Abstract:
Spin current generators and systems and methods for employing spin current generators. A spin current generator may be configured to generate a spin current polarized in one direction, or a spin current selectively polarized in two directions. The spin current generator may by employed in spintronics applications, wherein a spin current is desired.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/555,940, which was filed on Jul. 23, 2012, now U.S. Pat. No. 8,462,544, which issued on Jun. 11, 2013, which is a continuation of U.S. patent application Ser. No. 13/012,661, which was filed on Jan. 24, 2011, now U.S. Pat. No. 8,228,717, which issued on Jul. 24, 2012, which is a continuation of U.S. patent application Ser. No. 12/242,228, which was filed on Sep. 30, 2008, now U.S. Pat. No. 7,876,603, which issued on Jan. 25, 2011. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The invention relates generally to current generators, and more particularly, to spin current generators for spintronics applications. 
     2. Description of Related Art 
     This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art. 
     The development of microelectronics has led to large increases in integration density and efficiency. However, the conventional electronic methods of operation by applying voltage to control electron charge are fundamentally limited. Further improvements in nonvolatility, speed, and size of electronic devices may require advancements in new technology. Spintronics, or spin electronics (also known as spin transport electronics and magnetoelectronics), refers to the study of the spin of an electron in solid state physics and the possible devices that may advantageously use electron spin properties instead of, or in addition to, the conventional use of electron charge. 
     The spin of an electron has two states and is characterized as being either “spin up” or “spin down.” Conventional spintronics devices have relied on systems that provide bidirectional current to alter the electron spins in the device. For example, one spintronics application involves data storage through a spintronics effect known as giant magnetoresistance (GMR). The GMR structure includes alternating ferromagnetic and nonmagnetic metal layers, and the magnetizations and electron spins in each of these magnetic layers provide resistance changes through the layers. The resistance of the GMR may change from low (if the magnetizations are parallel) to high (if the magnetizations are antiparallel), and the inducing and detecting of such magnetoresistance changes are the basis for writing and reading data. Another example of spintronics devices includes spin torque transfer magnetic random access memory (STT-MRAM). STT-MRAM also exploits electron spin polarity by utilizing the electron spin to switch the magnetization of ferromagnetic layers to provide two programmable states of low and high resistance. 
     This alteration of magnetization typically employs a bidirectional programming current to change the magnetizations of the layers in a memory cell. However, bidirectional programming logic requires more cell space. A transistor select device is required for each memory cell, and this also increases the cell area. Furthermore, bidirectional programming logic is generally more complicated and less efficient than unidirectional programming logic. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Certain embodiments are described in the following detailed description and in reference to the drawings in which: 
         FIG. 1  depicts a block diagram of a processor-based system in accordance with an embodiment of the present technique; 
         FIG. 2  depicts a device architecture and method by which a spin current generator may enable a spintronics device in accordance with embodiments of the present technique; 
         FIG. 3  depicts a spin current generator capable of generating non-polarized or adjustably polarized current in accordance with embodiments of the present invention; and 
         FIG. 4  depicts a spin current generator capable of generating adjustably polarized current in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Spintronics devices write and store information by manipulating electron spin in a particular orientation. As previously discussed, information may be stored by programming magnetic layers in a memory cell into low resistance and high resistance states. Switching between the two resistance states typically employs a bidirectional programming current, where a current passed in one direction may orient the magnetization of memory cell layers to a low resistance state, and a current passed in an opposite direction may orient the magnetization of memory cell layers to a high resistance state. Since bidirectional programming logic requires more complicated circuitry and more chip space, a method of generating electron currents with desired spin polarizations may reduce the complexity and size of memory cell area or other devices requiring currents of different polarities by facilitating unidirectional programming. The following discussion describes the systems and devices, and the operation of such systems and devices in accordance with the embodiments of the present technique. 
       FIG. 1  depicts a processor-based system, generally designated by reference numeral  10 . As is explained below, the system  10  may include various electronic devices manufactured in accordance with embodiments of the present technique. The system  10  may be any of a variety of types such as a computer, pager, cellular phone, personal organizer, control circuit, etc. In a typical processor-based system, one or more processors  12 , such as a microprocessor, control the processing of system functions and requests in the system  10 . As is explained below, the processor  12  and other subcomponents of the system  10  may include resistive memory devices manufactured in accordance with embodiments of the present technique. 
     The system  10  typically includes a power supply  14 . For instance, if the system  10  is a portable system, the power supply  14  may advantageously include a fuel cell, a power scavenging device, permanent batteries, replaceable batteries, and/or rechargeable batteries. The power supply  14  may also include an AC adapter, so the system  10  may be plugged into a wall outlet, for instance. The power supply  14  may also include a DC adapter such that the system  10  may be plugged into a vehicle cigarette lighter, for instance. 
     Various other devices may be coupled to the processor  12  depending on the functions that the system  10  performs. For instance, a user interface  16  may be coupled to the processor  12 . The user interface  16  may include buttons, switches, a keyboard, a light pen, a mouse, a digitizer and stylus, and/or a voice recognition system, for instance. A display  18  may also be coupled to the processor  12 . The display  18  may include an LCD, an SED display, a CRT display, a DLP display, a plasma display, an OLED display, LEDs, and/or an audio display, for example. Furthermore, an RF sub-system/baseband processor  20  may also be coupled to the processor  12 . The RF sub-system/baseband processor  20  may include an antenna that is coupled to an RF receiver and to an RF transmitter (not shown). One or more communication ports  22  may also be coupled to the processor  12 . The communication port  22  may be adapted to be coupled to one or more peripheral devices  24  such as a modem, a printer, a computer, or to a network, such as a local area network, remote area network, intranet, or the Internet, for instance. 
     The processor  12  generally controls the system  10  by implementing software programs stored in the memory. The software programs may include an operating system, database software, drafting software, word processing software, and/or video, photo, or sound editing software, for example. The memory is operably coupled to the processor  12  to store and facilitate execution of various programs. For instance, the processor  12  may be coupled to the system memory  26 , which may include spin torque transfer magnetic random access memory (STT-MRAM), magnetic random access memory (MRAM), dynamic random access memory (DRAM), and/or static random access memory (SRAM). The system memory  26  may include volatile memory, non-volatile memory, or a combination thereof. The system memory  26  is typically large so that it can store dynamically loaded applications and data. In some embodiments, the system memory  26  may include STT-MRAM devices, such as those discussed further below. 
     The processor  12  may also be coupled to non-volatile memory  28 , which is not to suggest that system memory  26  is necessarily volatile. The non-volatile memory  28  may include STT-MRAM, MRAM, read-only memory (ROM), such as an EPROM, resistive read-only memory (RROM), and/or flash memory to be used in conjunction with the system memory  26 . The size of the ROM is typically selected to be just large enough to store any necessary operating system, application programs, and fixed data. Additionally, the non-volatile memory  28  may include a high capacity memory such as a tape or disk drive memory, such as a hybrid-drive including resistive memory or other types of non-volatile solid-state memory, for instance. As is explained in greater detail below, the non-volatile memory  28  may include STT-MRAM devices manufactured in accordance with embodiments of the present technique. 
     Both the system memory  26  and the non-volatile memory  28  may include memory cells programmable by manipulation of electron spin or other spintronics components. For example, the memory cells may include MRAM cells, STT-MRAM cells, or memory cells that utilize the giant magnetoresistive (GMR) effect. The system memory  26  and the non-volatile memory  28  may further include a spin current generator to generate single-spin polarity current (i.e., a current that can be generated with a spin polarity in only one direction), bi-spin polarity current (i.e., a current that can be generated with a spin polarity in either direction), non-polarized current or arbitrary spin-polarized current to program the memory cells, as will be further described below. 
       FIG. 2  depicts an example of a portion of a spintronics device and a method by which a spin current generator  100  may be used to program the device in accordance with embodiments of the present technique. The portion of the spintronics device illustrated here includes an array  102  of memory cell components  104  with magnetic layers  106  and  108 . As will be appreciated, each memory cell component  104  may form the memory portion of a single memory cell in the array  102 . The memory cell components  104  may include magnetic tunnel junctions (MTJs), stacks of ferromagnetic and nonmagnetic layers, or any other structure in which magnetization may be manipulated to alter the structure&#39;s magnetoresistance state. Furthermore, the memory cell components  104  may be components of magnetic random access memory (MRAM) cells, spin torque transfer magnetic random access memory (STT-MRAM) cells, or any other device exploiting the manipulation of electron spin to program the cell. 
     In this example, the memory cell component  104  includes a pinned layer  106  and a free layer  108 . A memory cell may be “written” or “programmed” by switching the magnetization of the free layer  108  in the memory cell component  104 , and the cell may be read by determining the resistance across the free layer  108  and the pinned layer  106 . The layers  108  and  106  may comprise ferromagnetic materials, such as Co, Fe, Ni or its alloys, NiFe, CoFe, CoNiFe, or doped alloys CoX, CoFeX, CoNiFeX (X═B, Cu, Re, Ru, Rh, Hf, Pd, Pt, C), or other half-metallic ferromagnetic material such as Fe3O4, CrO2, NiMnSb and PtMnSb, and BiFeO, for instance. The pinned layer  106  is so named because it has a magnetization with a fixed or pinned preferred orientation, and this is represented by the unidirectional arrow illustrated in the pinned layer  106 . An additional layer of antiferromagnetic material may be deposited below the pinned ferromagnetic layer to achieve the pinning through exchange coupling. The bidirectional arrow illustrated in the free layer  108  represents that the free layer  108  may be magnetized either in a direction parallel to the pinned layer  106 , which gives a low resistance, or in a direction antiparallel to the pinned layer  106 , which gives a high resistance. The memory cell component  104  may also include a nonmagnetic layer between the free layer  108  and the pinned layer  106  to serve as an insulator between the two layers  108  and  106 , thereby forming a MTJ structure in this example. The nonmagnetic layer may include materials such as AlxOy, MgO, AlN, SiN, CaOx, NiOx, HfO2, Ta2O5, ZrO2, NiMnOx, MgF2, SiC, SiO2, SiOxNy, for example. 
     The spin current generator  100  is connected to each memory cell in the array  102  through source lines  110 . In the presently illustrated embodiment, each of the memory cell components  104  is coupled in series to form a string, such that each of the memory cell components  104  is coupled to a common source line  110 . When a memory cell is selected to be programmed, the spin current generator  100  sends a spin polarized current through the source line  110  to the selected memory cell and memory cell component  104 . If the memory cell is to be programmed to a low resistance state (“write 1 operation”)  114 , the spin current generator  100  will generate a current polarized in one direction (e.g., to the left)  116 , and the left-polarized current will switch the magnetization of the free layer  108  to the left. Because the magnetization of the pinned layer  106  is also directed to the left, the magnetizations of the free layer  108  and the pinned layer  106  are parallel, and the memory cell is programmed to a low resistance state. Likewise, if the memory cell is to be programmed to a high resistance state (“write 0 operation”)  118 , the spin current generator  100  will generate a current polarized in an opposite direction (to the right)  120 , and the right-polarized current will switch the magnetization of the free layer  108  to the right. Because the magnetization of the pinned layer  106  is directed to the left, the magnetizations of the free layer  108  and the pinned layer  106  are antiparallel, and the memory cell is programmed to a high resistance state. 
     The method depicted in accordance with embodiments of the present technique thus enables the memory cells or other spintronics devices to be programmed by a unidirectional current, allowing for simpler unidirectional programming logic. As previously discussed, conventional spintronics devices, including STT-MRAM devices, typically use bidirectional programming logic, meaning the write current is driven in opposite directions through a device cell stack to switch the cell between different programmable states. For example, in a STT-MRAM cell, a write current may be driven from a transistor source to a transistor drain, and then through a MTJ to program the memory cell to a high resistant state. To program a memory cell to a low resistance state, a write current may be driven from a MTJ to a transistor drain to a transistor source. Unidirectional programming logic may be simpler and more efficient than bidirectional programming logic. Also, the array  102  may be fabricated without a separate transistor for each cell, which further decreases cell size and cost. By utilizing a spin current generator  100  which may generate a spin current polarized in either direction (a bi-spin polarity current), the memory cell component  104  may be programmed with a unidirectional current, as described further below. Further, in certain embodiments, a single-spin polarity current, or a non-polarized spin current may be utilized to program a memory cell component  104 , by adding certain features or layers to the memory cell component  104 , such that the memory cell component  104  is able to exploit the properties of the current to facilitate the changing of the magnetization of a free ferromagnetic layer, therein. For example, in  FIG. 2 , if a non-polarized current is passed through the memory cell component  104 , the pinned layer  106  may reflect the current towards the free layer  108  and switch the magnetization direction of the free layer  108  to the opposite direction of the pinned layer  106 . 
     One embodiment of the present invention, a spin current generator configured to generate a unidirectional current to adjust polarization direction in a spintronics device, is illustrated in  FIG. 3 , where a spin current generator  200  can generate a non-polarized current  212  or a single-spin polarized current  214 . The bidirectional arrows depicting the non-polarized current  212  represent that the current is not yet polarized in any direction. Conversely, the unidirectional arrow depicting the polarized current  214  represent that the current is polarized in one direction (single-spin polarized). The spin current generator  200  includes a spin-polarizing layer  202 , which may comprise ferromagnetic materials, such as Co, Fe, Ni or its alloys, NiFe, CoFe, CoNiFe, or doped alloys CoX, CoFeX, CoNiFeX (X═B, Cu, Re, Ru, Rh, Hf, Pd, Pt, C), or other half-metallic ferromagnetic material such as Fe3O4, CrO2, NiMnSb and PtMnSb, and BiFeO, for instance. The spin current generator  200  may also include a nonmagnetic layer  204  which may be nonconductive and include some combination of AlxOy, MgO, AlN, SiN, CaOx, NiOx, HfO2, Ta2O5, ZrO2, NiMnOx, MgF2, SiC, SiO2, or SiOxNy, for example, or conductive and include some combination of Cu, Au, Ta, Ag, CuPt, CuMn or other nonmagnetic transition metal, for example. The spin-polarizing layer  202  and the nonmagnetic layer  204  may be isolated from a material  206  by an insulative material  208 . In some embodiments, the material  206  may generate heat (“heater material”), and in other embodiments, the material  206  may comprise piezoelectric materials (“piezoelectric material”). 
     In some embodiments, the material  206  may incorporate some combination of heat generating and piezoelectric materials, or the material  206  may comprise more than one heat generating and/or piezoelectric material. As used in the present specification, the term “layer” refers to materials formed in parallel, with one material disposed over another (e.g., layers  204 ,  202 , and  210  of  FIG. 3 ). In contrast other materials, not referred to as layers, may be formed perpendicular to a stack of parallel materials (e.g., layers  206  and  208  are perpendicular to layers  204 ,  202 , and  210  of  FIG. 3 ), as spacers other structures formed adjacent to the layers. As also used herein, it should be understood that when a layer is said to be “formed on” or “disposed on” another layer, the layers are understood to be parallel to one another, but there may be intervening layers formed or disposed between those layers. In contrast, “disposed directly on” or “formed directly on” refers to layers in direct contact with one another. Similarly, if materials are said to be “adjacent” to other materials, the materials are in the same cross-sectional plane (e.g., the layer  206  is adjacent to the layers  202 ,  204  and  210 ). Further, if a material is said to be adjacent to another material or layer, there may be intervening materials therebetween, while “directly adjacent,” connotes no intervening materials therebetween. 
     Since heat decreases magnetization and spin-polarization efficiency in magnetic materials, a heater material  206  may apply heat to decrease or eliminate the magnetization or spin polarization of the spin-polarizing layer  202 , and the spin current generator  200  may output a non-polarized or less spin polarized current  212 . Specifically, when voltage is applied to the spin current generator  200  through the transistor  216 , the heater material  206  may heat up the spin-polarizing layer close to or above its curie temperature, which may be in a range of approximately 160° C. to 300° C. The spin-polarizing layer  202  would then substantially lose its magnetization, and current would be non-polarized or not highly polarized after it passes through the demagnetized spin-polarizing layer  202 . The spin-polarizing layer  202  may retain its magnetization through an exchange interaction with the antiferromagnetic layer  210  when the spin-polarizing layer  202  is cooled to approximately room temperature. Thus, the spin current generator  200  may produce a unidirectional non-polarized current to program a spintronics device. One example of how a unidirectional non-polarized current may program a spintronics device is to pass non-polarized current that becomes spin polarized by magnetic layers of fixed magnetization in a spintronics device. Further, magnetic layers may be switched by reflected currents polarized by other layers in a spintronics device. 
     Alternatively, the spin current generator  200  may produce polarized current  214  of various polarization degrees through a transient stress effect induced by the piezoelectric stress material  206 . The piezoelectric stress material  206  may apply varying stress to adjust the spin polarization of the spin-polarizing layer  202 . When voltage is applied to the piezoelectric stress material  206  through the transistor  216 , the piezoelectric stress material  206  may induce a stress that modulates the spin-polarization efficiency of the spin-polarizing layer  202  such that the current output of the spin current generator  200  may be polarized to a desired degree. 
     Specifically, the spin polarization degree of the output current is determined by the spin-polarization efficiency of the spin-polarizing layer  202 , which may be adjusted by either heat or stress to the spin-polarizing layer  202 . If the spin current generator  200  sends a polarized current  214  to a spintronics device, voltage may be applied to the spin current generator  200 , and the piezoelectric material  206  may generate a transient stress in the spin-polarizing layer  202 . The transient stress influences the spin-polarization efficiency of the spin-polarizing layer  202 , which affects the degree of polarization of the output current. Thus, embodiments in accordance with the present technique may produce unidirectional single-spin polarized current to switch the magnetization of a spintronics device. The direction of the spin current that may be output by the spin current generator  200  is dependent on the arrangement of the transistor  216 , as will be appreciated. 
     The heater material  206  may comprise refractory metals including, for example, nitride, carbide, and Boride, TiN, ZrN, HfN, VN, NbN, TaN, TiC, ZrC, HfC, VC, NbC, TaC, TiB2, ZrB2, HfB2, VB2, NbB2, TaB2, Cr3C2, Mo2C, WC, CrB2, Mo2B5, W2B5, or compounds such as TiAlN, TiSiN, TiW, TaSiN, TiCN, SiC, B4C, WSix, MoSi2, or elemental materials such as doped silicon, carbon, Pt, Niobium, Tungsten, molybdenum, or metal alloys such as NiCr, for example. In some embodiments, the piezoelectric material  206  may be composed of a conductive piezoelectric material, such as (TaSe4)2I, multi-layered AlxGal-xAs/GaAs, BaTiO3/VGCF/CPE composites, or other piezoelectric/conductive material composites. In other embodiments, the piezoelectric material  206  may be an insulative material, such as berlinite (AlPO 4 ), quartz, gallium orthophosphate (GaPO 4 ), langasite (La 3 Ga 5 SiO 14 ), ceramics with perovskite or tungsten-bronze structures such as barium titanate (BaTiO 3 ), SrTiO3, bismuth ferrite (BiFeO 3 ), lead zirconate titanate (Pb[Zr x Ti 1-x ]O 3  0&lt;x&lt;1), Pb 2 KNb 5 O 15 , lead titanate (PbTiO 3 ), lithium tantalate (LiTaO 3 ), sodium tungstate (Na x WO 3 ), potassium niobate (KNbO 3 ), lithium niobate (LiNbO 3 ), Ba 2 NaNb 5 O 5 , and other materials such as ZnO, AlN, polyvinylidene fluoride (PVDF), lanthanum gallium silicate, potassium sodium tartrate, sodium potassium niobate (KNN). The nonmagnetic layer  204  may insulate the magnetic layers of the spin current generator  200  from other magnetic layers and may be either conductive or nonconductive. The conductive nonmagnetic layer  204  may comprise Cu, Au, Ta, Ag, CuPt, CuMn, or other nonmagnetic transition metals, or any combination of the above nonmagnetic conductive materials. The nonconductive nonmagnetic layer  204  may comprise Al x O y , MgO, AlN, SiN, CaO x , NiO x , HfO 2 , Ta 2 O 5 , ZrO 2 , NiMnO x , MgF 2 , SiC, SiO 2 , SiO x N y , or any combination of the above nonmagnetic nonconductive materials. 
     Another embodiment of the present invention is illustrated in  FIG. 4 , where a spin current generator  300  can generate arbitrary spin current or spin current polarized in either direction (bi-spin polarity). As used in the present specification, arbitrary spin current refers to spin current polarized in either direction with any desired polarization degree. The spin current generator  300  uses two structures  302  and  304  of opposite magnetization. Each structure has a respective spin polarizing layer  306  and a nonmagnetic layer  308 . The spin-polarizing layer  306  in the structures  302  and  304  have opposite magnetizations, and this enables the spin current generator  300  to generate current spin-polarized in an arbitrary degree for a spintronics device, or in a specified direction based on the selection of the appropriate transistor  314  or  316 . The spin current generator  300  may be employed in applications and systems benefiting from a spin current generator capable of producing a bi-spin polarity current (i.e., a current with a spin-polarity in either direction), such as the memory cells of  FIG. 1 . 
     For example, if a memory cell (as in  FIG. 2 ) is selected to be programmed to a low resistance state, a current would pass through the structure  304  of the spin current generator  300 , via the transistor  316 , where the spin-polarizing layer  306  polarizes the spin of the electrons to the left. The spin current generator  300  then outputs a programming current spin polarized to the left  310 , and the left-polarized current  310  switches the magnetization of free layer  108  (of  FIG. 2 ) to the left, parallel to the pinned layer  106 , writing the cell in a low resistance state. If a memory cell is selected to be programmed to a high resistance state, a current would pass through the structure  302  of the spin current generator  300 , via the transistor  314 , where the spin-polarizing layer  306  polarizes the spin of the electrons to the right. The programming current is spin polarized to the right  312 , and the right-polarized current  312  switches the magnetization of free layer  108  to the right, antiparallel to the pinned layer  106 , writing the cell in a high resistance state. 
     The spin-polarizing layer  306  may comprise ferromagnetic materials, such as Co, Fe, Ni or its alloys, NiFe, CoFe, CoNiFe, or doped alloys CoX, CoFeX, CoNiFeX (X═B, Cu, Re, Ru, Rh, Hf, Pd, Pt, C), or other half-metallic ferromagnetic material such as Fe3O4, CrO2, NiMnSb and PtMnSb, and BiFeO, for instance. The nonmagnetic layer  308  may insulate the magnetic layers of the spin current generator  300  from other magnetic layers and may be either conductive or nonconductive. The conductive nonmagnetic layer  308  may comprise Cu, Au, Ta, Ag, CuPt, CuMn, or other nonmagnetic transition metals, or any combination of the above nonmagnetic conductive materials. The nonconductive nonmagnetic layer  308  may comprise Al x O y , MgO, AlN, SiN, CaO x , NiO x , HfO 2 , Ta 2 O 5 , ZrO 2 , NiMnO x , MgF 2 , SiC, SiO 2 , SiO x N y , or any combination of the above nonmagnetic nonconductive materials. 
     This embodiment and other embodiments in accordance with the present technique may be used in spintronics applications, or in conjunction with or incorporated in any device using electron spin properties. As an example, STT-MRAM cells are programmed into low or high resistance states by switching the magnetization of a free ferromagnetic layer in the memory cell. As previously discussed, the memory cell is programmed to a low resistance state when a programming current switches the magnetization of the free layer to be parallel with the magnetization of a pinned layer in the STT-MRAM cell. The memory cell is programmed to a high resistance state when a programming current switches the magnetization of the free layer to be antiparallel with the magnetization of the pinned layer in the STT-MRAM cell. The typical STT-MRAM cell is structured with bidirectional programming logic, as the free layer requires programming current polarized in both directions, depending on the resistance state it will be switched to. In the embodiments of the present technique, a spin current generator capable of generating current polarized in either direction, or not polarized at all, may allow for simpler unidirectional programming logic in the STT-MRAM cell or other spintronics components. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.