Patent Publication Number: US-6909630-B2

Title: MRAM memories utilizing magnetic write lines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is claiming under 35 USC 119(e) the benefit of provisional patent application Ser. No. 60/431,742 filed on Dec. 9, 2002. 
   The present application is related to co-pending U.S. patent application Ser. No. 60/444,881 (2817P), entitled HIGH DENSITY AND HIGH PROGRAMMING EFFICIENCY MRAM DESIGN, filed on Feb. 5, 2003, and assigned to the assignee of the present application. The present application is related to co-pending U.S. patent application Ser. No. 10/606,557(2818P), entitled MRAM ARCHITECTURE AND A METHOD AND SYSTEM FOR FABRICATING MRAM MEMORIES UTILIZING THE ARCHITECTURE, filed on Jun. 26, 2003, and assigned to the assignee of the present application. The present application is related to co-pending U.S. patent application Ser. No. 10/646,455(2780P), entitled MRAM ARRAY WITH MAGNETIC WRITE LINES, filed on Aug. 21, 2003, and assigned to the assignee of the present application. 

   FIELD OF THE INVENTION 
   The present invention pertains to magnetic memories, and more particularly to a method and system for providing a magnetic random access memory (MRAM) that is preferably high density, nonvolatile and that incorporates write-lines having improved writing efficiencies, ease of manufacturing, and better reliability against electromigration. 
   BACKGROUND OF THE INVENTION 
   DRAM, FLASH, and SRAM are the three major semiconductor memories on the market. Although the manufacturing cost of DRAM is the lowest, DRAM has several shortcomings. DRAM is volatile, and, therefore, loses data when the power is turned off. Furthermore, DRAM needs refreshment, has a relatively low speed and has a high power consumption. FLASH memory offers non-volatility, but its speed is very low. In addition, the write cycle endurance of FLASH memories is typically less than 1000,000 cycles. These drawbacks limit the application of FLASH memories in some high data rate markets. SRAM is a fast memory, but is volatile and takes a relatively large amount of silicon area per cell. In search of a universal random access memory that offers high speed, non-volatility, small cell area, and good endurance, thin film Magnetic Random Access Memories (MRAM) have been developed. 
   Conventional thin film Magnetic Random Access Memories can be fabricated with a variety of conventional memory cell types, including an Anisotropic Magnetoresistance (AMR) cell, a Giant Magnetoresistance (GMR) cell, and a Magnetic Tunneling Junction (MTJ) cell. Because the conventional MTJ cell is the easiest to manufacture and use, it will be used as the primary example throughout this disclosure. However, one of ordinary skill in the art will readily understand that these concepts also apply to other MRAM cells and arrays. The conventional MTJ cell essentially includes an MTJ stack. The MTH stack includes a pair of magnetic layers with an insulating layer sandwiched there between. One of the magnetic layers, the pinned layer, has a fixed magnetic vector (fixed magnetization). The other magnetic layer (free layer) has a changeable magnetic vector (changeable magnetization) that is stable either aligned parallel to or substantially antiparallel to the fixed magnetic vector in the pinned layer. When the magnetic vectors are aligned, the resistance of the conventional MTJ stack and thus the conventional MTJ cell, i.e. the resistance to current flow between the magnetic layers, is a minimum. When the magnetic vectors are opposed or misaligned, the resistance of the conventional MTJ cell is a maximum. 
   Data is stored in the conventional MTJ cell by applying a magnetic field to the conventional MTJ cell. The applied magnetic field has a direction chosen to move the changeable magnetic vector of the free layer to a selected orientation. Stated differently, the conventional MTJ cell is typically written by applying a magnetic field that can alter the direction of the magnetic vector of the free layer. Generally, the aligned orientation can be designated a logic 1 or 0, while the misaligned orientation is the opposite, i.e., a logic 0 or 1, respectively. Stored data is read or sensed by passing a current through the conventional MTJ cell from one magnetic layer to the other. The amount of current passing through the conventional MTJ cell, or the voltage drop across the conventional MTJ cell will vary according to the orientation of the changeable magnetic vector. 
   The magnetic field for changing the orientation of the changeable magnetic vector is usually supplied by two conductor lines that are substantially orthogonal to each other. When electrical current passes through the two conductor lines at the same time, two magnetic fields associated with the currents in the two conductor lines are generated. These two magnetic fields act on the changeable magnetic vector of the free layer to orient the direction of the changeable magnetic vector. 
     FIG. 1  depicts a portion of a conventional magnetic memory including conventional orthogonal conductor lines  10  and  12 , conventional magnetic storage cell  11  and conventional transistor  13 . The conventional magnetic storage cell  11  is located at the intersection of and between the conventional conductor lines  10  and  12 . The magnetic storage cell  11  depicted in  FIG. 1  is a conventional MTJ cell consisting of a conventional MTJ stack. Conventional line  10  and conventional line  12  are often referred to as the word line and the bit line respectively. The names, however, are interchangeable. Other names, such as row line, column line, digit line, and data line, may also be used. 
   The conventional MTJ  11  stack primarily includes the free layer  1104  with the changeable magnetic vector (not explicitly shown), the pinned layer  1102  with the fixed magnetic vector (not explicitly shown), and the insulator  1103  in between the two magnetic layers  1104  and  1102 . The insulator  1103  typically has a thickness that is low enough to allow tunneling of charge carriers between the magnetic layers  1102  and  1104 . Layer  1101  is usually a composite of seed layers and an anti-ferromagnetic layer that is strongly coupled to the pinned magnetic layer. 
   During writing, the electrical current I 1  flowing in the conventional bit line  12  and I 2  flowing in the conventional word line  10  yield two magnetic fields on the free layer  1104 . In response to the magnetic fields, the magnetic vector in free layer  1104  is oriented in a direction that depends on the direction and amplitude of I 1  and I 2  and the properties and shape of the free layer  1104 . Generally, writing a zero (0) requires the direction of either I 1  or I 2  to be different than when writing a one (1). During reading, the conventional transistor  13  is turned on and a small tunneling current flows through the conventional MTJ cell. The amount of the current flowing through the conventional MTJ cell  11  or the voltage drop across the conventional MTJ cell  11  is measured to determine the state of the memory cell. In some designs, the conventional transistor  13  is replaced by a diode, or completely omitted, with the conventional MTJ cell  11  in direct contact with the conventional word line  10 . 
   Although the above conventional MTJ cell  11  can be written using the conventional word line  10  and conventional bit line  12 , one of ordinary skill in the art will readily recognize that the amplitude of I 1  or I 2  is in the order of several milli-Amperes for most designs. Therefore, one of ordinary skill in the art will also recognize that a smaller writing current is desired for many memory applications. 
     FIG. 2  depicts a portion of a conventional magnetic memory that has a lower writing current. Similar systems are described in U.S. Pat. Nos. 5,659,499, 5,940,319, 6,211,090, 6,153,443, and U.S. patent application Ser. No. 2002/0127743. The conventional systems and conventional methods for fabricating the conventional systems disclosed in these references encapsulate bit lines and word lines with soft magnetic cladding layer on the three surfaces not facing MTJ cell  11 ′. Many of the portions of the conventional memory depicted in  FIG. 2  are analogous to those depicted in FIG.  1  and are thus labeled similarly. 
   The system depicted in  FIG. 2  includes the conventional MTJ cell  11 ′, conventional word line  10 ′ and bit line  12 ′. The conventional word line  10 ′ is composed of two parts: a copper core  1001  and a soft magnetic cladding layer  1002 . Similarly, the conventional bit line  12 ′ is composed of two parts: a copper core  1201  and a soft magnetic cladding layer  1202 . 
   Relative to the design in  FIG. 1 , the soft magnetic cladding layers  1002  and  1202  can concentrate the magnetic flux associated with I 1  and I 2  onto the MTJ cell  11 ′ and reduce the magnetic field on the surfaces which are not facing the MTJ cell  11 ′. Thus, the sot magnetic cladding layers  1002  and  1202  concentrate the flux on the MTJ that makes up the MTJ cell  11 ′, making the free layer  1104  easier to program. Although this approach works well theoretically, one of ordinary skill in the art will readily recognize that the magnetic properties of the portions of the soft cladding layers  1002  and  1202  on the vertical sidewalls of the conventional lines  10 ′ and  12 ′, respectively, are hard to control. One of ordinary skill in the art will also recognize that the process of making the conventional word line  10 ′ and the conventional bit line  10 ′ is complicated. The complicated fabrication methods pose significant challenge to scaling to higher densities. Accordingly it is highly desirable to provide an MRAM architecture which is scalable and easy to fabricate, and offers high writing efficiency. 
   Furthermore, the conventional write lines  10 ,  10 ′,  12 , and  12 ′ of the conventional designs depicted in both FIG.  1  and  FIG. 2  limit scalability. In these conventional designs, the conventional write lines  10 ,  10 ′,  12 , and  12 ′ are mostly made of either aluminum or copper. The current density limits for aluminum and copper are in the order of 1×10 6  A/cm 2  or less. As the line width decreases to increase the memory density, the electromigration current density limit poses severe challenges for scaling. 
   Other conventional systems attempt to propose different solutions, each of which has its drawbacks. As an example, U.S. patent application Ser. No. 2002/0080643 proposed that, after a write operation, a reverse current is applied to the write lines to prevent electromigration. But such conventional methods compromise performance by reducing the speed of the memory and add complexities. Thus, it is also highly desirable to have write line made of materials with high reliability in electromigration, which will allows for easy scalability to high density memory arrays. 
   Conventional thin bit lines, which might be used for smaller or more efficient memories have shortcomings. Thinner conventional bit lines have higher resistances. This adversely affects the performance of the overall memory array. However, there are many conventional methods of overcoming this issue. One common practice is to break up the long bit lines in the memory array into global bit lines that are made of thick metals, and connect the global bit lines into local bit lines that are made of thinner metals, and thus have a higher resistance. Examples of such design are taught by U.S. Pat. No. 6,335,890 and U.S. patent application Ser. No. 2002/0034117. However, the other problems described above, such as the electromigration are still not overcome. 
   Accordingly, what is needed is a system and method for providing a scalable, efficient, low current magnetic memory. The present invention addresses such a need. 
   SUMMARY OF THE INVENTION 
   A method and system for providing and using a magnetic random access memory are disclosed. The method and system include providing a plurality of magnetic memory cells, a first plurality of write lines, and a second plurality of write lines. The first plurality of write lines is a plurality of magnetic write lines. At least one of the plurality of magnetic lines and at least one of the second plurality of write lines each carrying a current for writing to at least one of the plurality of magnetic memory cells. Preferably, the plurality of magnetic write lines have soft magnetic properties and are preferably magnetic bit lines. For magnetic tunneling junction stacks within the magnetic memory cells, the magnetic bit lines are preferably significantly thicker than and closely spaced to the free layers of the magnetic memory cells. 
   According to the system and method disclosed herein, the present invention provides a magnetic memory having an improved efficiency, improved reliability against electromigration, while being simpler to fabricate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a three-dimensional view of a portion of a conventional magnetic memory including a MTJ cell, located at the intersection of a bit line and a word line. 
       FIG. 2  is a three-dimensional view of a portion of a conventional magnetic memory including a MTJ cell, located at the intersection of a bit line and a word line, where the bit line and the word line have magnetic cladding to improve write efficiency. 
       FIG. 3   a ,  FIG. 3   b , and  FIG. 3   c  are, respectively, the side view, the cross-sectional view, and the plane view of a portion of one embodiment of a magnetic memory in accordance with the present invention including a MTJ stack in an MRAM cell with a magnetic bit line. 
       FIG. 4  is the plane view of the MTJ cell and the bit line, as well as schematic representations of the magnetic vectors of free layer of the MTJ cell and the magnetic bit line in the quiescent states for one embodiment of a magnetic memory in accordance with the present invention. 
       FIG. 5  is the plane view of the MTJ cell and the bit line, as well as the schematic representation of the magnetic vectors of free layer of the MTJ cell and the magnetic bit line when a write current is flowing in the bit line for one embodiment of a magnetic memory in accordance with the present invention. 
       FIG. 6  is the cross-sectional view of the MTJ cell and the bit line, showing the field produced by the portion of the current in the metal spacer layer for one embodiment of a magnetic memory in accordance with the present invention. 
       FIG. 7  is the cross-sectional view of a portion of one embodiment of a MRAM in accordance with the present invention including a memory cell. 
       FIG. 8  is the cross-sectional view of a portion of another embodiment of a MRAM in accordance with the present invention including a memory cell. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates to an improvement in magnetic memories. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
   A method and system for providing and using a magnetic random access memory are disclosed. The method and system include providing a plurality of magnetic memory cells, a first plurality of write lines, and a second plurality of write lines. The first plurality of write lines is a plurality of magnetic write lines. At least one of the plurality of magnetic lines and at least one of the second plurality of write lines each carrying a current for writing to at least one of the plurality of magnetic memory cells. Preferably, the plurality of magnetic write lines have soft magnetic properties and are preferably magnetic bit lines. For magnetic tunneling junction stacks within the magnetic memory cells, the magnetic bit lines are preferably significantly thicker than and closely spaced to the free layers of the magnetic memory cells. 
   The present invention will be described in terms of particular types of magnetic memory cells, particular materials, and a particular configuration of elements. However, one of ordinary skill in the art will readily recognize that this method and system will operate effectively for other magnetic memory cells, and other materials and configurations non inconsistent with the present invention. For example, although MTJ stacks are described as including single magnetic layers, nothing prevents the use of other materials, other alloys and synthetic layers. One of ordinary skill in the art will also readily recognize that although the present invention are described in terms of magnetic bit lines, the method and system are consistent with the magnetic word lines, digit lines, or simply write lines. 
   To more particularly illustrate the method and system in accordance with the present invention, refer now to  FIGS. 3   a ,  3   b  and  3   c , depicting a portion of one embodiment of a magnetic memory, such as an MRAM in accordance with the present invention.  FIG. 3   a  depicts a side view of the portion of the magnetic memory in accordance with the present invention. The MRAM includes word lines (not shown), bit lines, such as the magnetic bit line  32 , and MRAM cells, of which one is shown. The MRAM cell  31  shown includes a MTJ stack  31 . The MTJ stack  31  includes two magnetic layers: a pinned layer  3101  and a free layer  3103 . The free layer  3103  has a changeable magnetic vector, while the pinned layer  3101  has a fixed magnetic vector. The magnetic layers  3101  and  3103  are preferably made of magnetic materials such as nickel, cobalt, iron, or alloys thereof. Other elements, such as boron and nitrogen, can also be added to produce desirable magnetic properties. While the free layer  3103  and the pinned layer  3101  are shown each in the figure as a single layer, it should be clear to those of ordinary skill in the art that each layer can also be a composite of several magnetic layers. The pinned layer  3101  is often pinned by an anti-ferromagnetic layer underneath  3101 , not shown here, or other means familiar to those of ordinary skill in the art. 
   An insulator layer  3102  separates the two magnetic layers. The insulator  3102  is preferably aluminum oxide, other oxide, or composite of two or more oxide layers. The insulator  3102  is also preferably thin enough to allow tunneling of charge carriers between the magnetic layers  3101  and  3103 . Free layer  3103  is preferably capped by a metal layer  3104 . In the shown, the capping layer is preferably made of high conductivity metals such as copper, gold, silver, rhodium, ruthenium, aluminum, and tantalum. If copper is used as the capping layer, cobalt-iron (not shown) is usually used as a diffusion barrier. In such case, the free layer  3103  should at least include a thin cobalt-iron layer of several angstroms on the top to prevent the diffusion of copper from the capping layer  3104  into free layer  3103 . This diffusion barrier finds particular utility when nickel-iron is part of the free layer  3101 . 
   The magnetic bit line  32  crosses the MTJ stack  31  and preferably makes contact with the metal capping layer  3104 . In the present invention the bit line  32  is magnetic. A significant portion, and preferably all, of the magnetic bit line  32  is made of magnetic materials, such as nickel, cobalt, iron, or alloy thereof, such as NiFe. The magnetic material making up the magnetic bit line  32  is a soft magnetic material. In one embodiment, the core (central portion) of the magnetic bit line  32  is magnetic. In a preferred embodiment, the bulk of or all of magnetic bit line  32 , except layers such as seed layers, is magnetic. The magnetic bit line  32  can further be a composite of several magnetic layers. If copper is used as the metal capping layer  3104 , the magnetic bit line  32  should at least include a cobalt iron layer (not separately shown) as a diffusion barrier between the copper metal capping layer  3104  and the magnetic bit line  32 . While the magnetic bit line  32  preferably is made mostly of magnetic material, the magnetic bit line  32  can also have a non-magnetic metal seed layer. However, for reasons discussed below, it is still preferred that the spacing between the magnetic portion of the magnetic bit line  32  and the free layer  3103  is approximately three hundred Angstroms or less. In a preferred embodiment, the combined thickness of the metal seed layer (not shown) and the non-magnetic capping layer  3104  of the MTJ stack  34  is still in the order of three hundred Angstroms or less. 
     FIG. 3   b  shows a cross-sectional view of the structure of the magnetic bit line  32  and the MTJ stack  31  in accordance with the present invention. The magnetic bit line  32  has a width W 32 . The MTJ stack  31  has a width W 31 . In the present invention, the magnetic bit line width W 32  is preferably equal or greater than the MTJ stack width W 31 . In  FIG. 3   b , the magnetic bit line width W 32  is greater than the MTJ stack width W 31 . However, self-aligned patterning processes can be employed to produce a substantially equal widths of the magnetic bit line  32  and the MTJ stack  31  and substantially perfect alignment between the two. In such a process, the magnetic bit line width W 32  and the MTJ stack width W 31  are determined in one etching process, such as ion milling. 
     FIG. 3   c  is a plane view of the embodiment of the magnetic bit line  32  and the MTJ stack  31  in accordance with the present invention. In  FIG. 3   c , the MTJ stack  31  is shown to have a rectangular shape having its long axis substantially aligned with the magnetic bit line  32 . Thus, the MTJ stack  31  has a shape anisotropy such that the magnetic vector of the free layer  3103  lies substantially along the magnetic bit line  32  in the lengthwise direction. In the following descriptions, we will continue to assume this shape anisotropy. However, it should be noted that other shapes and other orientations of the magnetic vector of the free layer  3103  are consistent with the present invention. For example, it should be noted that the invention also works with the free layer easy axis oriented orthogonal to the bit line lengthwise direction. 
     FIG. 4  is a plane view of the embodiment of the portion of the MRAM of the free layer  3103  and the magnetic bit line  32  described in  FIGS. 3   a ,  3   b , and  3   c . In  FIG. 4 , a schematic representation of the magnetic vectors M 321 , M 322 , and M 323  of the magnetic bit line  32  is shown. According to the present invention, the magnetic vectors are to orient substantially parallel to the bit line in the lengthwise direction. Also shown in  FIG. 4  is the magnetic vector M 311 /M 312  of the free layer  3103 . In the quiescent state, the free layer magnetic vector M 311 /M 312  is to lie in the easy axis direction. Consequently, for exemplary purposes, the free layer vector is shown as either M 311  or M 312 , representing the two logic states of the memory cell  31 . In addition,  FIG. 4  depicts easy axis of the free layer  3103  of the MTJ stack  31  being parallel with a long axis substantially aligned with the magnetic bit line  32  because of the shape of the MTJ stack  31 . However, the easy axis of the free layer  3103  could be induced in another manner, such as due to intrinsic and/or stress anisotropy. In a preferred embodiment, the easy axis of the free layer is induced by a combination of shape, intrinsic anisotropy, and stress induced anisotropy. 
     FIG. 5  depicts one embodiment of the portion of the magnetic memory during writing.  FIG. 5  displays the magnetic vectors of the magnetic bit line  32  and the free layer  3103 , as described in  FIG. 4 , in the presence of a write current I 32  flowing in the magnetic bit line  32 . It is assumed that the logic state corresponds to the free layer magnetic vector M 312 . The current I 32  induces a magnetic field that rotates the free layer magnetic vector M 312  according to the right hand rule. Thus, the magnetic vector  312  of the free layer  3103  rotates down as shown in FIG.  5 . Because the magnetic vector M 322  of the magnetic bit line  32  is in close proximity to the free layer  31 , M 322  has a strong magnetostatic coupling with the free layer magnetic vector M 312 . As a result, M 322  also rotates, up as shown in  FIG. 5 , to form a flux closure in the direction perpendicular to the magnetic bit line  32 . 
   In a preferred embodiment, the magnetic moment of the magnetic bit line  32  is much greater than the magnetic moment of the free layer  3103 . Preferably, this is ensured by making the thickness of the magnetic bit line  32  much greater than that of the free layer  3103 . Because the moment of the magnetic bit line  32  is much greater than that of the free layer  3103 , the angle of rotation, θ 322 , of the bit line magnetic vector M 322  is much smaller than the angle of rotation, θ 312 , of the free layer magnetic vector M 312 . In a preferred embodiment, the magnetic vectors M 322  of the magnetic bit line  32  thus remain substantially in the magnetic bit line  32  lengthwise direction throughout the write operation. In other words, during reversal of the free layer magnetic vector, the magnetic vector M 322  of the magnetic bit line  32  only deviates from the bit line lengthwise direction by a very small angle. In a preferred embodiment, the ratio of the sine of the rotation angle θ 322  of the bit line magnetic vector M 322  to the sine of the rotation angle θ 312  of the free layer magnetic vector M 312  is roughly as follows,
 
sin(θ 322 )/sin(θ 312 )˜(total moment of the free layer  3103 )/(total moment of the magnetic bit line  32 ).
 
By ensuring that the magnetic moment of the magnetic bit line  32  at least ten times that of the free layer  3103 , a small rotation angle for the bit line magnetic vectors is allowed. In practice, this difference in magnetic moments is preferably achieved by providing a line thickness for the bit line  32  that is at least three hundred Angstroms or larger.
 
   A small rotation angle θ 322  for the bit line magnetic vector M 322  of the magnetic bit line  32  is desired to improve the writing efficiency of the magnetic bit line  32  without compromising the writing efficiency of the word line  30 . As shown in  FIG. 5 , the magnetic memory also includes a word line  30  that, in a preferred embodiment, runs substantially orthogonal to the magnetic bit line  32 . During writing, a current I 30  flows in the word line  30  while the current I 32  flows in the magnetic bit line  32 . The combination of the two currents (I 30  and I 32 ) and the polarity of the word line current I 30  determines the final direction of the magnetic vector (M 311 /M 312 ) of the free layer  3103 . Stated differently, the two currents I 30  and I 32  combine to write to the MTJ stack  31 . During writing, I 30  produces a magnetic field F 30  that lies substantially lengthwise, along the magnetic bit line  32 . The polarity of the field F 30 , which is left or right as shown in  FIG. 5 , is determined by the polarity of I 30 . The polarity of the field F 30  determines the logic state to be written to the MTJ cell  31 . 
   The field F 30  produces a torque on both the bit line magnetic vector M 322  and the free layer magnetic vector M 312 . When θ 312  is much greater than θ 322 , the torque produced on the free layer magnetic vector M 312  by magnetic field F 30  is much greater than the torque on the bit line magnetic vector M 322 . Moreover, when θ 322  is negligibly small, the torque on M 322  is negligible. As a result, the bit line magnetic vector M 322  remains substantially lengthwise, along the magnetic bit line  32  and flux closure is maintained. In this way, the write efficiency of the magnetic bit line  32  is greatly improved by the flux closure, while the write efficiency of the word line  30  is not compromised. 
   In addition to the magnetic bit line  32  having a much greater thickness than the free layer  3103 , to achieve more efficient flux closure between the magnetic bit line magnetic vector M 322  and the free layer magnetic vector M 311 /M 312 , the spacing between the magnetic bit line  32  and the free layer  3103  is sufficiently small. Consequently, in a preferred embodiment, the thickness of the magnetic bit line  32  is much greater than the thickness of the free layer  3103  and the spacing between the free layer  3103  and the magnetic bit line  32  is sufficiently small. However, in alternate embodiments, one or more of these features may be omitted. For example, the spacing between the magnetic bit line  32  and the free layer  3103  may be sufficiently small, but the difference between the thicknesses of the magnetic bit line  32  and the free layer  3103  may be small. An estimation of the desired spacing can be made using the conventional characteristic length for the flux closure of two magnetic layers. Because the magnetic bit line  32  is preferably much thicker than the free layer  31 , the characteristic length is roughly (μgt/2) 0.5 , where μ is the permeability of the free layer  3103 , g is the spacing between the magnetic bit line  32  and the free layer  3103 , and t is the thickness of the free layer  32 . The width W 31  of the MTJ stack  31  is preferably much larger than the characteristic length to avoid significant edge curling walls. As a result, the desired spacing can be determined from the following relationship:
 
(μ gt/ 2) 0.5   &lt;W   31 
 
In today&#39;s applications, this relationship means that for the spacing to be as small as desired, the spacing should be in the order of three hundred Angstroms or less. Consequently, in a preferred embodiment, the spacing between the free layer  3103  and the magnetic bit line  32  is less than three hundred Angstroms, while the thickness of the magnetic bit line  32  is greater than three hundred Angstroms.
 
     FIG. 6  shows a cross-sectional view of one embodiment of the magnetic bit line  32  and the MTJ stack  31  as depicted in  FIG. 3   b . During writing, portion of the current I 32  in the magnetic bit line  32  is shunted by the metal capping layer  3104 . The current in the metal capping layer I 61 , produces magnetic fields, F 61  and F 62 , in the magnetic bit line  32  and in the free layer  3103 , respectively. These two magnetic fields F 61  and F 62  are opposite in direction, further enhancing the magnetic flux closure between the magnetic bit line  32  and the free layer  3103 . Therefore, it is desirable to have high conductivity metal as the capping layer  3104  to enhance the flux closure between the free layer  3103  and the magnetic bit line  32 . In a preferred embodiment, the capping layer  3104  includes gold, copper, silver, ruthenium, rhodium, aluminum, other well know good conductors, and alloys thereof. 
   If copper is used as the capping layer  3104 , cobalt-iron (not shown) or other suitable material is usually used as a diffusion barrier between the capping layer  3104  and the free layer  3103 . In such an embodiment, the free layer  3103  preferably includes a thin cobalt-iron layer (not shown) of at least several angstroms on the top to prevent the diffusion of copper into free layer  3103 . This diffusion barrier may be particularly useful when nickel-iron is part of the free layer  3103 . For similar reasons, a cobalt-iron layer (not shown) may be placed between the copper capping layer  3104  and the magnetic bit line  32 , especially when nickel-iron is used as part of the magnetic bit line  32 . 
   To reduce the resistance of the magnetic bit line  32 , a thin layer of high conductivity non-magnetic metal (not shown) can also be placed as the seed layer (not shown) of the magnetic bit line  32 . However, the thickness of this seed layer is preferably small enough that the combined thickness of the non-magnetic seed layer and the capping layer  3104  is about 300 angstrom or thinner. 
   Although the method and system in accordance with the present invention is described using a MTJ stack  31  with free layer  3103  on top of the thin insulator layer  3102 , one of ordinary skill in the art will readily recognize that the method and system also function for other magnetic structures. Such magnetic structures include, but are not limited to, a MTJ stack (not shown) having the free layer beneath the insulator tunneling layer, and a MTJ stack (not shown) having two insulator tunneling layers and a free layer sandwiched between the two insulator layers. In each configuration, it is preferred that the spacing between the magnetic bit line and the free layer is in the order of three hundred Angstroms or less. In different configurations, this preferred thickness takes into account any layer that lies between the magnetic bit line  32  and the free layer  3103 , such as a capping layer, a pinned layer, or other layers. 
   Furthermore, one of ordinary skill in the art will also readily recognize that the magnetic bit line need not be placed on top of the MTJ stack  31 . For example, the magnetic bit line  32  can also lie beneath the MTJ structure. Nor is the electrical connection between the magnetic bit line  32  and the MTJ stack  31  required. In such an embodiment, the magnetic line may be called a write line. Moreover, in another embodiment, the write line and the bit line may both be magnetic. However, in all embodiments, it is preferred that the spacing between the magnetic write line, if any, and the free layer with the changeable magnetic vector be about three hundred Angstroms or less and that the magnetic write line thickness is greater than three hundred Angstroms. 
     FIG. 7  depicts one of the preferred embodiments of a portion of a MRAM memory in accordance with the present invention. Many of the components are analogous to the magnetic bit line  32 , memory cell/MTJ stack  31 , and word line  30  depicted in  FIGS. 3   a-   3   c ,  4 ,  5 , and  6 . Consequently, many components are labeled similarly. For example, the MTJ stack  31 ′ in  FIG. 7  corresponds to the MTJ stack  31  depicted in  FIGS. 3   a-   3   c ,  4 ,  5 , and  6 . Referring to  FIG. 7 , in addition to the magnetic bit line  32 ′, and the MTJ stack  31 ′ as described above, a word line  30 ′, a by-pass connection  78 , a conductive layer  79 , a ground line  77 , and a transistor  81  in a substrate  72  are depicted. The word line  30 ′ is shown to run substantially orthogonal to the magnetic bit line  32 ′. The MTJ stack  31 ′ is connected through a conductive layer  79  and a by-pass connection  78 , to the source  74  of the transistor  81 . The transistor  81  is preferably a FET transistor. The drain  73  of the FET transistor  81  is connected, through a contact  75  to the ground line  77 . The gate  76  of the FET transistor  81  is connected to a read word line (not shown). Although the configuration shown is preferred, one of ordinary skill in the art will readily recognize that many other different configurations incorporating the magnetic bit lines according to the present invention are possible. For example, diode (not shown) can be used to replace the transistor  81 . MRAM cells (not shown) without transistor or diodes are also possible. 
     FIG. 8  shows another preferred embodiment of a portion of an MRAM in accordance with the present invention. Many of the components are analogous to the magnetic bit line  32 ″, memory cell/MTJ stack  31  and  31 ′, and word line  30  and  30 ″ depicted in  FIGS. 3   a-   3   c ,  4 ,  5 ,  6 , and  7 . Consequently, many components are labeled similarly. For example, the MTJ stack  31 ″ in  FIG. 8  corresponds to the MTJ stack  31  and  31 ′ depicted in  FIGS. 3   a-   3   c ,  4 ,  5 ,  6 , and  7 . Similarly, the transistor  81 ′ corresponds to the transistor  81  in FIG.  7 . The word line  80  corresponds to the word lines  30  and  30 ′ in  FIGS. 3   a-   3   c ,  4 ,  5 ,  6 , and  7 . Referring to  FIG. 8 , the MRAM includes the magnetic bit line  32 ″, the MTJ stack  31 ″, the bypass connection  78 ′, the ground line  77 ′, and the FET  81 ′ in the substrate  72 ′. The bit line  32 ″, the MTJ  31 ″, the FET  81 ′, and the word read line  80  are substantially the same as described with respect to FIG.  7 . However, unlike the word line  30  of  FIG. 7 , the word line  80 ′ in  FIG. 8  is placed above the magnetic bit line  32 ″. To maintain the write efficiency of the word line  80 ′, the thickness of the magnetic bit line  32 ″ is preferably as small as possible. Making a conventional bit line this thin is difficult in normal design, where the bit line is made of aluminum or copper. However, such a small thickness is practical in the present invention. 
   As discussed above, conventional thin bit lines have two shortcomings. First, thinner conventional bit lines have higher resistances. This adversely affects the performance of the overall memory array. However, there are many conventional methods of overcoming this issue. One common practice is to break up the long bit lines in the memory array into global bit lines that are made of thick metals, and connect the global bit lines into local bit lines that are made of thinner metals, and thus have a higher resistance. However, such conventional methods do not use magnetic bit lines. In addition, such conventional methods still suffer from other drawbacks, such as electromigration discussed below. 
   Second, electromigration of conventional thin metal lines adversely affect reliability. For example, copper has a relatively low electromigration limit for current density of about 1×10 6  A/cm 2  or less. Aluminum lines have even lower capability to carry current without suffering adverse affects due to electromigration. In comparison, magnetic materials such as nickel-iron have much higher capability to carry current without suffering adverse effects due to electromigration. For example, nickel-iron films have been used for magneto-resistive read sensors in hard drives, and have been shown to have capability to carry current in excess of 10 8  A/cm 2 . That is far greater than that of aluminum and copper, and thus allows for much thinner and/or narrower bit lines. 
   Due to the excellent electromigration properties of soft magnetic films such as nickel-iron, it is practical to have thin bit lines in the order of several hundred angstroms. This will greatly enhance the efficiency of the word line  80 . Although the word line  80  is shown as a single line in  FIG. 8 , it should be understood by those skilled in the art that the invention includes more efficient word line structures, such as the use of cladding layers for the word line  80 . 
   A method and system has been disclosed for providing a magnetic memory having improved writing efficiency, better reliability, and simpler fabrication. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.