Patent Publication Number: US-7902579-B2

Title: Magnetic memory devices using magnetic domain dragging

Description:
PRIORITY STATEMENT 
     This non-provisional U.S. patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2006-0017876, filed on Feb. 23, 2006, in the Korean Intellectual Property Office (KIPO), the entire contents of which is incorporated herein by reference. 
     BACKGROUND 
     Description of the Related Art 
     Magnetic random access memory (MRAM) is a type of non-volatile memory device. Related art MRAMs may include a transistor switching device and a magnetic tunnel junction (MTJ) cell for storing data. The MTJ cell includes a pinned ferromagnetic layer having a pinned or constant magnetization direction, and a free ferromagnetic layer having a magnetization direction that may change to be parallel or non-parallel to the magnetization direction of the pinned ferromagnetic layer. A non-magnetic layer may be disposed between the pinned ferromagnetic layer and the free ferromagnetic layer. Related art MRAMs may have limited data storage capacity because each MTJ cell stores only a single bit of data. 
     SUMMARY 
     Example embodiments relate to magnetic memory devices, for example, magnetic memory devices capable of reading and/or writing multi-bit data using magnetic domain dragging. At least one example embodiment provides a magnetic memory device capable of storing data of a plurality of states by positioning a magnetic domain wall formed at a free layer of the memory device using magnetic domain dragging. 
     According to at least one example embodiment, a magnetic memory device may include a memory region. The memory region may include a free layer having adjoined sectors in which a magnetic domain wall may be formed. A pinned layer may be formed to correspond to the sectors and may have a fixed magnetization direction. A non-magnetic layer may be formed between the free layer and the pinned layer. A magnetic domain wall stopper for stopping the magnetic domain wall may be formed at each boundary between the sectors. The magnetic memory device may further include an input part or port electrically connected to an end of the free layer for inputting a dragging signal for magnetic domain dragging, and a sensor or sensing part for measuring current flowing through the memory region. 
     In at least some example embodiments, the magnetic domain wall stopper may be a pinning groove formed at the boundary of each sector. Each pinning groove may be a pair of pinning grooves formed facing each other at the boundary between each sector. The sector may be trapezoidal in shape, and the magnetic domain wall stopper may be a boundary line formed where a short side of one sector and a long side of an adjacent sector meet. Both ends of the free layer may have a sharp shape. For example, both ends of the free layer may be pointed or have a pointed shape. 
     In at least some example embodiments, a length of each sector may be greater than a length of the magnetic domain wall. The magnetic memory device may include a bit line connected to one end of the free layer and a transistor connected to another end of the free layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will be described in more detail with reference to the attached drawings in which: 
         FIG. 1  is a cross-sectional view of a magnetic memory device, according to an example embodiment of the present invention; 
         FIG. 2  is a perspective view of a memory region of a magnetic memory device, according to an example embodiment of the present invention; 
         FIG. 3  is a series of diagrams illustrating the dependence of the resistance of the memory region shown in  FIG. 2  on the position of a magnetic domain wall, according to an example embodiment of the present invention; 
         FIG. 4A through 4C  are diagrams illustrating an operating method of a magnetic memory device, according to an example embodiment of the present invention; 
         FIG. 5  is a perspective view of a memory region, according to another example embodiment of the present invention; and 
         FIG. 6  is a series of diagrams illustrating the dependence of the resistance of the memory region as shown in  FIG. 5  on the position of a magnetic domain wall, according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Various example embodiments will now be described more fully with reference to the accompanying drawings in which some example embodiments of the invention are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
     Detailed illustrative example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. This invention may, however, may be embodied in many alternate forms and should not be construed as limited to only the example embodiments set forth herein. 
     Accordingly, while example embodiments are capable of various modifications and alternative forms, example embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but on the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to limit the example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed, concurrently, substantially concurrently or executed in the reverse order, depending upon the functionality/acts involved. 
       FIG. 1  is a cross-sectional view of a magnetic memory device, according to an example embodiment. 
     Referring to  FIG. 1 , a source S and a drain D doped with impurities may be formed in a substrate  10 . A gate oxide film  21  and a gate electrode  20  may be stacked (e.g., sequentially stacked) between the source S and the drain D on a portion of the substrate  10 . A memory region  30  for storing data may be formed above the gate electrode  20 . An upper portion of the memory region  30  may have one end electrically connected to a bit line  51  and another end electrically connected to the drain D via a conducting line  52 . 
     The memory region  30  may be formed of, for example, a giant-magneto-resistance (GMR) material; although any other suitable material may be used. The memory region  30  may include a pinned layer  31  having a fixed magnetization direction, a non-magnetic metal layer  32  formed on the pinned layer  31  and/or a free layer  33  formed on the non-magnetic metal layer  32 . The free layer  33  may have a variable magnetization direction (e.g., a magnetization direction and that may be switched and/or may change). The non-magnetic metal layer  32  may be formed of, for example, copper (Cu) and may include a plurality of sectors. A magnetic domain wall DW (not shown in  FIG. 1 ) may be formed in the free layer  33 . The resistance of the memory region  30  may vary based on the position of the magnetic domain wall DW. Data may be represented using a change of resistance in the memory region  30 . The magnetic memory device may further include an insulating layer  60 . 
       FIG. 2  is a perspective view of a memory region, according to an example embodiment of the present invention. The memory region of  FIG. 2  may be used as the memory region  30  in  FIG. 1 . 
     Referring to  FIG. 2 , a plurality of sectors (e.g., five) S 0 ˜S 4  may be formed within the free layer  33 . Pairs of pinning grooves P 1 ˜P 4  may be formed at respective boundaries of the sectors. Sector S 0  may be connected to bit line  51  and sector S 4  may be electrically connected to the drain D. Two magnetic areas or magnetic domains may be formed facing each other at the free layer  33 . At least one magnetic domain wall DW may be formed between the magnetic domains. The magnetic domain wall DW may be positioned (e.g., variably positioned) at a magnetic domain wall stopper. The magnetic domain wall stopper may include pairs of pinning grooves P 1 ˜P 4 . 
     The magnetic domain wall DW may be positioned at each of the pinning grooves P 1 , P 2 , P 3  and P 4 , and each position of the magnetic domain wall DW may represent a (e.g., one) portion and/or piece of information. A magnetic domain having one magnetization direction may be formed at the pinned layer  31 . 
     When a pulse current smaller than (e.g., having a magnitude less than) a critical current is applied to one end of the free layer  33 , the magnetic domain wall DW may not move, but remain in a position located between one of the pairs of pinning grooves. On the other hand, when a pulse current greater than or equal to (e.g., having a magnitude larger than) or equal to the critical current is applied to the free layer  33 , the magnetic domain wall DW may move from between one pair of pinning grooves, where the magnetic domain wall DW has been located prior to the application of the pulse current, to between a pair of adjacent pinning groove. The direction in which the magnetic domain wall DW moves may be determined by the direction of the pulse current. The critical current may be determined, for example, according to the shape of the pinning groove. 
     Ends of the sectors S 0  and S 4  may be tapered to a point in order to suppress (e.g., prevent) other magnetic domain walls DW from forming at sectors S 0  and S 4 . 
     A length of the magnetic domain wall DW may vary depending on the magnetic material used to form the free layer  33 . For example, the length of the magnetic domain wall DW may range from about 30 nm to about 80 nm, inclusive. In at least some example embodiments, a length L, which represents the minimum length of each sector, may be longer than the length of the magnetic domain wall DW. 
     In example embodiments discussed herein, pinning grooves may be formed in pairs; however, example embodiments are not limited thereto. In least one example embodiment, each magnetic domain wall stopper may be formed of one pinning groove or a plurality (e.g., two or more) pinning grooves. 
       FIG. 3  is a series of diagrams illustrating the resistance dependence of the memory region  30  of  FIG. 2  on the position of the magnetic domain wall DW position according to an example embodiment. 
     Referring to  FIG. 3 , in each of the plurality of states STATE  0 ˜ 3 , the magnetic domain wall DW may be positioned between one of the pairs of pinning grooves P 1 ˜P 4  of the free layer  33 . As shown in  FIG. 3 , in STATE  0 , when the magnetization direction of the pinned layer  31  corresponding to the sectors S 1 ˜S 3  is in a direction indicated by arrow B, the magnetization direction of the free layer  33  contained in the memory region  30  may be parallel to the magnetization direction of pinned layer  31 , and the resistance of the memory region  30  may be reduced (e.g., to a minimum). 
     In STATE  1 , the magnetic domain wall DW may be positioned between the pair of pinning grooves P 2 , the magnetization direction of the free layer  33  in sectors S 0  and S 1  may be different from the magnetization direction of the free layer  32  in sectors S 2 , S 3  and S 4 . For example, in sectors S 0  and S 1 , the magnetization direction of the free layer  33  may be different from the magnetization direction of the pinned layer  31 , and in sectors S 2  and S 3  the magnetization direction of the free layer  33  may be the same as the magnetization direction of the pinned layer  31 . 
     As the memory region  30  changes from STATE  1  to STATE  2  and to STATE  3 , the magnetic domain wall DW may move to from left to right in  FIG. 3 . Accordingly, the region having a magnetization direction different from the magnetization direction of the free layer  33  may increase. This may increase the resistance of the memory region  30 . 
     Because the resistance of the memory region  30  changes based (or depending) on the position of the magnetic domain wall DW, which of states STATE  0 ˜ 3  the memory region  30  is in may be determined by measuring the current flowing through the memory region  30 . An information storage device may be realized by writing and/or reading states of in the memory region  30 . 
     A method of operating the magnetic memory device, according to an example embodiment, will now be described in more detail. 
       FIG. 4A through 4C  illustrate an operating method of a magnetic memory device, according to an example embodiment. Referring to  FIG. 4A , the magnetic domain wall DW may be positioned at the pinning groove P 1  in the memory region  30 . In this position, resistance of the memory region  30  may be reduced (e.g., to a minimum) because the magnetization direction of the sectors S 1 ˜S 3  is the same as the magnetization direction of the pinned layer  31 . 
     When a positive pulse current greater than or equal to a critical current (e.g., magnetic drain dragging pulse current) and that overcomes the pinning groove P 1  is applied to an input end of the free layer  33 , the magnetic domain wall DW may move from left to right in  FIG. 4B . The duration of the pulse current may range from about 0.2 ns to about 1.0 ns, inclusive. The duration may vary depending on the length of the sector. The critical current may vary depending on the shape of the pinning groove. 
     When a negative pulse current great than or equal to the critical current is applied to the input end of the free layer  33 , the magnetic domain wall DW may move from right to left in  FIG. 4B . In this example, the magnetic domain wall DW may be moved by applying a positive or negative pulse current to the input end of the free layer  33 . This example process may correspond to a writing process. 
     Referring back to  FIG. 1 , the information or data writing process may address a memory region  30  by applying a positive current greater than or equal to the critical current to the bit line  51  connected to the input end of the free layer  33  and applying a voltage greater than or equal to a threshold voltage at the gate electrode  20  of a transistor. The bit line  51  may serve as an input for inputting a dragging signal used for magnetic domain dragging. 
     A method of reading information stored in the memory region  30 , according to an example embodiment, will now be described with regard to  FIG. 4C . Referring to  FIG. 4C , a voltage may be applied to the input end of the free layer  33 , and current flowing through the memory region  30  may be measured. Because resistance of the memory region  30  may be calculated from the measured current, information stored in the memory region  30  may be read using the measured current. 
     Referring back to  FIG. 1 , a memory region  30  may be addressed by applying a voltage to the bit line  51  and the gate electrode  20 . A current flowing through the memory region  30  may be measured using a sensor (or sensing part, not shown) connected to the source electrode S. The sensor may be, for example, a galvanometer or ammeter. 
       FIG. 5  is a perspective view of a memory region (e.g., memory region  130 ), according to another example embodiment. 
     Referring to  FIG. 5 , sectors S 0 ˜S 4  may be formed in a free layer  133 , and sensor magnetic domain wall stoppers DWS 1 ˜DWS 4  may be formed between sectors S 0 ˜S 4 . Sectors S 0 ˜S 3  may have a trapezoid shape including shorter and longer sides in parallel. A portion where the shorter side of a sector (e.g., S 1 ) contacts the longer side of another sector (e.g., S 2 ) may correspond to a magnetic domain wall stopper (e.g., DWS 2 ). 
     The sector S 0  may be connected to the line  51  (as shown in  FIG. 1 ), and the sector S 4  may be electrically connected to the drain D (as shown in  FIG. 1 ). Two magnetic domains may be formed facing each other at the free layer  133 , and a magnetic domain wall DW may be formed. Each positioning of the magnetic domain wall DW at magnetic domain wall stoppers DWS 1 , DWS 2 , DWS 3  and DWS 4  may represent at least one piece of information. A magnetic domain having a fixed magnetization direction may be formed in a region corresponding to sectors S 0 ˜S 4  at pinned layer  131 . 
     When a pulse current less than a critical current is applied to an input end of the free layer  133 , the magnetic domain wall DW may not pass through the stopper and may not move. On the other hand, when a pulse current greater than or equal to the critical current is applied to the input end of the free layer  133 , the magnetic domain wall DW may pass through the stopper and stop at an adjacent stopper. Because a region of the longer side has a higher energy than that of the shorter side in a sector, after entering a sector the magnetic domain wall DW may move toward the stopper position, which may be the position of the shorter side having a lower energy. The magnetic domain wall may stop at the shorter side of the sector. 
     The length of the magnetic domain wall DW may vary depending on the magnetic material used to form the free layer  33 , and may range from about 30 nm to about 80 nm, inclusive. A length L, which may be the length of at least one or all sectors, may be longer than the length of the magnetic domain wall DW according to an example embodiment. 
       FIG. 6  is a series of diagrams illustrating a dependence of the resistance of the memory region  130  on the position of a magnetic domain wall DW. 
     Referring to  FIG. 6 , in states STATE  0 ˜ 3 , the magnetic domain wall DW may be positioned at one of the magnetic domain wall stoppers DWS 1 ˜DWS 4  in the free layer  133 . When the magnetization direction of the pinned layer  131  corresponding to the sectors S 0 ˜S 4  is in the direction indicated by arrow B, at least some (e.g., most) sectors (e.g., S 1 ˜S 4 ) may have magnetization directions of the free layer  133  in the memory region  130  for STATE  0  may be the same as the pinned layer  131 , and the resistance of the memory region  130  may be lower or relatively low (e.g., at a minimum). 
     In STATE  1 , the domain wall DW may be positioned at the magnetic domain wall stopper DWS 2 , and the magnetization direction of sectors S 0  and S 1  may be different from the magnetization direction of the pinned layer  131 . Accordingly, the resistance of the memory region in STATE  1  may be larger than the resistance of the memory region in STATE  0 . 
     As the memory region  130  changes from STATE  1  to STATE  2  and to STATE  3 , the magnetic domain wall DW may move from left to right in  FIG. 6 . The region having a magnetization direction different from the magnetization direction of the free layer  133  may increase, which may increase the resistance of the memory region  130 . 
     Because the resistance of the memory region  130  depends on the position of the magnetic domain wall DW, determining which of states STATE  0 ˜ 3  the memory region  130  is in may be determined by measuring the amount of current flowing through the memory region  130  due to an applied voltage. Therefore, an information storage apparatus may be realized by writing and/or reading the STATEs  0 ˜ 3  in the memory region  130 . 
     The method of writing information on the memory region and reading the information, according at least the example embodiment illustrated in  FIGS. 2-4 , may be the same or substantially the same as the example method shown in  FIGS. 5 and 6 , and therefore, detailed descriptions thereof will be omitted for the sake of brevity. 
     As described above, magnetic memory devices using magnetic domain dragging, according to at least some example embodiments may be used to store multi-bit data (e.g., a number of pieces of information or data), the number being the same or substantially the same as the number of magnetic drain wall stoppers formed in a memory region. 
     Magnetic memory devices, according to at least some example embodiments, may increase data storage capacity. Accordingly, the magnetic device such as the MRAM having an increased data storage capacity may be realized using example embodiments. 
     Although discussed herein as being comprised of copper Cu, the non-magnetic metal layer may be formed of any suitable metallic material. 
     While the present invention has been particularly shown and described with reference to example embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims, and the present invention is not limited to the example structures and arrays illustrated herein.