Information storage device

Abstract: A magnetic memory element (10) is written to by heating the memory element (10) and applying at least one magnetic field (Hx, Hy) to the memory element (10). More reliable storage is afforded thereby.

Description: The present invention relates to information storage devices, such as Magnetic Random Access Memory ("MRAM") devices. Consider the example of an MRAM device including a resistive cross-point array of spin dependent tunnelling (SDT) junctions, word lines extending along rows of the SDT junctions, and bit lines extending along columns of the SDT junctions. Each SDT junction is located at a cross-point of a word line and a bit line. The magnetisation of each SDT junction assumes one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent logic values of '0' and '1'. The magnetisation orientation, in turn, affects the resistance of the SDT junction. Resistance of the SDT junction is a first value (R) if the magnetisation orientation is parallel and a second value (R+ΔR) if the magnetisation orientation is anti-parallel. The magnetisation orientation of the SDT junction and, therefore, its logic value may be read by sensing its resistance state. A write operation on a selected SDT junction is performed by supplying write currents to the word and bit lines crossing the selected SDT junction. The currents create two external magnetic fields that, when combined, switch the magnetisation orientation of the selected SDT junction from parallel to anti-parallel or vice versa. Too small a write current might not cause the selected SDT junction to change its magnetisation orientation. In theory, both external fields combined should be sufficient to flip the magnetisation orientation of the selected SDT junction. In practice, however, the combined magnetic fields do not always flip the magnetisation orientation. If the magnetisation orientation of the selected SDT junction is not flipped, a write error is made and an increased burden on error code correction can result. SDT junctions that see only one magnetic field (that is, SDT junctions along either a selected word line or a selected bit line) are "half-selected". In theory, a single magnetic field should not flip the magnetisation orientation of an SDT junction. In practice, however, the magnetisation orientation can be flipped by a single magnetic field. If the magnetisation orientation of a half-selected SDT junction is flipped, an undesirable erasure occurs and an increased burden on error code correction can result Document WO 00/79540 discloses a magnetic memory structure having thermally isolated bit structures. Heat is applied during writing by supplying current on word and sense lines to cause a desired temperature rise. Document JP 04023293 discloses a magento-resistant element having a resistivity that can be varied via a heating circuit. The present invention seeks to provide an improved memory device. According to an aspect of the present invention, there is provided an information storage device as specified in claim 1. The present invention also extends to a method of writing to a magnetic memory element which includes the steps of heating the memory element and applying at least one magnetic field to the memory element. The preferred embodiments can improve the reliability of writing to SDT junctions, in particular the reliability of writing to magnetic memory elements of MRAM devices. Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an illustration of an SDT junction; Figures 2a and 2b are illustrations of hysteresis loops for the SDT junction; Figure 3 is an illustration of an MRAM device that can perform thermally-assisted switching; Figure 4 is an illustration of a heating line for the MRAM device; Figures 5a, 5b, 5c and 5d are illustrations of different patterns of heating lines for the MRAM device; and Figure 6 is an illustration of a multi-level MRAM chip. As shown in the drawings, the described embodiment relates to a MRAM device including an array of magnetic memory elements. During data storage, the MRAM device performs thermally-assisted switching of selected memory elements. The thermally-assisted switching improves the reliability of storing data in the MRAM device. A magnetic memory element of the MRAM device could be any element having a resistance that is dependent upon the state of its magnetic film. Examples of such elements include magnetic tunnel junctions (the SDT junction is a type of magnetic tunnel junction) and giant magnetoresistance ("GMR") spin valves. For the purposes of illustration, the memory elements arc described herein as SDT junctions. Reference is made to Figure 1, which shows an SDT junction 10. The SDT junction 10 includes a pinned layer 12 having a magnetisation that is oriented in the plane of the pinned layer 12 but fixed so as not to rotate in the presence of an applied magnetic field in a range of interest. The SDT junction 10 also includes a "free" layer 14 having a magnetisation orientation that is not pinned. Rather, the magnetisation can be oriented in either of two directions along an axis (the "easy" axis) lying in the plane of the free layer 14. If the magnetisation of the pinned and free layers 12 and 14 are in the same direction, the orientation is said to be "parallel" (as indicated by the arrow P). If the magnetisation of the pinned and free layers 12 and 14 are in opposite directions, the orientation is said to be "anti-parallel" (as indicated by the arrow A). The pinned and free layers 12 and 14 are separated by an insulating tunnel barrier 16. The insulating tunnel barrier 16 allows quantum mechanical tunnelling to occur between the pinned and free layers 12 and 14. This tunnelling phenomenon is electron spin dependent, making the resistance of the SDT junction 10 a function of the relative orientations of the magnetisation of the pinned and free layers 12 and 14. For instance, resistance of the SDT junction 10 is a first value (R) if the magnetisation orientation of the pinned and free layers 12 and 14 is anti-parallel and a second value (R+ΔR) if the magnetisation orientation is parallel. Magnetic fields (Hx, Hy) may be applied to the SDT junction 10 by supplying currents (Iy, Ix) to first and second conductors 18 and 20 contacting the SDT junction 10. If the conductors 18 and 20 are orthogonal, the applied magnetic fields (Hx, Hy) will also be orthogonal. When sufficiently large currents (Ix, Iy) are passed through the conductors 18 and 20, the combined magnetic field (Hy+Hx) in the vicinity of the free layer 14 cause the magnetisation of the free layer 14 to rotate from the parallel orientation to the anti-parallel orientation, or vice-versa. For example, a sufficient current Ix will cause the magnetisation orientation to be anti-parallel, whereas a sufficient current Iy will cause the magnetisation orientation to be parallel. Current magnitudes may be selected so that the combined magnetic field (Hx+Hy) exceeds the switching field of the free layer 14 but does not exceed the switching field of the pinned layer 12. However, the magnitude of one or both write currents (Ix, Iy) may be reduced if the SDT junction 10 is heated. Coercivity of a magnetic film decreases with increasing temperature. Raising the temperature of the SDT junction 10 reduces the coercivity (Hc) of the SDT junction 10, as shown in Figures 2a and 2b. Figure 2a shows the coercivity (Hc) at room temperature, while Figure 2b shows the coercivity (Hc) at 50°C above room temperature. At the elevated temperature, the SDT junction 10 switches from a high resistance state to a low resistance state and vice-versa in the presence of a lower combined magnetic field (Hx+Hy). Therefore, heating the SDT junction 10 allows the magnitudes of one or both of the write currents (Ix, Iy) to be reduced. If, on the other hand, the magnitudes of the write currents (Ix, Iy) are not reduced, the SDT junction 10 will switch more reliably in the presence of the combined magnetic field (Hx+Hy). The temperature and write current can be varied to achieve a desired switching reliability. Heat may be applied and removed before the combined magnetic field (Hx+Hy) is applied, or the heat may be applied at the same time as the combined magnetic field (Hx+Hy). The free layer 14 may be heated to about 10°C to 50°C above ambient. More generally, the maximum heating temperature may be about 50°C less than the Blocking temperature T B (the temperature above which the anti-ferromagnetic layer looses its pinning properties). Returning to Figure 1, the heat may be applied to the free layer 14 by a third conductor 22, which is separated from the first conductor 18 by a layer 24 of an electrically insulating, thermally conductive material (e.g., silicon nitride). Even though the current flowing through the third conductor 22 creates an additional magnetic field, the third conductor 22 is far enough away from the SDT junction 10 so that the additional magnetic field does not adversely affect the switching. Although Figure 1 shows the third conductor 22 being above the SDT junction 10, the third conductor 22 may instead be below the SDT junction 10. Third conductors 22 may even be above and below the SDT junction 10. Reference is now made to Figure 3, which illustrates an information storage device 110 including a resistive cross point array 112 of memory elements 114. The memory elements 114 are arranged in rows and columns, with the rows extending along an x-direction and the columns extending along a y-direction. Only a relatively small number of memory elements 114 is shown to simplify the illustration of the information storage device 110. In practice, arrays of any size may be used. Traces functioning as word lines 116 extend along the x-direction in a plane on one side of the memory cell array 112. Traces functioning as bit lines 118 extend along the y-direction in a plane on an adjacent side of the memory cell array 112. There may be one word line 116 for each row of the array 112 and one bit line 118 for each column of the array 112. Each memory element 114 is located at a cross point of a word line 116 and a bit line 118. Traces functioning as heating lines 120 extend diagonally across the array 112. The heating lines 120 may be provided on the top of the array 112, on the bottom of the array 112 or on both the top and the bottom of the array 112. An exemplary construction of a heating line 120 is described below in connection with Figure 4. The information storage device 110 includes a read circuit for sensing the resistance states of selected memory elements 114 during read operations and a write circuit for supplying write currents to selected word lines 116, bit lines 118 and heating lines 120 during write operations. The read circuit is not shown in order to simplify the illustration of the information storage device 110. The write circuit includes a first current source 122 coupled to the word lines 116 by a first group of transistors 124, a second current source 126 coupled to the bit lines 118 by a second group of transistors 128, and a third current source 130 coupled to the heating lines 120 by a third group of transistors 132. During a write operation, a decoder 134 decodes addresses Ax and Ay to select a word line 116, a bit line 118 and a heating line 120. The decoder 134 selects a word line 116 by commanding a transistor 124 of the first group to connect the word line 116 to the first current source 122, a bit line 118 by commanding a transistor 128 of the second group to connect the bit line 118 to the second current source 126, and a heating line 120 by commanding a transistor 132 of the third group to connect the heating line 120 to the third current source 130. Currents flow through the selected word, bit and heating lines 116, 118 and 120. The memory element 114 at the cross-point of the selected word and bit lines 116 and 118 is exposed to the combined magnetic field (Hx+Hy). This selected memory element 114 is also heated by the selected heating line 120. An advantage of diagonally-extending heating lined 120 is that the selected element is heated, but the half-selected elements are not. Figure 3 shows a single current source 122 for the word lines 116, a single current source 126 for the bit lines 118 and a single current source 130 for the heating lines 120. In large arrays, multiple current sources 122 may be provided for the word lines 116, multiple current sources 126 may be provided for the bit lines 118 and multiple currents sources 130 may be provided for the heating lines 120, whereby each current source 122 is shared by multiple word lines 116, each current source 126 is shared by multiple bit lines 118 and each current source 130 is shared by multiple heating lines 120. This allows simultaneous writes to multiple memory elements 114. Other elements of the write circuit are not shown. For example, Figure 3 does not show transistors for connecting "free ends" of the word, bit and heating lines 116, 118 and 120 to a reference potential. Moreover, the transistors 124, 128 and 132 and current sources shown in Figure 3 are a simplification of the write circuit. Circuitry for supplying current to word, bit and heating lines 116,118 and 120 may be implemented in a variety of different ways. Reference is now made to Figure 4, which shows an exemplary construction of the heating line 120. The heating line 120 includes copper traces 120a separated by heating elements 120b made of tungsten or platinum or other highly resistive metal. The heating elements 120b are positioned over the memory elements 114. Figures 5a, 5b, 5c and 5d show different patterns for the heating lines 120. In these patterns the heating lines 120 extend diagonally across the array 112. Moreover, groups of heating lines 120 are tied together to form loops. Current is supplied to one end of a loop and the other end of the loop is tied to a reference potential. This reduces the number of transistors. It also allows heat to be applied to multiple elements crossed by the same bit line. Figure 5a shows the heating lines 120 arranged in a plurality of paths. Each path includes a pair of series-connected heating lines 120. One end of each path is tied to a reference potential, and the other end of each path is coupled to a current source 130 by a transistor 132. In this configuration heat is applied to the selected memory element 114 but not to half-selected memory elements 114. This configuration improves the half-select margin and reduces the likelihood of unwanted erasures. Figure 5b shows multiple heating lines 120 connected in series to form a single path. One end of the single path is tied to a reference potential and the other end of the single path is coupled to a current source 130 by a transistor 132. Each heating line 120 covers memory elements 114 in adjacent rows. Figure 5c shows a pattern similar to that shown in Figure 5b, except that the angle of the heating lines 120 is different. The heating lines of Figure 5c do not cover memory elements 114 in adjacent rows. Instead, each heating line 120 covers a memory element 114 in every other column. Figure 5d shows multiple heating lines having first ends that are tied together. Switches 132a allow currents to be supplied to selected second ends of the heating lines, and switches 132b allow other selected second ends to be connected to a reference potential. This arrangement allows any two heating lines 120 to be selected to form a path. For example, switches 132a and 132b could be selected to form a current path indicated by the dashed line. The switches 132a and 132b may be selected to allow current to flow through multiple heating lines 120 in parallel. This configuration allows for simultaneous writes. Blocks of the patterns described above may be repeated across a large array. For example, a large array might include a plurality of write circuits and groups of bit lines coupled to each write circuit. A pattern of heating lines 120 may be applied to each group of bit lines. Reference is now made to Figure 6, which illustrates a chip 200 having multiple levels or planes 202 of resistive cross point memory cell arrays. The planes 202 are stacked on a substrate 204 and separated by insulating material (not shown) such as silicon dioxide. Read and write circuits may be fabricated on the substrate 204. The read and write circuits may include additional multiplexers for selecting the levels that are read from and written to. The current sources may be on-chip or off-chip. An information storage device of the type described herein may be used in a wide variety of applications. For example, the device may be used for long-term data storage in a computer. Such a device offers many advantages (e.g., faster speed, smaller size) over hard drives and other conventional long-term data storage devices. The device may be used in digital cameras for long-term storage of digital images. It may even replace DRAM and other fast, short-term memory in computers. The device is not limited to switching a memory element by applying two orthogonal magnetic fields to the memory element. For example, a selected memory element may be switched by heat and only a single magnetic field. The present invention is not limited to the specific embodiments described and illustrated above but is to be construed according to the claims. The disclosures in United States patent application no. 09/758,757, from which this application claims priority, and in the abstract accompanying this application, are included herein by reference.

Claims: An information storage device (110) including: an array (112) of magnetic memory elements (114), the memory elements (114) being arranged in rows and columns and are addressed by respective word lines (116) and bit lines (118); and, a plurality of heating elements (120b) for the memory elements (114), characterised in that the plurality of heating elements (120b) are arranged in heating lines (120) extending diagonally with respect to the word (116) and bit (118) lines. A device as in claim 1, wherein the heating elements (120b) are spaced from the memory elements (114). A device as in claim 1 or 2, wherein each heating line (120) includes conductive traces (120a) separated by the heating elements (120b). A device as in claim 1, 2 or 3, wherein groups of heating lines (120) are connected together to form at least one path. A device as in claim 4, wherein the memory elements are arranged in rows and columns, each of the heating lines (120) extending substantially under a memory element (114) in every other column. A device as in claim 4, wherein the memory elements are arranged in rows and columns, each of the heating lines (120) extending substantially under memory elements (114) in adjacent rows. A device as in claim 4, 5 or 6, wherein the heating lines (120) of each group are connected in series. A device as in any one of the preceding claims, wherein the heating lines (120) have first ends that are tied together. A device as in any one of the preceding claims, including switches (124,128, 132) for allowing currents to be supplied to selected ends of the heating lines (120). A device as in any preceding claim, wherein the heating elements (120b) are able to raise the temperature of selected memory elements (114) by about 10°C to 50°C above ambient temperature during write operations.

Inventors: NICKEL JANICE H, TRAN LUNG T, NICKEL, JANICE H., TRAN, LUNG T.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 2001
Main classification: G11C11
IPC codes: H01L27, G11C11, H01L43, H01L21