domain structure of antiferromagnetically coupled submicron elements”, Journal of Applied Physics, vol. 93, No. 10, 15 May 2003, pp. 7441-7443). The recording layer according to this technique is shaped to have equal lengths in the easy axis direction and the hard axis direction. In this case, shape anisotropy is not obtained, and thus, the recording layer is made to have a stacked structure of ferromagnetic/non-ferromagnetic/ferromagnetic layers, and two ferromagnetic layers are coupled antiparallel to each other so as to control the magnetization distribution within the plane of the recording layer and to thereby provide magnetic anisotropy. The switching field of the recording layer with this configuration is approximated by the following expression (1):
 
 Hsw= 2 Ku ( t 2+ t 1)/| M 2 t 2− M 1 t 1|+4π C ( k )| M 2 t 2− M 1 t 1 |/w   (1)
 
 where Hsw represents the switching field of the recording layer, Ku represents anisotropic energy of the recording layer, t1 and t2 represent thicknesses of the respective ferromagnetic layers, and M 1  and M 2  represent saturation magnetizations of the respective ferromagnetic layers. Further, k represents an aspect ratio of the recording layer, and C(k) is a coefficient dependent thereon, and t and w represent thickness and width, respectively, of the recording layer. 
 
         [0012]     C(k) can be regarded as “1” for the shape having an infinite length, and “0” for the isotropic shape. The first term in the right side of the expression (1) is a term by anisotropic energy, and the second term is a term describing the influence of the demagnetizing field generated by shape anisotropy. Herein, C(k)=0, and the influence of the demagnetizing field generated by the shape anisotropy can be ignored. As such, it is possible to suppress the increase in switching field due to the miniaturization of the recording layer.  
         [0013]     In the above-described configuration, providing a difference between the products of saturation magnetization and thickness of the two ferromagnetic layers can decrease the magnetization switching field, as seen from the expression (1). However, it is reported in Document 5 that, if the difference in thickness between the two ferromagnetic layers increases, the effect of antiparallel coupling decreases, making it difficult to control the magnetization distribution in the recording layer.  
         [0014]     In the recording layer having such a stacked structure, it is difficult to suppress the increase in switching field due to the miniaturization of the element and to control the magnetization distribution in the recording layer.  
       SUMMARY OF THE INVENTION  
       [0015]     In view of the foregoing, a main object of the present invention is to provide a magnetic storage element capable of suppressing an increase in magnetization switching field due to miniaturization of the element and capable of controlling magnetization distribution in a recording layer.  
         [0016]     A magnetic storage element according to the present invention includes a recording layer arranged between two writing lines crossing each other and having a magnetization direction changed in accordance with directions of currents flown on the two writing lines. The recording layer has a length in a hard axis direction approximately equal to a length in an easy axis direction. The recording layer includes a first ferromagnetic layer, a first non-magnetic layer, a second ferromagnetic layer, a second non-magnetic layer, and a third ferromagnetic layer stacked successively, and the first ferromagnetic layer and the second ferromagnetic layer, and the second ferromagnetic layer and the third ferromagnetic layer are coupled antiparallel to each other.  
         [0017]     In the magnetic storage element of the present invention, the recording layer has the length in the hard axis direction that is made approximately equal to the length in the easy axis direction. This can suppress an increase in magnetization switching field due to miniaturization of the element. Further, the recording layer is formed of the first ferromagnetic layer, the first non-magnetic layer, the second ferromagnetic layer, the second non-magnetic layer, and the third ferromagnetic layer, which are successively stacked. The first and second ferromagnetic layers are coupled antiparallel to each other, and the second and third ferromagnetic layers are also coupled antiparallel to each other. This enables control of the magnetization distribution within the plane of the recording layer.  
         [0018]     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a circuit diagram showing a main part of an MRAM that uses a ferromagnetic tunnel junction element according to an embodiment of the present invention.  
         [0020]      FIG. 2  is a cross sectional view showing a configuration of a memory cell shown in  FIG. 1 .  
         [0021]      FIGS. 3A and 3B  are cross sectional views each showing a configuration and a storage state of the ferromagnetic tunnel junction element shown in  FIG. 1 .  
         [0022]      FIG. 4  is a top plan view showing the shape of the recording layer shown in  FIGS. 3A and 3B .  
         [0023]      FIG. 5  is a top plan view showing the vicinity of the ferromagnetic tunnel junction element shown in  FIG. 1 .  
         [0024]      FIGS. 6A, 6B  and  6 C illustrate the effect of the present embodiment.  
         [0025]      FIG. 7  is a top plan view showing a modification of the present embodiment.  
         [0026]      FIGS. 8A and 8B  are cross sectional views showing another modification of the present embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]      FIG. 1  is a circuit diagram showing a main part of an MRAM using a ferromagnetic tunnel junction element according to an embodiment of the present invention. In  FIG. 1 , a reading bit line  1 , a write line  2  and a writing bit line  3  extend in a horizontal direction in the figure, with a plurality of sets of lines  1 - 3  arranged in a vertical direction in the figure. A word line  4  extends in the vertical direction in the figure to cross the plurality of sets of lines  1 - 3 , with a plurality of such word lines  4  arranged in the horizontal direction in the figure. A plurality of reading bit lines  1  are commonly connected to an input node of a sense amplifier  5 .  
         [0028]     A memory cell MC is provided at each crossing point of the set of lines  1 - 3  and word line  4 , with a plurality of such memory cells MC arranged in a matrix. Each memory cell MC includes a transistor for selecting an element (hereinafter, “element selecting transistor”)  6  and a ferromagnetic tunnel junction element  7  serving as a magnetic storage element, connected in series. More specifically, ferromagnetic tunnel junction element  7  is arranged at the crossing point of write line  2  and writing bit line  3 .  
         [0029]      FIG. 2  is a schematic cross sectional view showing a configuration of memory cell MC. Element selecting transistor  6  is formed at an upper surface of a semiconductor substrate  10 . Word line  4  serves as a gate electrode of transistor  6 , with a gate insulating film  6   g  provided between word line  4  and semiconductor substrate  10 . A sidewall  6   w  is provided on each side of word line  4 . Element selecting transistor  6  has its drain  6   d  connected to ferromagnetic tunnel junction element  7  via a contact plug  11  and a conductive layer  14 , and has its source  6   s  connected to reading bit line  1  via a contact plug  15 . Write line  2  is provided between conductive layer  14  and semiconductor substrate  10 , insulated via an interlayer insulating film  16 . Contact plugs  11  and  15  are each stacked in a plurality of stages in interlayer insulating film  16 , for example. The respective stages of contact plugs  11  and  15 , reading bit line  1 , write line  2 , and writing bit line  3  each include, e.g., a copper interconnection  12  and a barrier metal  13  surrounding copper interconnection  12 .  
         [0030]     Ferromagnetic tunnel junction element  7  has a fixed layer  20 , a tunneling insulating layer  21 , and a recording layer  22  stacked in this order from the side of semiconductor substrate  10 . Fixed layer  20  is conductive with contact plug  11 , and recording layer  22  is conductive with writing bit line  3 . Writing bit line  3  has an opening portion  3   a  for contact with recording layer  22 .  
         [0031]      FIGS. 3A and 3B  are cross sectional views each showing a configuration and a storage state of ferromagnetic tunnel junction element  7 . In  FIGS. 3A and 3B , magnetization of fixed layer  20  is fixed in advance in a prescribed direction, for example in the extending direction of write line  2 . Recording layer  22  has its magnetization direction changed by an external magnetic field. It is assumed that the state where the magnetization direction of fixed layer  20  is the same as the magnetization direction of a ferromagnetic layer  25  constituting recording layer  22  and in contact with tunneling insulating layer  21 , as shown in  FIG. 3A , corresponds to the state where ferromagnetic tunnel junction element  7  stores data “0”, and the state where the magnetization direction of fixed layer  20  is opposite to the magnetization direction of ferromagnetic layer  25  of recording layer  22 , as shown in  FIG. 3B , corresponds to the state where ferromagnetic tunnel junction element  7  stores data “1”.  
         [0032]     Fixed layer  20  has its magnetization direction fixed by a stacked structure of an antiferromagnetic layer  23  and a ferromagnetic layer  24 , for example. Specifically, antiferromagnetic layer  23  fixes the spin direction of ferromagnetic layer  24  to thereby fix the magnetization direction of ferromagnetic layer  24 . Antiferromagnetic layer  23  is provided beneath ferromagnetic layer  24  (i.e., on the side opposite to recording layer  22 ). For example, CoFe may be used for ferromagnetic layer  24 , and PtMn may be used for antiferromagnetic layer  23 .  
         [0033]     Recording layer  22  is formed of a ferromagnetic layer  25 , a non-magnetic layer  26 , a ferromagnetic layer  27 , a non-magnetic layer  28 , and a ferromagnetic layer  29  stacked in this order from the side of tunneling insulating layer  21 . For ferromagnetic layers  25 ,  27  and  29 , a CoFe layer, for example, may be used. For non-magnetic layers  26  and  28 , a Ru film may be used, for example. The respective ferromagnetic layers are coupled antiparallel to each other via the Ru film. Specifically, ferromagnetic layers  25  and  27 , and ferromagnetic layers  27  and  29  are antiparallel coupled to each other. Here, when the thicknesses of ferromagnetic layers  25 ,  27  and  29  are represented as t 1 , t 2  and t 3 , respectively, t 2 &gt;t 1 +t 3 , and when saturation magnetization of the respective CoFe films is represented as M, the magnetization of the entire recording layer  22  is M●{t 2 −(t 1 +t 3 )}, which magnetization is switched by receiving torque of the external magnetic field.  
         [0034]     A process of providing magnetic anisotropy for determination of an easy axis of recording layer  22  is carried out at the time of formation of ferromagnetic layers  25 ,  27  and  29 , and also carried out upon heat treatment in a post-process. For example, upon formation of ferromagnetic layers  25 ,  27  and  29 , a uniform magnetic field of 100 Oe is applied in the film surface direction. Patterning is performed such that this direction corresponds to the easy axis. After formation of ferromagnetic tunnel junction element  7  as well, the magnetic field is applied in the similar direction to carry out heat treatment. At this time, to simultaneously determine the magnetization direction of fixed layer  20 , the magnetic field that can saturate magnetization of ferromagnetic layer  24  of fixed layer  20  as well as recording layer  22  is applied. For example, 5 kOe is applied and held at 300° C. for one hour.  
         [0035]     As shown in  FIG. 4 , recording layer  22  is in a circle shape in two dimensions. The radius R 1  of recording layer  22  is 50 nm, for example. AlOx, for example, may be used for tunneling insulating layer  21 . Tunneling insulating layer  21  and fixed layer  20  may each have the same shape as recording layer  22 , or they may each have a larger area than recording layer  22  covering the shape of recording layer  22 .  
         [0036]     A writing operation to ferromagnetic tunnel junction element  7  will now be described.  FIG. 5  is a top plan view showing the vicinity of ferromagnetic tunnel junction element  7 . Writing bit line  3  and write line  2  extend in directions orthogonal to each other. Ferromagnetic tunnel junction element  7  is arranged at the crossing point of write line  2  and writing bit line  3  as seen in two dimensions. It is noted that ferromagnetic tunnel junction element  7  is arranged above write line  2  (opposite from the side of semiconductor substrate  10 ) and arranged below writing bit line  3  (on the side of semiconductor substrate  10 ), as shown in  FIG. 2 .  
         [0037]     A ferromagnetic material generally has a direction in which it is easily magnetized (low state of energy) depending on the crystal structure, shape or the like. This direction is called the “easy axis”. By comparison, the direction in which magnetization is difficult is called the “hard axis”. The easy axis and the hard axis of recording layer  22  are set to the extending directions of write line  2  and writing bit line  3 , respectively.  
         [0038]     At the time of writing, a current is flown on each of writing bit line  3  and write line  2 . On writing bit line  3 , the current is flown in the direction shown by an arrow  31 , for example, which causes a magnetic field to be generated in the direction surrounding writing bit line  3 . With this magnetic field, a magnetic field  33  in the easy axis direction is applied to recording layer  22  that is arranged beneath writing bit line  3 . On the other hand, on write line  2 , the current is flown in the direction shown by an arrow  32 , for example, so that a magnetic field is generated in the direction surrounding write line  2 . With this magnetic field, a magnetic field  34  in the hard axis direction is applied to recording layer  22  that is positioned above write line  2 . As such, at the time of writing, a composite magnetic field  35  of magnetic fields  33  and  34  is applied to recording layer  22 .  
         [0039]     Meanwhile, the magnitude of the magnetic field required for switching the direction of magnetization of recording layer  22  becomes an asteroid curve shown by a curve  36 . In the direction of magnetic field  35 , when magnetic field  35  takes a value greater than that of curve  36 , recording layer  22  is magnetized in the direction shown by arrow  32  corresponding to the easy axis direction.  
         [0040]     In the case where fixed layer  20  is magnetized in advance in the same direction as magnetic field  33 , in ferromagnetic tunnel junction element  7 , the magnetization direction of fixed layer  20  and that of ferromagnetic layer  25  of recording layer  22  are parallel to each other (state of  FIG. 3A : “0” is stored). In this case, the resistance value in the thickness direction of ferromagnetic tunnel junction element  7  (in the direction in which recording layer  22  and fixed layer  20  are stacked) is small.  
         [0041]     When fixed layer  20  is magnetized in advance in the opposite direction from magnetic field  33 , in ferromagnetic tunnel junction element  7 , the magnetization direction of fixed layer  20  and that of ferromagnetic layer  25  of recording layer  22  are antiparallel to each other (state of  FIG. 3B : “1” is stored). In this case, the resistance value in the thickness direction of ferromagnetic tunnel junction element  7  is large. This state also occurs in the case where fixed layer  20  is magnetized in advance in the same direction as magnetic field  33  in the figure and a current is flown on writing bit line  3  in the direction opposite to the direction shown by arrow  31 .  
         [0042]     A reading operation will now be described. At the time of reading, a prescribed word line  4  is selected and driven to cause element selecting transistor  6  connected to the relevant word line  4  to attain an on state. Further, a current is flown through a prescribed writing bit line  3  to cause a tunneling current to be flown on ferromagnetic tunnel junction element  7  connected to element selecting transistor  6  of the on state. Stored data is determined based on the resistance value of ferromagnetic tunnel junction element  7  at this time. More specifically, ferromagnetic tunnel junction element  7  has a small resistance value when the magnetization direction is parallel, while it has a large resistance value when the magnetization direction is antiparallel. Sense amplifier  5  utilizes such properties to determine whether the output signal of selected memory cell MC is greater or smaller with respect to the output signal of a reference cell (not shown). In this manner, it is determined whether the stored data in selected memory cell MC is “0” or “1”.  
         [0043]     As shown in  FIG. 4 , recording layer  22  does not have shape anisotropy, and thus, there is no increase in magnetization switching field even if recording layer  22  is miniaturized. When the present configuration is used, the thickest ferromagnetic layer  27  is coupled in the vertical direction in the antiparallel manner, and thus, the magnetization distribution becomes uniform within the plane of recording layer  22 , so that stable magnetic characteristics can be obtained.  FIG. 6C  shows the magnetization distribution in recording layer  22  obtained with the present configuration, which is compared with the magnetization distribution of the conventional structure shown in  FIGS. 6A and 6B . Here, the magnetization distribution in ferromagnetic layer  25  in contact with tunneling insulating film  21  is shown. In the state of  FIGS. 6A and 6B , the magnetization is closed within the plane of the film, which is unlikely to receive torque from the external magnetic field, leading to an increased switching field. In the state of  FIG. 6A , magnetization of the entire recording layer is 0, so that it is not possible to obtain a magnetoresistance change rate. In the first embodiment, the magnetization distribution shown in  FIG. 6C  is obtained, and there is no increase in magnetization switching field due to the above-described reasons.  
         [0044]     Although ferromagnetic layers  25 ,  27  and  29  are formed of a CoFe film in the present embodiment, all that is needed is that ferromagnetic layers  25 ,  27  and  29  are made of a film having a Co or Fe element as its main component, like CoFeB. Further, non-magnetic layers  26  and  28  are not restricted to the Ru film, but may be Cu, Ta or other metal film.  
         [0045]     Furthermore, recording layer  22  does not necessarily have to be of a circular shape. It may have a square shape with truncated comers, as shown in  FIG. 7 . In FIG.  7 , the contour of recording layer  22  is formed with four straight-line parts  22   a  and four arcs  22   b  that constitute a closed curve. Here, straight-line part  22   a  may have a length of 50 nm, for example, and arc  22   b  may have a radius R 2  of 50 nm, for example. The stacked structure of recording layer  22  is as shown in  FIGS. 3A and 3B .  
         [0046]      FIGS. 8A and 8B  show cross sectional views of a modification of the present embodiment, which figures are in contrast with  FIGS. 3A and 3B . In  FIGS. 8A and 8B , in ferromagnetic tunnel junction element  7 , thicknesses t 1 , t 2  and t 3  of respective ferromagnetic layers  25 ,  27  and  29  satisfy t 1 +t 3 &gt;t 2 . When the saturation magnetization of each of the CoFe films is represented as M, magnetization of the entire recording layer  22  is M●{(t 1 +t 3 )−t 2 ). This magnetization is switched by receiving the torque from the external magnetic field. The other configurations and effects are similar to those of the above-described embodiment, and thus, description thereof will not be repeated.  
         [0047]     In the present embodiment, the magnetic field generated by the interconnection current is used as means for switching the magnetization. Alternatively, spin-polarized electrons may be introduced into recording layer  22  over tunneling insulating film  21  to switch the magnetization by the torque, in which case similar effects can be obtained as well.  
         [0048]     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.