Abstract:
A magnetic random access memory (MRAM) device may include a substrate, a first magnetic layer on the substrate, and a digit line on the first magnetic layer. A magnetic tunnel junction structure may be provided adjacent the digit line, and a bit line may be provided on the magnetic tunnel junction structure such that the magnetic tunnel junction structure is between the bit line and the digit line. In addition, a second magnetic layer may be provided on the bit line.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of priority as a divisional of U.S. application Ser. No. 11/092,362 filed Mar. 29, 2005, which claims the benefit of and priority to Korean Patent Application No. 2004-0054939, filed Jul. 14, 2004. The disclosures of both of the above-referenced applications are incorporated herein in their entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of semiconductor memory devices and, more particularly, to magnetic random access memory devices and methods. 
       BACKGROUND 
       [0003]    A magnetic random access memory (MRAM) is a non-volatile memory device that uses a magnetoresistance effect such that resistance of an electric conductor is changed in response to a circumferential magnetic field to thereby store information. An MRAM may include a plurality of MRAM cells with each MRAM cell including a magnetic tunnel junction (MTJ) on a single transistor. 
         [0004]    An MTJ may include multiple thin layers such that electrons may tunnel through a very thin insulating layer sandwiched between two ferromagnetic electrodes when an external electrical signal is applied. A top electrode may be referred to as a free layer and a bottom electrode may be referred to as a pinned layer. 
         [0005]    When magnetization directions within the free layer and the pinned layer are arranged parallel with respect to each other, a tunneling current flowing through the MTJ may have a relatively high value, and, the tunneling resistance may have a relatively low value. In contrast, when the magnetization directions within the free layer and the pinned layer are arranged anti-parallel with respect to each other, a tunneling current flowing through the MTJ may have a reduced value, and a tunneling resistance may have a relatively high value. 
         [0006]    An MRAM may use magnetic charges to store information while a conventional memory may use electrical charges. Digital data represented as “0” and “1” may thus be differentiated to store a low resistance state where the magnetization directions of the two electrodes are parallel with respect to each other and a high resistance state where the magnetization directions are anti-parallel with respect to each other. 
         [0007]    An anti-ferromagnetic layer which may be referred to as a pinning layer may be added to the pinned layer. The pinning layer may serve to fix the magnetization direction of the pinned layer. More particularly, the pinned layer attached to the pinning layer may have a relatively large switching field, and the magnetization direction of the pinned layer may be fixed always in a same direction when an applied magnetic field is smaller than the switching field. Thus, data of the MRAM cell may be determined by the magnetization direction within the free layer. The magnetization direction of the free layer may be changed by applying a magnetic field to its circumference. To change the magnetization direction of the free layer to a desired direction, conductive layers such as a bit line and a digit line may be formed to be orthogonal with respect to each other above/below the MTJ, and a current may flow through each conductive layer. A magnetic field induced by the current may be used to change the magnetization direction. 
         [0008]    In this case, when the magnetization direction of an MTJ selected to store data is changed, a magnetization direction of an MTJ which is not selected should not be changed. To enhance integration density within a limited space, however, not only the size of each MTJ but also spaces between MTJs should be reduced. As a result of reductions in space between MTJs, a magnetic field which is generated to change a magnetization direction of a selected MTJ may increasingly affect adjacent MTJs which are not selected. If this effect becomes sufficient to invert a magnetization direction of adjacent MTJs, it may not be possible to perform a normal data storage operation. 
         [0009]    To cope with the above-mentioned problems, alternate writing methods referred to as toggle switching have been proposed. A writing method for MRAMs using toggle switching is disclosed in U.S. Pat. No. 6,545,906 entitled “Method of writing to scalable magnetoresistance random access memory element “to Savtchenko et al. The disclosure of U.S. Pat. No. 6,545,906 is hereby incorporated herein in its entirety by reference. 
         [0010]    According to U.S. Pat. No. 6,545,906, a digit line may be provided above a predetermined region of a semiconductor substrate. A bit line may be disposed on the digit line and may cross the digit line. An MTJ may be interposed at an intersection of the digit line and the bit line. The MTJ may include a second magnetic region, a tunneling barrier, and a first magnetic region which are sequentially stacked. Each of the first and second magnetic regions may include a synthetic anti-ferromagnetic (SAF) structure. The SAF structure may be formed of a top ferromagnetic layer, a bottom ferromagnetic layer, and an anti-ferromagnetic coupling spacer layer interposed therebetween. 
         [0011]      FIG. 1  is a switching characteristic diagram of an MRAM cell fabricated as discussed in with U.S. Pat. No. 6,545,906. 
         [0012]    Referring to  FIG. 1 , the horizontal axis indicates a magnetic field (Hw) induced on the bit line measured in units of Oersted (Oe) which is a unit of magnetoresistance and which is proportional to the magnitude of the bit line current. The longitudinal axis indicates a magnetic field (Hw) induced on the digit line also measured in units of Oersted (Oe). There are three regions in the switching characteristic diagram, i.e., a no switching region  92 , a direct switching region  95 , and a toggle switching region  97 . 
         [0013]    A method of writing using toggle switching region  97  may begin with reading an initial state of the MTJ. For example, in the case that the initial state of the MTJ is read as “1”, when a bit line positive current pulse is applied to the bit line and a digit line positive current pulse is applied to the digit line, a magnetization direction of the MTJ may be changed to write a “0” state. Next, when the bit line positive current pulse is applied again to the bit line and the digit line positive current pulse is applied again to the digit line, the magnetization direction of the MTJ may also be changed to write a “1” state. In this case, the bit line positive current pulse and the digit line positive current pulse may be applied with a time delay, but, these pluses may include a sequence which has an overlapped region therebetween. In addition, the magnitude of the current pulse may correspond to the toggle switching region  97 . 
         [0014]    The toggle switching region  97  may have a large switching region as shown in  FIG. 1 , but it may require a relatively high switching current. The relatively high switching current may in turn cause an undesirable increase in power consumption. 
       SUMMARY 
       [0015]    According to embodiments of the present invention, a magnetic random access memory (MRAM) device may include a substrate, a first magnetic layer on the substrate, a digit line on the first magnetic layer, and a magnetic tunnel junction structure adjacent the digit line. A bit line may be provided on the magnetic tunnel junction structure such that the magnetic tunnel junction structure is between the bit line and the digit line. A second magnetic layer may be provided on the bit line. 
         [0016]    More particularly, the magnetic tunnel junction structure may include a synthetic anti-ferromagnetic (SAF) free layer, and the synthetic anti-ferromagnetic (SAF) free layer may include an exchange spacer layer between first and second free layers such that the first free layer is between the exchange spacer layer and the digit line. The first and second free layers may include a ferromagnetic material, and the exchange spacer layer may include ruthenium. The magnetic tunnel junction structure may include a tunneling insulating layer between the synthetic anti-ferromagnetic free layer and the digit line, a pinned layer between the tunneling insulating layer and the digit line, and a pinning layer between the pinned layer and the digit line. Moreover, the pinned layer may include an exchange spacer layer between first and second pinned sub-layers such that the first pinned sub-layer is between the exchange spacer layer and the pinning layer and the second pinned sub-layer is between the exchange spacer layer and the tunneling insulating layer. Each of the first and second pinned sub-layers may include a ferromagnetic material. 
         [0017]    In addition, portions of the first magnetic layer may be between the digit line and the substrate and portions of the first magnetic layer may extend on sidewalls of the digit line. The bit line may be between portions of the second magnetic layer and the substrate, and portions of the second magnetic layer may extend on sidewalls of the bit line. Moreover, at least one of the first magnetic layer and/or the second magnetic layer may be a permanent magnet, and/or at least one of the first magnetic layer and/or the second magnetic layer may include at least one of Co and/or CoFe. In an alternative, at least one of the first magnetic layer and/or the second magnetic layer may include an electromagnet. In addition, a longest dimension of the magnetic tunnel junction structure parallel with respect to a surface of the substrate may be arranged non-parallel with respect to the digit line and the bit line. 
         [0018]    The magnetic random access memory device may also include an insulating passivation layer and at least two magnets. More particularly, the insulating passivation layer may be provided on the bit line such that the bit line is between the insulating passivation layer and the substrate, and the at least two magnets may be provided on the insulating passivation layer such that the insulating passivation layer is between the at least two magnets and the substrate. Moreover, each of the at least two magnets may include a permanent magnet and/or an electromagnet. 
         [0019]    According to additional embodiments of the present invention, a magnetic random access memory (MRAM) device may include a substrate, a plurality of digit lines on the substrate, and a plurality of bit lines crossing the plurality of digit lines on the substrate. Each of a plurality of magnetic tunnel junction structures may be provided at a junction of respective digit and bit lines and between the respective digit and bit lines. An insulating passivation layer may be provided on the plurality of digit lines, on the plurality of bit lines and on the plurality of magnetic tunnel junction structures such that the plurality of digit lines, the plurality of bit lines and the plurality of magnetic tunnel junction structures are between the insulating passivation layer and the substrate. Moreover, at least two magnets may be provided on the insulating passivation layer such that the insulating passivation layer is between the at least two magnets and the substrate. 
         [0020]    Each of the magnetic tunnel junction structures may include a respective synthetic anti-ferromagnetic free layer. Each of the synthetic anti-ferromagnetic free layers may include an exchange spacer layer between first and second free layers such that the first free layer is between the exchange spacer layer and the substrate and the second free layer is between the exchange spacer layer and the insulating passivation layer. More particularly, each of the first and second free layers may include a ferromagnetic material, and the exchange spacer layer may include ruthenium. 
         [0021]    Each of the magnetic tunnel junction structures may include a tunneling insulating layer between the synthetic anti-ferroelectric free layer and the substrate, a pinned layer between the tunneling insulating layer and the substrate, and a pinning layer between the pinned layer and the substrate. Moreover, the pined layer may include an exchange spacer layer between first and second pinned sub-layers such that the first pinned sub-layer is between the exchange spacer layer and the pinning layer and the second pinned sub-layer is between the exchange spacer layer and the tunneling insulating layer. Each of the first and second pinned sub-layers may include a ferroelectric material. 
         [0022]    In addition, a first plurality of cladding layers may be provided with portions of each cladding layer being provided between a respective digit line and the substrate. A second plurality of cladding layers may be provided between respective bit lines and the insulating passivation layer. Moreover, each of the cladding layers may include a ferroelectric material. 
         [0023]    A longest dimension of each of the magnetic tunnel junction structures parallel with respect to a surface of the substrate may be arranged non-parallel with respect to the respective digit and bit lines. Each of the at least two magnets may include a permanent magnet and/or an electromagnet, and first and second ones of the at least two magnets may be orthogonal with respect to each other. In addition, the at least two magnets may includes a plurality of pairs of magnets. 
         [0024]    Embodiments of the invention may provide relatively stable operating characteristics for MRAMs at relatively low switching currents. In one embodiment of the invention, an MRAM may include a digit line disposed above a predetermined region of a semiconductor substrate. A bottom surface and sidewalls of the digit line may be surrounded by a first magnetic layer pattern. A bit line may be disposed to cross over the digit line, and a top surface and sidewalls of the bit line may be surrounded by a second magnetic layer pattern. An MTJ may be interposed between the bit line and the digit line. The MTJ may be electrically connected to the bit line and may be insulated from the digit line. The MTJ may have a synthetic anti-ferromagnetic (SAF) free layer. 
         [0025]    Each of the first and second magnetic layer patterns may be formed of a permanent magnet and/or an electromagnet. The permanent magnet may be formed of at least one material selected from a group consisting of Co, CoFe, and/or NiFe. A magnetic field generated by the current flowing through the digit line and the bit line may be synthesized with that generated in the first and second magnetic layer patterns, thereby producing a larger synthesized magnetic field. As a result, a relatively large magnetic field may be generated with a relatively small current. 
         [0026]    The SAF free layer may include a bottom free layer and a top free layer which are separated by a second exchange spacer layer. Each of the bottom and top free layers may be formed of a ferromagnetic layer. The second exchange spacer layer may be a ruthenium (Ru) layer. The SAF free layer may have a characteristic that the magnetization directions of the bottom and top free layers are arranged to be anti-parallel with respect to each other due to the effect of the second exchange spacer layer. In addition, the magnetization direction of the SAF free layer may be changed by the magnetic field which is generated in the digit line and the bit line. 
         [0027]    In another embodiment of the invention, an MRAM may include a digit line disposed above a predetermined region of a semiconductor substrate. A bit line may be disposed to cross over the digit line. An MTJ may be interposed between the bit line and the digit line. The MTJ may be electrically connected to the bit line and may be insulated from the digit line, and the MTJ may have a SAF free layer. A passivation insulating layer may be provided to cover the bit line. At least one pair of external magnets may be disposed on the semiconductor substrate covered by the passivation insulating layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0028]      FIG. 1  is a switching characteristic diagram of a conventional MRAM cell. 
           [0029]      FIG. 2  is a plan view illustrating MRAM cells according to embodiments of the present invention. 
           [0030]      FIG. 3  is a cross-sectional view taken along the line I-I′ of  FIG. 2 . 
           [0031]      FIG. 4  is a cross-sectional view taken along the line II-II′ of  FIG. 2 . 
           [0032]      FIG. 5  is a switching characteristic diagram of an MRAM cell according to embodiments of the present invention. 
           [0033]      FIG. 6  is a plan view illustrating an MRAM according to other embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0034]    Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
         [0035]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0036]    It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
         [0037]    Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “lateral” or “vertical” may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. 
         [0038]    Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention. 
         [0039]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0040]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0041]      FIG. 2  is a plan view illustrating MRAM cells according to embodiments of the present invention,  FIG. 3  is a cross-sectional view taken along the line I-I′ of  FIG. 2 , and  FIG. 4  is a cross-sectional view taken along the line II-II′ of  FIG. 2 . 
         [0042]    Referring to  FIGS. 2 ,  3  and  4 , a first inter layer dielectric layer  12  may be disposed on a predetermined region of a semiconductor substrate  11  including lower circuits (not shown). A digit line  22  may be disposed within the first inter layer dielectric layer  12 . A bottom surface and sidewalls of the digit line  22  may be surrounded by a first magnetic layer pattern  21 . The digit line  22  may be a conductive layer such as a copper layer and/or an aluminum layer. The first magnetic layer pattern  21  may be formed of a permanent magnet or an electromagnet. The permanent magnet may be formed of at least one material selected from a group consisting of Co, CoFe, and/or NiFe. A magnetic field generated by current flowing through the digit line  22  may be synthesized with that generated in the first magnetic layer pattern  21 , thereby producing a larger synthesized magnetic field. As a result, a relatively large magnetic field may be generated with a relatively small current. 
         [0043]    An entire surface of the semiconductor substrate including the digit line  22  may be covered by a second inter layer dielectric layer  13 . A bottom electrode  31  may be disposed on the second inter layer dielectric layer  13  to pass over the digit line  22 . The bottom electrode  31  may be electrically connected to a lower circuit through a contact hole (not shown) which penetrates the second inter layer dielectric layer  13 . 
         [0044]    An entire surface of the semiconductor substrate including the bottom electrode  31  may be covered by a third inter layer dielectric layer  14 . A bit line  52  may be disposed on the third inter layer dielectric layer  14  to cross the digit line  22 . A top surface and sidewalls of the bit line  52  may be surrounded by a second magnetic layer pattern  51 . Both sides of the bit line  52  and the second magnetic layer pattern  51  may be surrounded by a top inter layer dielectric layer  15 . The bit line  52  may be a conductive layer such as a copper layer or an aluminum layer. The second magnetic layer pattern  51  may be formed of a permanent magnet or an electromagnet. A permanent magnet may be formed of at least one material selected from a group consisting of Co, CoFe, and/or NiFe. A magnetic field generated by current flowing through the bit line  52  may be synthesized with that generated in the second magnetic layer pattern  51 , thereby producing a larger synthesized magnetic field. As a result, a relatively large magnetic field may be generated with a relatively small current. 
         [0045]    The bit line  52  and the bottom electrode  31  may be electrically connected by a MTJ  50  which penetrates the third inter layer dielectric layer  14 . That is, the MTJ  50  is electrically connected between the bit line  52  and the bottom electrode  31 . The MTJ  50  may include a pinning layer  32 , a pinned layer  37 , a tunneling insulating layer  39 , and a synthetic anti-ferromagnetic (SAF) free layer  47  which are sequentially stacked. 
         [0046]    The pinning layer  32  is an anti-ferromagnetic layer, and the pinned layer  37  is a ferromagnetic layer. As described above, the pinning layer  32  serves to fix the magnetization direction of the pinned layer  37 . That is, the pinned layer  37  attached to the pinning layer  32  may have a relatively large switching field, and the magnetization direction of the pinned layer  37  may be fixed always in the same direction when an applied magnetic field is smaller than the switching field. The pinned layer  37  may have a structure composed of one ferromagnetic layer, or a SAF structure composed of three layers. An SAF structure may include a bottom pinned layer  34  and a top pinned layer  36  which are separated by a first exchange spacer layer  35 . Each of the bottom pinned layer  34  and the top pinned layer  36  may be formed of a ferromagnetic layer. The first exchange spacer layer  35  may be a ruthenium (Ru) layer. An SAF structure has a characteristic that magnetization directions of the bottom and top pinned layers  34  and  36  are arranged to be anti-parallel to each other due to the effect of the first exchange spacer layer  35 . That is, when an initial magnetization direction is applied to the bottom pinned layer  34  attached to the pinning layer  32 , the magnetization direction of the top pinned layer  36  is arranged in an anti-parallel direction. 
         [0047]    The tunneling insulating layer  39  may be an insulating layer such as an aluminum oxide layer. A read current for the MRAM cell flows through the tunneling insulating layer  39 . An amount of the read current flowing through the tunneling insulating layer  39  may have a relatively high correlation with respect to a thickness of the tunneling insulating layer  39 . Thus, the tunneling insulating layer  39  may have a relatively uniform thickness. 
         [0048]    The SAF free layer  47  may include a bottom free layer  44  and a top free layer  46  which are separated by a second exchange spacer layer  45 . Each of the bottom free layer  44  and the top free layer  46  may be formed of a ferromagnetic layer. The second exchange spacer layer  45  may be a Ru layer. The SAF free layer  47  may have a characteristic that magnetization directions of the bottom and top free layers  44  and  46  are arranged to be anti-parallel with respect to each other due to an effect of the second exchange spacer layer  45 . In addition, a magnetization direction of the SAF free layer  47  may be changed by magnetic fields generated in the digit line  22  and the bit line  52 . 
         [0049]    Each of the ferromagnetic layers  34 ,  36 ,  44 , and  46  may be formed of at least one material selected from a group consisting of Cobalt (Co), iron (Fe), and/or nickel (Ni), or may be formed of an alloy including at least two materials selected from the group. 
         [0050]    The MTJ  50  may have an elliptical shape, a rectangular shape, a U-shape, a cross shape, or a shield shape when viewed in a plan view. In addition, the MTJ  50  may be disposed at an intersection of the digit line  22  and the bit line  52 , may be disposed parallel to the digit line  22 , may be disposed parallel to the bit line  52 , or may be disposed to have an angle of intersection with the digit line  22  and the bit line  52 . Alternatively, the MTJ  50  may be disposed to have an intersection angle of 45° with the digit line  22  and the bit line  52 . 
         [0051]      FIG. 5  is a switching characteristic diagram of an MRAM cell according to embodiments of the present invention. Referring to  FIG. 5 , the horizontal axis indicates a magnetic field (Hw) generated by the bit line  52 , and the magnetic field is measured in units of Oersted (Oe) which is a unit of magnetoresistance. The magnetic field generated by the bit line is proportional to the magnitude of the bit line current. The longitudinal axis indicates a magnetic field (Hw) generated by the digit line  22 , and the magnetic field is measured in units of Oersted (Oe) which is a unit of magnetoresistance. The magnetic field generated by the bit line is proportional to the magnitude of the digit line current. 
         [0052]    There are three types of regions in the switching characteristic diagram, i.e., no switching regions  54 , direct switching regions  55 , and toggle switching regions  56 . The toggle switching regions  56  are present in the first quadrant and the third quadrant of the switching characteristic diagram. The direct switching regions  55  are also present in the first quadrant and the third quadrant. Regions of the first and third quadrants other than the toggle switching regions  56  and the direct switching regions  55  are no switching regions  54 . The second and fourth quadrants have only the no switching regions  54 . The switching region in the first quadrant is a region where the MTJ  50  is switched in response to the magnetic field induced when a positive current pulse is applied to each of the bit line  52  and the digit line  22 . The switching region in the third quadrant is a region where the MTJ  50  is switched in response to the magnetic field induced when a negative current pulse is applied to each of the bit line  52  and the digit line  22 . 
         [0053]    Switching methods according to embodiments of the present invention may use the toggle switching regions  56 . Writing methods using the toggle switching region  56  may begin with reading an initial state of the MTJ  50 . For example, in the case that the initial state of the MTJ  50  is read as “1”, when a bit line positive current pulse is applied to the bit line  52  and a digit line positive current pulse is applied to the digit line  22 , the magnetization direction of the MTJ  50  may be changed to write a “ 0 ” state. Next, when the bit line positive current pulse is applied again to the bit line  52  and the digit line positive current pulse is applied again to the digit line  22 , the magnetization direction of the MTJ is also changed to write a “1” state. In this case, the bit line positive current pulse and the digit line positive current pulse may be applied with a time delay, however, these pluses may be provided as a sequence which has an overlapped region therebetween. In addition, magnitudes of the current pulses may correspond to the toggle switching region  56 . 
         [0054]    Due to an effect of the magnetic field generated in the first and second magnetic layer patterns  21  and  51  which respectively surround the digit line  22  and the bit line  52 , an origin of the switching characteristic diagram may be moved parallel to a point (W, D) in the first quadrant. Consequently, the digit line positive current pulse and the bit line positive current pulse may have a relatively small magnitude. 
         [0055]    Alternatively, direct switching regions  55  may be used to change the magnetization direction of the MTJ  50 . However, the direct switching regions  55  may have relatively narrow switching regions as shown in  FIG. 5 , and both of the positive current pulse and the negative current pulse may need to be used. Writing methods using the direct switching regions  55  may begin with reading an initial state of the MTJ  50 . For example, in the case that the initial state of the MTJ  50  is read as “1”, when the bit line positive current pulse is applied to the bit line  52  and the digit line positive current pulse is applied to the digit line  22 , the magnetization direction of the MTJ  50  is changed to write a “0” state. Next, in order to change the state from “0” to “1, ” the bit line negative current pulse should be applied to the bit line  52  and the digit line negative current pulse should be applied to the digit line  22 . In this case, the bit line negative current pulse and the digit line negative current pulse to be applied are present in the third quadrant as shown in  FIG. 5 , so that they may require a relatively high current. The relatively high current in turn may cause an increase in power consumption. 
         [0056]      FIG. 6  is a plan view illustrating an MRAM according to other embodiments of the present invention. Referring to  FIG. 6 , digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  are disposed above a predetermined region of a semiconductor substrate including lower circuits (not shown). Bottom surfaces and sidewalls of the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  may be surrounded by respective first cladding patterns. Each of the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  may be a conductive layer such as a copper layer and/or an aluminum layer. Each of the first cladding patterns may be a ferromagnetic layer such as NiFe. The first cladding patterns may serve to focus a magnetic flux generated by current flowing through the respective digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6 . 
         [0057]    An entire surface of the semiconductor substrate having the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  may be covered by a first inter layer dielectric layer. Bottom electrodes may be disposed on the first inter layer dielectric layer to pass over the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6 . The bottom electrodes may be electrically connected to the lower circuits through contact holes which penetrate the first inter layer dielectric layer. 
         [0058]    The entire surface of the semiconductor substrate including the bottom electrodes may be covered by a second inter layer dielectric layer. Bit lines B 1 , B 2 , B 3 , and B 4  are disposed on the second inter layer dielectric layer to cross over the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6 . Top surfaces and sidewalls of the bit lines B 1 , B 2 , B 3 , and B 4  may be surrounded by second cladding patterns. Each of the bit lines B 1 , B 2 , B 3 , and B 4  may be a conductive layer such as a copper layer and/or an aluminum layer. The second cladding patterns may be ferromagnetic layers such as NiFe. The second cladding patterns may serve to focus a magnetic flux generated by current flowing through the bit lines B 1 , B 2 , B 3 , and B 4 . 
         [0059]    The bit lines B 1 , B 2 , B 3 , and B 4  and the bottom electrodes may be connected through respective MTJs  50  which penetrate the second inter layer dielectric layer. Each MTJ  50  includes a pinning layer, a pinned layer, a tunneling insulating layer, and a synthetic anti-ferromagnetic (SAF) free layer which are sequentially stacked. 
         [0060]    The pinning layer may be formed of an anti-ferromagnetic layer, and the pinned layer may be formed of a ferromagnetic layer. As described above, the pinning layer may fix the magnetization direction of the pinned layer. That is, the pinned layer attached to the pinning layer may have a relatively large switching field, and a magnetization direction of the pinned layer may always be fixed in a same direction when an applied magnetic field is smaller than the switching field. The pinned layer may have a structure including only one ferromagnetic layer, or a SAF structure including three layers. The SAF structure may include a bottom pinned layer and a top pinned layer which are separated by a first exchange spacer layer. Each of the bottom pinned layer and the top pinned layer may be formed of a ferromagnetic material. The first exchange spacer layer may be a Ru layer. The SAF structure may have a characteristic that the magnetization directions of the bottom and top pinned layers are arranged to be anti-parallel with respect to each other due to the effect of the first exchange spacer layer. That is, when an initial magnetization direction is applied to the bottom pinned layer attached to the pinning layer, the magnetization direction of the top pinned layer may be arranged in an anti-parallel direction. The tunneling insulating layer may be an insulating layer such as an aluminum oxide layer. 
         [0061]    The SAF free layer may include a bottom free layer and a top free layer which are separated by a second exchange spacer layer. Each of the bottom and top free layers may be formed of a ferromagnetic layer. The second exchange spacer layer may be a Ru layer. The SAF free layer may have a characteristic that the magnetization directions of the bottom and top free layers are arranged to be anti-parallel with respect to each other due to the effect of the second exchange spacer layer. The ferromagnetic layers may be formed of at least one material selected from a group consisting of Co, Fe, and/or Ni, and/or may be formed of an alloy including at least two materials selected from the group. 
         [0062]    In this case, the MTJ  50  may have an elliptical shape, a rectangular shape, a U-shape, a cross shape, or a shield shape when viewed in a plan view. In addition, the MTJs  50  may be disposed at respective intersections of the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  and the bit lines B 1 , B 2 , B 3 , and B 4 . Moreover, MTJs  50  may be disposed parallel with respect to the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6 ; MTJs  50  may be disposed parallel with respect to the bit lines B 1 , B 2 , B 3 , and B 4 ; or MTJs  50  may be disposed to have angles of intersection with respect to the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  and the bit lines B 1 , B 2 , B 3 , and B 4 . In addition, the MTJs  50  may be disposed to have respective intersection angles of 45° with respect to the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  and the bit lines B 1 , B 2 , B 3 , and B 4 . Consequently, an MRAM in accordance with other embodiments of the present invention may include a plurality of MTJs  50  which are interposed between respective digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  and bit lines B 1 , B 2 , B 3 , and B 4 . 
         [0063]    An entire surface of the semiconductor substrate including the MTJs  50 , the digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6 , and the bit lines B 1 , B 2 , B 3 , and B 4  may be covered by a passivation insulating layer. 
         [0064]    At least one pair of external magnets N 1  and S 1  may be disposed on the semiconductor substrate covered by the passivation insulating layer. In addition, two pairs of external magnets N 1 , S 1 , and N 2 , S 2  may be disposed to be orthogonal with respect to each other as shown in  FIG. 6 . Alternatively, a plurality of pairs of the external magnets may be repeatedly disposed with predetermined intervals, respectively. Each external magnet may be a permanent magnet or an electromagnet. The external magnet(s) may move in parallel an origin of a switching characteristic diagram to a point near a toggle switching region as described with reference to  FIG. 5 . Consequently, magnetization directions of the MTJs  50  may be changed with only a relatively small current applied to the respective digit lines D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  and bit lines B 1 , B 2 , B 3 , and B 4 . 
         [0065]    In MRAMs according to yet other embodiments of the present invention, a digit line may be disposed above a predetermined region of a semiconductor substrate having lower circuits (not shown) as described with reference to  FIGS. 2 ,  3 , and  4 . A bottom surface and sidewalls of the digit line may be surrounded by a first magnetic layer pattern. The first magnetic layer pattern may be formed of a permanent magnet or an electromagnet. An entire surface of the semiconductor substrate having the digit line may be covered by an inter layer dielectric layer. A bottom electrode may be disposed on the inter layer dielectric layer to pass over the digit line. An entire surface of the semiconductor substrate including the bottom electrode may be covered by a second inter layer dielectric layer. A bit line may be disposed on the second inter layer dielectric layer to cross over the digit line. A top surface and sidewalls of the bit line may be surrounded by a second magnetic layer pattern. The second magnetic layer pattern may be formed of a permanent magnet or an electromagnet. A bit line and a bottom electrode may be connected to each other through a MTJ which penetrates the second inter layer dielectrics. Each MTJ may include a pinning layer, a pinned layer, a tunneling insulating layer, and a SAF free layer which are sequentially stacked. 
         [0066]    MRAMs according to yet other embodiments of the present invention may include a plurality of MTJs interposed between respective digit and bit lines. An entire surface of the semiconductor substrate including the MTJs, the digit lines, and the bit lines may be covered by an insulating passivation layer. 
         [0067]    At least one pair of external magnets may be disposed on the semiconductor substrate covered by the insulating passivation layer. In addition, two pairs of external magnets may be disposed to be orthogonal with respect to each other as shown in  FIG. 6 . Alternatively, a plurality of pairs of the external magnets may be repeatedly disposed with predetermined intervals, respectively. The external magnet may be a permanent magnet or an electromagnet. By means of the first and second magnetic layer patterns and the external magnet, an origin of a switching characteristic diagram may be moved parallel to a point relatively near the toggle switching region. As a result, magnetization directions of the MTJs may be changed with a relatively small current applied to the digit lines and the bit lines, 
         [0068]    According to embodiments of the present invention discussed above, a bottom surface and sidewalls of a digit line may be surrounded by a first magnetic layer pattern, and a top surface and sidewalls of a bit line may be surrounded by a second magnetic layer pattern. A magnetic field generated by current flowing through the digit line may be synthesized with that generated by the first magnetic layer pattern, thereby producing a larger synthesized magnetic field. Similarly, a magnetic field generated by current flowing through the bit line may also be synthesized with that generated by the second magnetic layer pattern, thereby producing a larger synthesized magnetic field. In addition, an external magnet may be added above the bit line. Moreover, a magnetization direction of the MTJ having a SAF free layer may be changed using a toggle switching region. In this case, the origin of the switching characteristic diagram may be moved parallel to a point near the toggle switching region by means of the first and second magnetic layer patterns and the external magnet. As a result, magnetization directions of the MTJs may be changed with relatively small currents applied to the digit line and the bit line. 
         [0069]    In the drawings and specification, there have been disclosed typical embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.