Patent Publication Number: US-2003227794-A1

Title: System and method for enabling chip level erasing and writing for magnetic random access memory devices

Description:
TECHNICAL FIELD  
       [0001] The present invention is directed to magnetic memory devices, specifically, magnetic random access memory devices. More particularly, the present invention is directed to a method and system for facilitating erasing and writing to magnetic memory devices.  
       BACKGROUND OF THE INVENTION  
       [0002] Magnetic or magnetoresistive random access memory (“MRAM”) devices offer advantages over conventional transistor-based random access memory (“RAM”) devices and rewriteable nonvolatile read only memory devices. MRAM devices exploit the inherent nonvolatility of magnetic storage, used in early RAM devices, and long used in sequential memory devices in disk and tape storage. Unlike dynamic random access memory (“DRAM”) devices which consume appreciable quantities of power in having to be continually refreshed to preserve the integrity of their memory contents, MRAM cells do not need to be refreshed. In fact, unlike transistor-based RAM devices, once a cell of an MRAM device is polarized to its desired state, the cell retains its polarity without having to be supplied with power. Furthermore, unlike nonvolatile flash electronically erasable programmable read only (“flash EEPROM”) memories, the contents of which can become corrupted with heavy use, MRAM devices are highly reliable. Moreover, while flash EEPROM devices can only be rewritten by erasing them and rewriting them in their entirety, cells in MRAM devices can be selectively written and rewritten without erasing the contents stored in the entire device.  
       [0003] Unlike previous uses of magnetic storage, such as disk and tape storage or bubble memory, MRAM devices provide direct, random access to their contents. Accordingly, MRAM devices provide the advantages of conventional RAM devices with the reliable nonvolatility of magnetic storage.  
       [0004] MRAM devices exploit the inherent interrelationship between the flow of electric current and corresponding magnetic fields. As is known in the art, a current flowing through a longitudinal conductor creates a magnetic field which encircles latitudinally about the axis of the longitudinal conductor. Specifically, MRAM devices exploit this interrelationship by using electric currents to generate magnetic fields which, in turn, are applied in close proximity to storage elements comprised of magnetically susceptible materials. Electric currents directed in a first direction results in a magnetic field having a corresponding first polarity. Exposed to the field of that corresponding polarity, if the field has sufficient magnitude, the magnetically susceptible element becomes magnetized in that same polarity. The magnetic field generated by the magnetized element then is capable of reacting to other applied magnetic fields, such as those caused by other currents flowing through the conductor. As a result, if an electric current of the same polarity was applied to the same conductor which first caused the element to become magnetized, the magnetic field of that magnetized element would not resist that current. On the other hand, if an electric current of opposite polarity was applied to the conductor, inducing a magnetic field of opposite polarity to encircle the conductor, those magnetic fields would conflict, and affect the resistivity of the conductor to the flow of current. Measuring the discrepancies in current caused by the differing resistance encountered as a result of the influence of these previously magnetized elements allows the stored polarity of these elements to be read.  
       [0005] It will be appreciated that, while a current of opposite polarity applied to the conductor will be opposed by the current induced by the magnetic element, that current of opposite polarity will not necessarily repolarize the magnetic field of that element. Magnetic materials exhibit a hysteresis effect in that a stronger current must be applied to repolarize them than might be required to polarize them initially. This principle is relied upon by MRAM devices: currents of lower magnitude can be used to detect the magnetic field created in the magnetic elements and thereby allow the bit written to that magnetic element to be read, while currents of greater magnitude generate magnetic fields which can be used to overcome hysteresis and write or rewrite the bit written to that magnetic element. However, as is understood in the art, an acknowledged problem in MRAM devices is that relatively high currents are required to induce a magnetic field of sufficient magnitude to reliably write and rewrite MRAM memory cells.  
       [0006] As shown in FIG. 1, an MRAM device comprises a Cartesian array  100  of MRAM memory cells  104 . Each MRAM memory cell  104  comprises an element of magnetically susceptible material  108  disposed at an intersection of a row line  112  or  116  and a column line  120 ,  124 , or  128 . Electrical current is selectively applied to the row lines  112  and  116  and column lines  120 ,  124 , or  128  to effect writing and reading of data to and from each of the memory cells. As is known in the art, writing these cells is accomplished by selectively and simultaneously directing the current in the row lines  112  and  116  and column lines  120 ,  124 , and  128  so as to subject a particular element  108  to a desired combination of magnetic fields generated by the current flowing through the conductive lines.  
       [0007]FIGS. 2A and 2B show how a magnetic element can become polarized and, therefore, written. FIG. 2A shows an MRAM cell  200  which, physically, is comprised of magnetic element  204  disposed at the intersection of the row line  208  and the column line  212 . An electrical row current of a first polarity  216  is applied to the row line  208  and thereby induces a magnetic flux field  220  of a first polarity to which the magnetic element  204  is exposed. At the same time, an electrical column current  224  is applied to the column line  212  and thereby induces a magnetic field  228  to which the magnetic element  204  is exposed. The combination of these complementary magnetic fields  220  and  228  cause the magnetic element to become polarized to radiate a composite magnetic field in a predetermined direction to represent a stored data bit. Once the magnetic element  204  has become polarized, the magnetic element  204  generates a magnetic field which, as previously described, will interact with the magnetic field generated by currents of a different polarity flowing through the row line  208  and column line  212 . It will be appreciated that, as in any Cartesian grid, selection of a single row and a single column singularly identify a single point on the grid. Correspondingly, applying the row current  216  to the row line  208  and the column current  224  to the column line  212  allow the individual MRAM cell  200  at the intersection of the row line  208  and the column line  212  to be programmed.  
       [0008]FIG. 2B, for the sake of completeness, shows the opposite case in which an MRAM cell  250  is programmed to store a magnetic field of the opposite polarity. If, in the example shown in FIG. 2A, the field stored in the magnetic element  204  of the MRAM cell  200  is considered to represent a logical zero, FIG. 2B shows the MRAM cell  250  being programmed to read as a logical one. The magnetically susceptible element  254  disposed at the intersection of the row line  258  and the column line  262  exposed to an electrical row current of a first polarity  266  applied to the row line  258  and induces a magnetic field  270  of a first polarity. At the same time, a column current  274  is applied to the column line  262  and induces a magnetic field  278  to which the magnetic element  254  is exposed. The composite magnetic field of magnetic fields  270  and  278  causes the magnetic element  254  to become polarized to radiate a magnetic fields of opposite polarity.  
       [0009] Once programmed, magnetic elements  204  and  254  in FIGS. 2A and 2B, respectively, will retain their magnetic fields in the absence of power. Accordingly, MRAM array  100  (FIG. 1) will retain the data stored therein where it can be retrieved upon being accessed by the system (not shown) served by the array  100  without having to be refreshed, reloaded, or rebooted.  
       [0010] Despite the advantages MRAM devices afford, however, they do present disadvantages. For example, because MRAM cells retain the data stored therein even when not supplied with power, affirmative steps must be taken to erase sensitive or otherwise unwanted data. One way to erase such data is to overwrite the contents of every cell in accordance with the steps described previously in connection with FIGS. 2A and 2B. Considering the hysteresis effect previously described, rewriting these cells could consume an appreciable amount of power. Writing the MRAM array with new data to be used by another application would necessarily overwrite and erase old data. However, in an age where data privacy and security becomes increasingly more important, and MRAM cells are nonvolatile, it would be desirable to be able to erase data from an MRAM array or a section thereof without having to rewrite the array with bogus data solely for the sake of erasing the data. Similarly, it would be desirable to facilitate the ability to write or rewrite MRAM cells without having to apply the high degree of current to the row and column lines required to overcome the hysteresis effect.  
       [0011] An additional concern arises from the possibility that the conductive row and column lines themselves could become magnetized through being exposed to the magnetic fields radiated by the magnetic elements. This could pose a problem in reading the MRAM cells. As previously described, the MRAM cells are read by applying electric currents to the row and column lines and measuring whether any resistance was encountered as a result of the magnetic fields stored in the magnetic elements at the intersections of those lines. If the conductive row and column lines were to become magnetized, thereby radiating their own magnetic fields that would affect the longitudinal flow of current through these lines, it could skew the reading of what was stored in the magnetic elements. It would be desirable to be able to demagnetize these lines.  
       [0012] It is to these ends that embodiments of the present invention are directed.  
       SUMMARY OF THE INVENTION  
       [0013] The present invention employs one or more switchable, close proximity electromagnets as part of the MRAM device circuit package to apply external magnetic fields to the magnetic elements and conductive lines of the MRAM array. An external magnetic field of sufficient magnitude could be induced to overwrite each of the targeted cells in the MRAM array. Alternatively, currents of decreasing magnitude and reversing polarity could be applied to the electromagnet to demagnetize the cells, as opposed to overwriting the contents of the cells.  
       [0014] In addition, the generated magnetic fields could be produced so as to complement the magnetic fields induced by application of current to the row and column lines of the MRAM array, thereby facilitating writing of data to magnetic elements while applying less power to the row and column lines. As described in the background of the invention, a relatively high current might be required to generate a magnetic field of sufficient magnitude to write or rewrite a memory cell. However, if an electromagnet of the present invention were used to generate an ambient magnetic field of the polarity desired to be programmed to the selected MRAM cells, a lesser current would be required. As long as the combination of the ambient field created by an electromagnet of the present invention and the localized magnetic fields created by applying appropriate row and column currents to the appropriate row and column lines, the selected MRAM cells could be written with application of currents of lesser magnitude applied to those conductive lines. In accordance with the first disclosed use of the invention, a magnetic field of sufficiently high magnitude would erase the MRAM array, while a magnetic field of lesser magnitude could be combined with the selectively applied row and column line currents to selectively write to the MRAM cells.  
       [0015] Further, diagonally disposed electromagnets could be used to generate these magnetic fields, and could also be used to demagnetize the conductive row and column lines. Because the conductive lines are not comprised of magnetically susceptible materials, any magnetic fields needed to demagnetize the row and column lines would not be of sufficient magnitude to affect the magnetic fields written to the magnetic elements of the MRAM cells. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a perspective diagram of a conventional MRAM memory array.  
     [0017]FIG. 2A is an enlarged perspective view of a single conventional MRAM memory cell to which a 0 is being written.  
     [0018]FIG. 2B is an enlarged perspective view of a single conventional MRAM memory cell to which a 1 is being written.  
     [0019]FIG. 3 is a cross-sectional view of an embodiment of the present invention employing a single electromagnetic device.  
     [0020]FIG. 4 is a hysteresis curve of an MRAM cell showing the magnetic field required to magnetize and remagnetize a magnetic element.  
     [0021]FIG. 5 is a hysteresis curve of an MRAM cell showing the magnetic field required to write to a magnetic element in the presence of an externally applied magnetic field.  
     [0022]FIG. 6 is a perspective view of an embodiment of the present invention employing a plurality of separate electromagnetic devices.  
     [0023]FIG. 7 is a perspective view of an embodiment of the present invention employing a diagonally disposed electromagnetic device.  
     [0024]FIG. 8 is a block diagram of a memory subsystem using MRAM devices and an embodiment of the present invention.  
     [0025]FIG. 9 is a block diagram of a computer system using MRAM memory devices and an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0026]FIG. 3 shows a cross-sectional view of an MRAM device  300  adapted to use an embodiment of the present invention. The circuit package  304  of the MRAM device  300  comprises two principal components. First, the MRAM device  300  comprises an MRAM array  308 , accessible by row lines  312  and column lines  316 , and supporting a number of magnetic elements  318 . Second, the MRAM device  300  comprises an electromagnet  320  driven by power lines  324  and  328  in accordance with a first embodiment of the present invention. Disposed within the circuit package  304 , the electromagnet  320  can selectively generate a magnetic field  332  which can influence magnetic elements  318  in the MRAM array  308 . FIG. 3 shows an embodiment of the present invention in which a single electromagnet  320  is used to work with the MRAM device  300 .  
     [0027] As shown, the flux of the magnetic field  332  generated by the electromagnet  320  interacts with magnetic fields generated by the magnetic elements  318 . The magnitude and polarity of the current applied to the electromagnet  320  through its power lines  324  and  328  controls the magnitude and polarity of the generated magnetic field  332 , respectively. As is known in the art, longitudinal flow of current through a conductor in a first direction results in a latitudinal flow of magnetic flux around the conductor in a first polarity. Reversing the flow of the current through the conductor results in a latitudinal flow of magnetic flux around the conductor in an opposite polarity. Thus, application of a current of one polarity may result in the electromagnet  320  generating the magnetic field  332  of the polarity depicted, and reversing the polarity of the current would result in the generation of a magnetic field of a polarity opposite to that of magnetic field  332  shown in FIG. 3. Also, as indicated by Maxwell&#39;s equations, the force of the magnetic field  332  generated is proportional to the current applied to the electromagnet  320 .  
     [0028] To better illustrate how embodiments of the present invention operate, FIG. 4 shows the hysteresis graph  400  for a typical magnetically susceptible element. The abscissa  404  represents the magnetic field applied to the magnetic element, while the ordinate  408  represents the magnetoresistance caused by the magnetic element as a result of the magnetic element being programmed by an applied magnetic force. As previously discussed, practically, the magnetoresistance of the magnetic element results in its contents being read as either a logical ZERO  412  or logical ONE  416 . As shown in the graph  400 , the application of a magnetic field of threshold magnitude B  420  is required to polarize the magnetic element to its ONE  416  state. Alternatively, the application of a magnetic field of the same magnitude but opposite polarity −B  424  is required to repolarize the magnetic element to its ZERO  412  state.  
     [0029] It will be appreciated that, because magnetic fields are vectorized forces, the strength of the magnetic field applied must not only reach the magnitude of B  420  or B- 424 , but also must overcome any existing, opposing magnetic field. For example, if an ambient magnetic field of magnitude −b  436  exists, a magnetic field of B+b  432  must be applied to the magnetic element to polarize the element to its ONE  416  state. This magnetic field of −b  436  could be ambient in the environment surrounding the magnetic element, or it could represent the magnetic field generated by the magnetic element itself, by an array of magnetic elements as a whole, by magnetization of the conductive row and column lines, or by a combination of these. In practice, therefore, a magnetic field of greater than B  420  will have to be applied to repolarize magnetic elements in the presence of an ambient magnetic field, including whatever field is generated by the magnetic element itself. In other words, the presence of the ambient magnetic field −b  436  shifts the hysteresis curve to repolarize the magnetic element from path  444  at B  420  to path  448  at B+b  432 .  
     [0030] Correspondingly, if an ambient magnetic field having the same polarity as the magnetic field required to magnetize the element to the desired state, an external magnetic field of lesser magnitude will be sufficient to repolarize the magnetic element. Again, if the ambient magnetic field is of magnitude −b  436 , but this time it is desired to repolarize the magnetic element to its ZERO  412  state, an external magnetic field of −B+b  440 , having a magnitude of less than −B  424 , is needed to repolarize the magnetic element. In other words, the presence of the ambient magnetic field −b  436  shifts the hysteresis curve to repolarize the magnetic element from path  452  at −B  424  to path  456  at −B+b  440 . It will be subsequently appreciated that embodiments of the present invention both exploit and can redress the effects of ambient magnetic fields such as that at −b  436 .  
     [0031] By varying the polarity and magnitude of the current applied to the control lines  324  and  328  of the electromagnet  320 , embodiments of the present invention can be put to a number of different uses. One embodiment of the present invention can be used to erase the contents of the MRAM array  308 . With regard to FIG. 3, it can be appreciated that the magnetic elements  318  of the MRAM array  308 , having been put to use, will have been programmed to differing magnetic states. An embodiment of the present invention could be used to erase the programming of the magnetic elements in two different ways.  
     [0032] First, an embodiment of the present invention could be used to generate a magnetic field which would overwrite the contents of each of the magnetic elements  318 . FIG. 4 shows hysteresis curves for a magnetic element used in an MRAM array. Considering the hysteresis conditions for such a system, allowing for ambient conditions, a hypothetical worst-case aggregate magnetic field generated by the plurality of magnetic elements  318  might have a magnitude of −b  436  (FIG. 4) or b  438 . If all or nearly all of the magnetic elements  318  (FIG. 3) had been written as ZERO  412  (FIG. 4), an aggregate initial magnetic field of −b  436  would result. Introduction of an external magnetic field of B+b  432  would be required to overcome and overwrite that initial magnetic field of −b  436  and repolarize each of the magnetic elements to a ONE  416  state. On the other hand, if all or nearly all of the magnetic elements  318  (FIG. 3) had been written as ONE  412  (FIG. 4), an aggregate initial magnetic field of b  438  would result. Introduction of an external magnetic field of −B−b  430  would be required to overcome and overwrite that initial magnetic field of b  438  and remagnetize each of the magnetic elements to a ZERO  412  state. Thus, applying a current to the control lines  324  and  328  (FIG. 3) of the electromagnet  320  such that it generates a magnetic field  324  of magnitude B+b  432  (FIG. 4) or −B−b  430  will remagnetize every magnetic element  318  (FIG. 3) to read as a logical ZERO  412  (FIG. 4) or ONE  416 , respectively. In other words, appropriately energizing the electromagnet  320  (FIG. 3) will overwrite and effectively erase all the data stored in the MRAM array  308 . Without such erasure, as previously described, because MRAM memory is nonvolatile, potentially sensitive data would remain stored in the MRAM array even after the system in which the MRAM array is used has long been powered off or disconnected.  
     [0033] Second, the electromagnet  320  could also be used to erase the contents stored in the MRAM array  308  not by overwriting the contents stored therein, but by actually demagnetizing the array. As is known in the art and as is used to demagnetize objects ranging from hand tools to audiotape, magnetically susceptible materials can be demagnetized through the application of magnetic fields of alternating polarity and successively decreasing magnitude. Each applied field must be large enough to counteract the initial magnetic field. Thus, considering FIG. 4, an external magnetic field alternating between magnitudes −B−b  430  and B+b  432  would have to be applied initially to ensure demagnetization of the MRAM array  308  (FIG. 3), the polarity of the external magnetic field and its magnitude being sequentially alternated and reduced, respectively. This external magnetic field can be applied and controlled by alternating the polarity and systematically reducing the magnitude of the current applied to the control lines  324  and  328  of the electromagnet  320 . Once the magnitude of the magnetic field drops to zero, the MRAM array  308  will be demagnetized, the magnetic elements  318  storing neither a ZERO  412  nor a ONE  416 .  
     [0034] The electromagnet  320  of the disclosed embodiment of the present invention can be used not only to erase the MRAM array  308 , but also to facilitate writing data bits to the magnetic elements  318  comprising storage of individual bits within the MRAM array  318 . As previously described and is known in the art, the MRAM memory cells  104  (FIG. 1) are written to by applying appropriately polarized currents to the row lines  112  or  116  and column lines  120 ,  124 , or  128  which intersect over the magnetic element  108  to be written. Because of the hysteresis effects shown in FIG. 4, a significant magnetic field must be induced to write to an MRAM memory cell  104  at an intersection of row lines  112  or  116  and column lines  120 ,  124 , or  128  at the desired MRAM memory cell  104 . Consequently, significant current must be applied to the desired row lines  112  or  116  and column lines  120 ,  124 , or  128 , which results in significant power being applied to and consumed by MRAM memory devices. Embodiments of the present invention can adjust the ambient magnetic field subsisting at each of the MRAM memory cells  104 , decreasing the power required to write and rewrite MRAM memory cells.  
     [0035]FIG. 5 shows a hysteresis curve for writing to a magnetic element in the presence of an induced ambient magnetic field. As in FIG. 4, the abscissa  500  represents the magnetic field applied to the magnetic element, while the ordinate  502  represents the magnetoresistance caused by the magnetic element as a result of the magnetic element being programmed by an applied magnetic force. As previously described with regard to FIG. 4, it takes an applied magnetic field of a particular magnitude to write to a magnetic element, and a magnetic field of a slightly greater magnitude to write to a magnetic element in the presence of an ambient magnetic field of opposing polarity. However, just as the presence of an ambient magnetic field can impede the ability to repolarize and/or write to a magnetic element, an ambient magnetic field can be introduced to enhance that process.  
     [0036] With regard to FIG. 5, consider the situation where a magnetic element presently is magnetized the state representing ZERO  504 , and it is desired to repolarize and thus rewrite that magnetic element to store a ONE  508 . As previously described, ordinarily it would require an externally applied magnetic field of magnitude B  520  to change the state of the magnetic element along the curve  512  to repolarize the magnetic element to the ONE  508  state. As shown in FIG. 4, if the same result were desired in the presence of an ambient magnetic field of magnitude −b  436 , an externally applied magnetic field of magnitude B+b  432  would be required to repolarize and rewrite the magnetic element. However, considering FIG. 5, if an ambient magnetic field of magnitude B E    528  and a constructive polarity enhancing the magnitude of the applied magnetic field subsisted at the point where the magnetic element was situated, the magnetic element could be repolarized from ZERO  504  to ONE  508  with an additional externally applied magnetic field of magnitude equal to the difference between B E    528  and B  520 , or B E −B.  
     [0037] In an embodiment of the present invention, the electromagnet  320  (FIG. 3) could actually be used to induce an external, facilitating magnetic field of magnitude BE  528  (FIG. 5). Thus, creation of a magnetic field of lesser magnitude of B E −B would only be required to repolarize the MRAM memory cell. Because the magnetic field needed from the row and column lines is reduced by the application of the externally applied facilitating magnetic field, less current needs to be applied to the row and column lines, saving power. At the same time, as long as the magnitude of the facilitating magnetic field B E    528  (FIG. 5) induced by the electromagnet  320  (FIG. 3) is less than B  520  (FIG. 5), the facilitating magnetic field induced by the electromagnet  320  (FIG. 3) by itself will not overwrite or rewrite any of the magnetic elements in the MRAM array. It should also be appreciated that increasing the magnitude of the facilitating magnetic field B E    528  (FIG. 5) can be used to increase the aggregate magnetic field applied to the magnetic elements in the MRAM array without increasing the current applied to the row and column lines. Thus, application of this field could be used not only in the interest of reducing the current applied to the row and column lines, but to increase the write reliability of the system.  
     [0038] It will be appreciated that the opposite process, to repolarize and rewrite magnetic element  318  (FIG. 3) from ONE  508  (FIG. 5) to ZERO  504  works in the same manner. The difference is one only of polarity. In this case, if the magnetic element stores a ONE  508 , in the absence of an ambient magnetic field, an externally applied magnetic field of −B  524  would have to be applied to shift the magnetic field of the magnetic element along the hysteresis curve  516  to be rewritten as a ZERO  504 . Using the electromagnet  320  (FIG. 3), applying a current to the control lines  324  and  328  equal in magnitude to that applied to generate a magnetic field of magnitude B E    528 , but of opposite polarity, thereby resulting in the generation of a magnetic field of magnitude −B E    532 . As a result of the application of that magnetic field, the magnitude of the magnetic field required to repolarize and rewrite the magnetic element would be −BE+B. As previously described, use of the electromagnet  320  (FIG. 3) to apply this external magnetic field reduces the amount of current and power that must be applied to the row and column lines of the MRAM device.  
     [0039] Another embodiment of the present invention is shown in FIG. 6. In this embodiment of an MRAM memory device  600 , the MRAM array  604  is equipped with a plurality of electromagnets  608 , each controlled by separate control lines  612 . Sizing, spacing, and powering of these electromagnets  608  are selected to confine the magnetic field (not shown) generated by the electromagnets  608  to a sector of the MRAM array  604 . It will be appreciated that the operation of these electromagnets for erasing or for facilitating writing data to the MRAM array  604  is equivalent to that of the previously described embodiment. The essential difference in operation between this embodiment and the previously described embodiment is this embodiment employs more confined magnetic fields generated by the use of a number of smaller electromagnets  608  as opposed to one larger electromagnet  320  (FIG. 3) covering an entire MRAM array  308 . While using separate electromagnets  608  (FIG. 6) makes control of this embodiment of the present invention more complex, it does afford greater control and power savings.  
     [0040] Selectively applying current to one or some of the control lines  612  of the appropriate electromagnets  608  can be used to erase or facilitate writing of one only one sector of the MRAM array  604 . This embodiment allows for erasing one section if the MRAM array  604  or facilitating writing to only one section of the MRAM array  604 . As a result, if the electromagnets  608  are to be used to erase data, the erasure can be confined to one sector of the MRAM array  604 . Moreover, using smaller, sector-confined electromagnets  608  to facilitate writing of data to the array saves power. If data is only to be written to one sector of the MRAM array  604 , only the electromagnet  608  disposed at the sector to be written needs to be powered. Less power is required to energize each of the smaller electromagnets  608  than is required to energize a larger electromagnet  320  (FIG. 3) spanning an entire MRAM array  308 . Therefore, this embodiment of the invention reduces power consumption by not using current to generate an unnecessarily large magnetic field spanning parts of the MRAM array  604  to which data is not being written.  
     [0041] An additional embodiment of the present invention is shown in FIG. 7. FIG. 7 shows an MRAM array  704  equipped with an electromagnet  708  disposed diagonally on a surface of the MRAM array  704  and, therefore, poles of the electromagnet are aligned diagonally with respect to poles of the magnetically susceptible elements (not shown) of the MRAM memory cells of the MRAM array  704 . Although a single electromagnet is shown in FIG. 7, it will be appreciated that the MRAM array  704  could be fitted with a plurality of diagonally disposed electromagnets  708 . The embodiment of the invention shown in FIG. 7 provides the same functions as previously disclosed embodiments of the invention. The embodiment of the invention shown in FIG. 7 allows for the erasing of the MRAM array  704  or for the generation of an externally applied electromagnetic field which facilitates magnetizing and writing of the magnetic elements in the MRAM array  704  with the application of lower power to the row lines and column lines of the MRAM array  704 . With its poles disposed diagonally with respect to the poles of the magnetically susceptible elements of the MRAM array  704 , vectorized components of the magnetic field generated by the electromagnet  708  can be used to create the desired magnetic fields previously described with regard to other embodiments of the invention.  
     [0042] In addition, this diagonally disposed electromagnet is well suited for demagnetizing row lines and column lines (not shown) of the MRAM array  704 . If the row and column lines become magnetized, it can create some spurious resistance which possibly can lead to erroneous reading of the data stored in the magnetic elements (not shown). Using the same techniques previously described for erasing an MRAM array  704  or sections thereof by applying a current of reversing polarity and diminishing magnitude, a magnetic field of reversing polarity and diminishing magnitude is created which can erase any magnetization of the row and column lines. The diagonal disposition of the electromagnet  708  in this embodiment applies component fields across the grid formed by the row and column lines to facilitate demagnetization of these lines. It will be appreciated that, because the row and column lines are not comprised of materials which are magnetically susceptible to the same degree as the magnetic elements (not shown), a magnetic field of lesser magnitude will serve to demagnetize the row and column lines. As a result, the row and column lines can be demagnetized without effecting the magnetic fields of the magnetic elements storing the data, because the magnetic field induced to demagnetize the row and column lines will be of insufficient magnitude to repolarize the magnetically susceptible elements as depicted in the hysteresis curves shown in FIGS. 4 and 5.  
     [0043]FIG. 8 depicts a section of a memory subsystem  800  using MRAM devices  804 ,  808 , and  812  for data storage. The MRAM devices  804 ,  808 , and  812 , as previously described, are equipped with electromagnetic devices  816 ,  820 , and  824 , to induce the magnetic fields as previously described. The subsystem  800  is connected to the system (not shown) through a bus  828 . A memory controller  832  receives addresses, data, and commands from the bus  828  and, as appropriate, passes addresses to an address decoder  836  which determines which device or devices store the data to be read or where the data is to be written.  
     [0044] The subsystem  800  includes a read/write control unit  840  to manage the MRAM devices  804 ,  808 , and  812 , coordinating the application of current to the row and column lines (not shown) to effect the magnetization of the magnetic elements (not shown) which store the data. In using embodiments of the present invention, an electromagnet control unit  844  is used to control the externally applied magnetic fields to facilitate erasing, reading, writing, and/or demagnetizing of the MRAM arrays. The electromagnet control unit  844  controls the magnitude and polarity of the currents applied to the electromagnets  816 ,  820 , and  824 , associated with each of the MRAM devices  804 ,  808 , and  812 , respectively. As previously described, the electromagnets  816 ,  820 , and  824 , could each be a network of electromagnets, separately addressable and controllable by the electromagnet control unit  844 , or possibly by control logic (not shown) associated with the individual MRAM devices  804 ,  808 , and  812 .  
     [0045] Working in concert with the read/write control unit  840 , the electromagnet control unit  844  can direct the generation of electromagnetic fields in the appropriate MRAM device  804 ,  808 , or  812 , or section thereof, to facilitate writing to the memory cells in accordance with the preceding descriptions of the embodiments of the present invention. The read/write control unit  840  and the electromagnet control unit  844  work in concert. For example, when data is to be written to MRAM device  804 , the electromagnet control unit  844  would direct the electromagnet  816  to generate a magnetic field of sufficient magnitude and appropriate polarity to facilitate writing of data to the appropriate row and column. As also previously described, the electromagnet control unit  844  could direct the electromagnet  816  to erase the contents of the MRAM device  804 , or to demagnetize the row and column lines of the MRAM device  804 .  
     [0046] Embodiments of the invention can be incorporated into a computer system by one skilled in the art. FIG. 9 is a block diagram of a computer system  910  that includes a processor  912  for performing various computing functions by executing software to perform specific calculations or tasks. The processor  912  is coupled to a processor bus  914  that normally includes an address bus, a control bus, and a data bus (not separately shown). In addition, the computer system  910  includes a system memory  916 , which might comprise the memory subsystem  800  (FIG. 8) employing the network of MRAM devices  804 ,  808 , and  812 . The system memory  916  is coupled to the processor bus  914  by a system controller  920  or similar device, which is also coupled to an expansion bus  922 , such as a Peripheral Component Interface (“PCI”) bus. A bus  926  coupling the system controller  920  to the system memory  916  also normally includes an address bus, a control bus, and a data bus (not separately shown), although other architectures can be used. For example, the data bus of the system memory  916  may be coupled to the data bus of the processor bus  914 , or the system memory  916  may be implemented by a packetized memory (not shown), which normally does not include a separate address bus and control bus.  
     [0047] The computer system  910  also includes one or more input devices  934 , such as a keyboard or a mouse, coupled to the processor  912  through the expansion bus  922 , the system controller  920 , and the processor bus  914 . Also typically coupled to the expansion bus  922  are one or more output devices  936 , such as a printer or a video terminal. One or more data storage devices  938  are also typically coupled to the expansion bus  922  to allow the processor  912  to store data or retrieve data from internal or external storage media (not shown). Examples of typical storage devices  938  include hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). The processor  912  is also typically coupled to cache memory  940  through the processor bus  914 .  
     [0048] It is to be understood that, even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only. Changes may be made in detail, and yet remain within the broad principles of the invention.