US20140159175A1 - Spin transfer torque magnetic memory device using magnetic resonance precession and the spin filtering effect - Google Patents

Spin transfer torque magnetic memory device using magnetic resonance precession and the spin filtering effect Download PDF

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US20140159175A1
US20140159175A1 US14/116,959 US201214116959A US2014159175A1 US 20140159175 A1 US20140159175 A1 US 20140159175A1 US 201214116959 A US201214116959 A US 201214116959A US 2014159175 A1 US2014159175 A1 US 2014159175A1
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magnetic layer
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memory device
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Kyung-Jin Lee
Soo-Man Seo
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Samsung Electronics Co Ltd
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Korea University Research and Business Foundation
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    • H01L43/02—
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
    • G11C11/16—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
    • G11C11/16—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect
    • G11C11/161—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/10—Magnetoresistive devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • H10N50/85—Materials of the active region

Definitions

  • the present invention relates to a magnetic memory device, and more particularly, to a spin transfer torque magnetic device that induces an alternating current magnetic field in itself in the injection of a current by inserting a free magnetic layer having horizontal anisotropy into a free layer having perpendicular anisotropy and that includes two fixed magnetic layers having magnetization directions opposite to each other not to deteriorate its characteristics by a stray field occurring from a fixed magnetic layer.
  • a ferromagnetic material means a material that is spontaneously magnetized even though a strong magnetic field is not applied from the outside.
  • a giant magnetic resistance effect that an electric resistance is changed depending on relative magnetization directions of two magnetic layers occurs in a spin valve structure having a non-magnetic material inserted between two ferromagnetic bodies (a first magnetic material/a non-magnetic material/a second magnetic material). This occurs because electric resistances experienced by up-spin and down-spin are different from each other in the spin valve structure.
  • the giant magnetic resistance effect is widely used as a core technique of a sensor for reading data stored in a hard disk.
  • While the giant magnetic resistance effect describes a phenomenon that relative magnetization directions of two magnetic layers control the flow of a current, it is also possible to control a magnetization direction of a magnetic layer using an applied current according to the law of action and reaction, which is Newton's third law.
  • a current is applied to the spin valve structure so that a current spin-polarized by the first magnetic material (a fixed magnetic layer) passes through the second magnetic material (a free magnetic layer) to transfer its spin angular momentum. This is called spin-transfer-torque.
  • International Business Machines Corporation IBM suggested a device having a free magnetic layer of which a magnetization is reversed or continuously rotated using the spin-transfer-torque. Thereafter, the device was experimentally identified. In particular, a magnetic memory device using the spin-transfer-torque is spotlighted as a new memory device replaced with a dynamics random access memory (DRAM).
  • DRAM dynamics random access memory
  • a basic magnetic memory device has the spin valve structure, as described above.
  • a conventional magnetic memory device 100 has a structure of a lower electrode/a first magnetic material 101 (the fixed magnetic layer)/a non-magnetic material 102 /a second magnetic material 103 (the free magnetic layer) of which a magnetization direction is changable by a current/an upper electrode, as shown in the following FIG. 1 .
  • the magnetization reversal of the second magnetic material is induced by a current or magnetic field applied from the outside, and a high resistance and a low resistance are shown by the giant magnetic resistance effect described above. These are applied as data of “0” or “1” written in the magnetic memory device.
  • a half-selected cell problem occurs with the reduction of a size of a device to limit high integration of the device.
  • the spin-transfer-torque occurring by applying a voltage to a device is used, the magnetization reversal of a selected cell is easy irrelevantly to a size of a device.
  • a magnitude of the spin-transfer-torque occurring in the free magnetic layer is proportional to the amount of an applied current density (or a voltage), and a critical current density for the magnetization reversal of the free magnetic layer exits. If all of the fixed layer and the free layer are composed of a material having perpendicular anisotropy, the critical current density J c is expressed by the following equation 1.
  • ⁇ denotes the Gilbert damping constant
  • ⁇ denotes a spin polarization efficiency constant determined by a material and a structure of a device
  • M s denotes a saturation magnetization of the free magnetic layer
  • d denotes a thickness of the free magnetic layer
  • ⁇ 0 denotes an inverse number of an attempt frequency and is about ins
  • V denotes a volume of the device
  • T denotes the Kelvin temperature.
  • K eff V/k B T is defined as a thermal stability factor ⁇ of the magnetic memory device.
  • ⁇ >50 a condition of ⁇ >50 should be satisfied in order that the magnetic memory device maintains its non-volatile characteristic. If the volume V of the free layer is reduced with the reduction of the size of the cell, the K eff should be increased to satisfy the condition of ⁇ >50. As a result, the J c increases according to the equation 1.
  • CMOS complementary metal-oxide-semiconductor
  • the magnetization reversal critical current density of the free layer should be reduced in order to reduce the size of the memory device and in order to realize high integration of the memory device. Also, the magnetization reversal critical current density of the free layer is reduced so that a power used for writing should be reduced.
  • the critical current density of the magnetic memory device is proportional to the effective magnetic anisotropy magnetic field H K,eff .
  • the effective magnetic anisotropy magnetic field H K,eff should be effectively reduced in order to reduce the critical current density of the device.
  • a method of applying a high frequency modulation magnetic field was suggested as the above method.
  • the high frequency modulation magnetic field was applied simultaneously with a magnetic field generated from a writing head of a hard disk drive, thereby reducing a magnitude of a writing magnetic field.
  • This uses a principle that a frequency of an applied AC magnetic field is closer to a resonance frequency of a magnetization of a writing medium to generate the magnetization reversal with a magnetic field lower than an original H K,eff .
  • a method of reducing the critical current density by applying the same principle to a current driving magnetic memory device was experimentally verified. However, the principle and the structure surely require an additional device for inducing the modulation magnetic field, and it was confirmed that a reduction effect of a driving power was less effective in an entire device.
  • the magnetization direction of the second magnetic material is varied by the stray field generated from the first magnetic material in the conventional art shown in the following FIG. 1 .
  • the magnetization direction of the first magnetic material is a +z-axis being a thickness direction of a layer
  • a direction of the stray field also becomes the +z-axis.
  • a ⁇ in the event that the magnetization direction of the second magnetic material is a ⁇ z-axis is smaller than a ⁇ in the event that the magnetization direction of the second magnetic material is the +z-axis, due to the influence of the stray field.
  • the magnetic memory device Since the magnetic memory device has all of the directions being +z and ⁇ z directions of the second magnetic material, thermal stability of the magnetization is determined by a smaller ⁇ of the two events. Thus, characteristics of the device are deteriorated by the stray field generated from the first magnetic material.
  • the saturation magnetization value of a magnetic layer is 650 emu/cm 3 or more, the influence of a corresponding magnetic layer on a neighboring magnetic layer increases to be likely to cause problems on characteristics of the device.
  • the present invention provides a magnetic memory device using spin-transfer-torque and having a lower critical current density and no deterioration of device characteristics caused by a stray field generated from a fixed magnetic layer in order to realize its high integration.
  • a magnetic memory device including: a first fixed magnetic layer; a first free magnetic layer; and a second free magnetic layer.
  • the first fixed magnetic layer is a thin layer formed of a material that has a fixed magnetization direction and that is magnetized in a perpendicular direction to a plane of the layer.
  • the first free magnetic layer is a thin layer formed of a material that has a magnetization direction changed by a current applied from the outside and that is magnetized in a perpendicular direction to a plane of the layer.
  • the second free magnetic layer is a thin layer formed of a material that has a magnetization direction changed by a current applied from the outside and that is magnetized in a horizontal direction to a plane of the layer.
  • a first non-magnetic layer and a second non-magnetic layer are disposed between the first fixed magnetic layer and the first free magnetic layer and between the first free magnetic layer and the second free magnetic layer, respectively.
  • the magnetic memory device may further include a second fixed magnetic layer; and a third non-magnetic layer between the second free magnetic layer and the second fixed magnetic layer.
  • the second fixed magnetic layer may be a thin layer formed of a material that has a fixed magnetization direction opposite to the first fixed magnetic layer and that is magnetized in a perpendicular direction to a plane of the layer.
  • a saturation magnetization value of the material magnetized in the horizontal direction may be in a range of 300 ⁇ 2000 kA/m.
  • the first fixed magnetic layer and the second fixed magnetic layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof.
  • the first fixed magnetic layer and the second fixed magnetic layer may be a multi-thin layer consisting of n stacked double layer (n ⁇ 1), the double layer may consist of an X layer and a Y layer, and the X layer and the Y layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Tb, Zr, Pt, Pd, and a mixture thereof.
  • At least one of the first fixed magnetic layer and the second fixed magnetic layer may have an anti-magnetic structure consisting of a first magnetic layer, a non-magnetic layer and a second magnetic layer.
  • the first magnetic layer and the second magnetic layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof.
  • At least one of the first magnetic layer and the second magnetic layer may be a multi-thin layer consisting of n stacked double layer (n ⁇ 1), the double layer may consist of an X layer and a Y layer, and the X layer and the Y layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof.
  • At least one of the first fixed magnetic layer and the second fixed magnetic layer may have an exchange biased anti-magnetic structure consisting of an antiferromagnetic layer, a first magnetic layer, a non-magnetic layer and a second magnetic layer.
  • the antiferromagnetic layer may be formed of a material selected from a group consisting of Ir, Pt, Mn, and a mixture thereof.
  • the first magnetic layer and the second magnetic layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof.
  • At least one of the first magnetic layer and the second magnetic layer may be a multi-thin layer consisting of n stacked double layer (n ⁇ 1), the double layer may consist of an X layer and a Y layer, and the X layer and the Y layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof.
  • the first fixed magnetic layer and the second fixed magnetic layer may be formed of different materials from each other and may have different multi-thin layer structures from each other.
  • the first free magnetic layer may be formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof.
  • the first free magnetic layer may be a multi-thin layer consisting of: a layer formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, Pd, and a mixture thereof; and a layer consisting of n stacked double layer (n ⁇ 1).
  • the double layer may consist of an X layer and a Y layer.
  • the X layer and the Y layer may be each independently formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, Pt, and Pd.
  • the second free magnetic layer may be formed of a material selected from a group consisting of Fe, Co, Ni, B, Si, Zr, and a mixture thereof.
  • the first non-magnetic layer, the second non-magnetic layer and the third non-magnetic layer may be formed of different materials from each other.
  • the first non-magnetic layer, the second non-magnetic layer and the third non-magnetic layer may be each independently formed of a material selected from a group consisting of Ru, Cu, Al, Ta, Au, Ag, AlO x , MgO, TaO x , ZrO x , and a mixture thereof.
  • electrical conductivities of the first non-magnetic layer, the second non-magnetic layer and the third non-magnetic layer may be higher or lower than those of the first fixed magnetic layer, the first free magnetic layer, the second free magnetic layer and the second fixed magnetic layer.
  • the magnetic memory device may further include: an upper electrode and a lower electrode supplying a current to a device.
  • the magnetic memory device having a new structure according to the present invention further includes the free magnetic layer constituting a horizontal direction variable magnetization layer having a fixed saturation magnetization value, whereby a switching current is markedly reduced as compared with conventional magnetic layers such that a high degree of integration of the device can be achieved and it is possible to lower a critical current density necessary for magnetization reversal thereby reducing a power consumption of the device. Also, a stray field effect occurring from a fixed magnetic layer is reduced such that a written magnetization data is thermally stable.
  • FIG. 1 is a cross-sectional view showing a structure of a conventional magnetic memory device using spin-transfer-torque
  • FIG. 2 is a cross-sectional view showing a structure of a magnetic memory device using spin-transfer-torque according to an embodiment of the present invention
  • FIG. 3 is a cross-sectional view showing a structure of a magnetic memory device using spin-transfer-torque according to another embodiment of the present invention.
  • FIG. 4A is a graph showing an applied current according to a time in the device of FIG. 2 ;
  • FIG. 4B is a graph showing magnetization behavior of a first free magnetic layer according to a time in the device of FIG. 2 ;
  • FIG. 4C is a graph showing magnetization behavior of a second free magnetic layer according to a time in the device of FIG. 2 ;
  • FIG. 4D is a graph showing an alternating current (AC) magnetic field occurring in a first free magnetic layer according to a precessional motion of a second free magnetic layer in the device of FIG. 2 ;
  • AC alternating current
  • FIG. 5A is a graph showing switching probabilities according to an applied current of the structures of FIGS. 1 and 2 ;
  • FIG. 5B is a graph showing values obtained by differentiating switching probabilities according to an applied current of the structures of FIGS. 1 and 2 with a current;
  • FIG. 6 is a graph showing a switching current of a magnetic memory device according to a saturation magnetization value and a spin polarization efficiency of a second free magnetic layer in the device of FIG. 2 ;
  • FIG. 7 (a) is a graph showing switching probabilities according to an applied current of the structures of FIGS. 1 and 3 ;
  • (b) is a graph showing values obtained by differentiating switching probabilities according to an applied current of the structures of FIGS. 1 and 3 with a current;
  • FIG. 8A is a graph showing a switching current density of a magnetic memory device having the structure of FIG. 1 and a switching current according to a saturation magnetization value of a second free magnetic layer of a magnetic memory device having the structure of FIG. 3 ;
  • FIG. 8B is a graph showing a distribution chart of a switching current density of a magnetic memory device having the structure of FIG. 1 and a distribution chart of a switching current density according to a saturation magnetization value of a second free magnetic layer of a magnetic memory device having the structure of FIG. 3
  • the present invention in a magnetic memory device, thermal stability is maintained and a critical current density is reduced to reduce a device size. Thus, high integration is realized and usage power consumption is reduced in writing. Additionally, characteristics of the device are not deteriorated by a stray field generated from a fixed magnetic layer. To achieve these, the present invention provides a new structural magnetic memory device that induces an alternating current (AC) magnetic field in itself and controls this.
  • AC alternating current
  • a magnetic memory device 200 includes a fixed magnetic layer 201 , a first non-magnetic layer 202 , a first free magnetic layer 203 , a second non-magnetic layer 204 and a second free magnetic layer 205 .
  • the fixed magnetic layer is a thin layer formed of a material that has a fixed magnetization direction and that is magnetized in a perpendicular direction to a plane of the layer.
  • the first free magnetic layer is a thin layer formed of a material that has a magnetization direction changed by a current applied from the outside and that is magnetized in a perpendicular direction to a plane of the layer.
  • the second free magnetic layer is a thin layer formed of a material that has a magnetization direction changed by a current applied from the outside and that is magnetized in a horizontal direction to a plane of the layer.
  • the second free magnetic layer 205 having horizontal anisotropy is additionally inserted in the first free magnetic layer 203 having perpendicular anisotropy, so that the magnetization of the second free magnetic layer is rotated by a spin-transfer-torque effect when a current is applied and so that a direct current (DC) is applied to rotate the magnetization in a plane of the free layer with a high frequency.
  • DC direct current
  • an alternating current (AC) magnetic field having the high frequency is autonomously generated to be possible to effectively reduce the critical current density of the device.
  • a magnetic memory device 300 includes a first fixed magnetic layer 301 , a first non-magnetic layer 302 , a first free magnetic layer 303 , a second non-magnetic layer 304 , a second free magnetic layer 305 , a third non-magnetic layer 306 and a second fixed magnetic layer 307 .
  • the first and second fixed magnetic layers are thin layers formed of materials that have fixed magnetization directions opposite to each other and that are magnetized in a perpendicular direction to planes of the layers.
  • the first free magnetic layer is a thin layer formed of a material that has a magnetization direction changed by a current applied from the outside and that is magnetized in a perpendicular direction to a plane of the layer.
  • the second free magnetic layer is a thin layer formed of a material that has a magnetization direction changed by a current applied from the outside and that is magnetized in a horizontal direction to a plane of the layer.
  • the second free magnetic layer 305 having horizontal anisotropy is additionally inserted in the first free magnetic layer 303 having perpendicular anisotropy, so that the second free magnetic layer performs a precessional motion through the spin-transfer-torque effect to generate a high frequency alternating current (AC) magnetic field.
  • the high frequency alternating current (AC) magnetic field is possible to reduce the critical current density of the device.
  • the alternating current (AC) magnetic field by the precessional motion of the second free magnetic layer is determined depending on the current applied from the outside and an effective magnetic field of the second free magnetic layer.
  • an induced magnetic field may be generated in the perpendicular direction to the plane of the second free magnetic layer.
  • the precessional motion of the second free magnetic layer may be controlled depending on a structure and a physical property value of the second fixed free layer. In other words, it is possible to control the alternating current magnetic field generated in the device by a structure and a physical property value of the device as well as an external current.
  • the magnetization direction of the first fixed magnetic layer and the magnetization direction of the second fixed magnetic layer may be controlled to reduce the stray field applied to the first free magnetic layer as compared with a conventional device.
  • the thermal stability of magnetization data written in the device may be improved.
  • FIG. 2 is a cross-sectional view showing a structure of a spin-transfer-torque magnetic memory device 200 according to an embodiment of the present invention.
  • the device according to the present invention basically has a structure including a lower electrode, a fixed magnetic layer 201 having magnetization of a perpendicular direction, a first non-magnetic layer 202 , a first free magnetic layer 203 having perpendicular anisotropy and a magnetization direction changed by a current, a second non-magnetic layer 204 , a second free magnetic layer 205 having horizontal magnetization and a magnetization direction changed by a current, and an upper electrode.
  • a shape anisotropy magnetic field is remarkably greater a magnetic anisotropy magnetic field such that the magnetization direction is stabilized in the plane of the layer.
  • a great angle is maintained between the magnetizations of the first free magnetic layer 203 and the second free magnetic layer 205 in a condition that a current is not applied.
  • the first free magnetic layer 203 receives spin-transfer-torque from electrons that are spin-polarized by the fixed magnetic layer 201 . Also, since the second non-magnetic layer 204 is disposed between the first free magnetic layer 203 and the second free magnetic layer 205 , the second free magnetic layer 205 receives spin-transfer-torque from electrons that are spin-polarized by the first free magnetic layer 203 . In other words, if a voltage is applied to the magnetic memory device, the magnetization of the second free magnetic layer 205 receives the spin-transfer-torque close to perpendicular such that the magnetization of the second free magnetic layer 205 is rotated at high speed.
  • the second free magnetic layer 205 rotated at the high speed provides a modulation magnetic field to the device, thereby obtaining an effect that effectively reduces a critical current density of the first free magnetic layer 203 . If a total resistance of the device is low, a larger amount of the current flows by the same applying voltage to reduce a power required for magnetization reversal.
  • the first non-magnetic layer 202 and the second non-magnetic layer 204 may use a metal having remarkably high electrical conductivity.
  • FIG. 3 is a cross-sectional view showing a structure of a spin-transfer-torque magnetic memory device 300 according to another embodiment of the present invention.
  • the device according to the present invention basically has a structure including a lower electrode, a first fixed magnetic layer 301 having magnetization of a perpendicular direction, a first non-magnetic layer 302 , a first free magnetic layer 303 having perpendicular anisotropy and a magnetization direction changed by a current, a second non-magnetic layer 304 , a second free magnetic layer 305 having horizontal anisotropy and a magnetization direction changed by a current, a third non-magnetic layer 306 , a second fixed magnetic layer 307 having magnetization of a perpendicular direction, and an upper electrode.
  • the second free magnetic layer 305 is stabilized in the plane of the layer by a shape anisotropy magnetic field. As a result, a great angle is maintained between the magnetizations of the two free magnetic layers even though an external magnetic field or a current is not applied.
  • the first free magnetic layer 303 receives spin-transfer-torque from a current spin-polarized by the first fixed magnetic layer 301 .
  • the second non-magnetic layer 304 is disposed between the first free magnetic layer 303 and the second free magnetic layer 305 , the second free magnetic layer 305 receives spin-transfer-torque from a current spin-polarized by the first free magnetic layer 303 .
  • the magnetization of the second free magnetic layer 305 receives the spin-transfer-torque of a component close to perpendicular such that the magnetization is rotated at high speed.
  • the third non-magnetic material 306 is disposed between the second free magnetic layer 305 and the second fixed magnetic layer 307 , spin-transfer-torque and an induced magnetic field are received from a current spin-polarized by the second fixed magnetic layer 307 , and they are parallel or anti-parallel to each other by the applied current and act in a perpendicular direction to the plane of the layer. A frequency and a magnitude of the high alternating current (AC) magnetic field are influenced.
  • AC alternating current
  • the second free magnetic layer 305 rotated at the high speed provides a modulation magnetic field to the device, thereby obtaining an effect that effectively reduces a critical current density of the first free magnetic layer 303 .
  • the modulation magnetic field may be effectively controlled using the current and the induced magnetic field applied through the second fixed magnetic layer.
  • the magnetization direction of the first fixed magnetic layer and a relative magnetization direction of the second fixed magnetic layer may be controlled to reduce a stray field applied to the first free magnetic layer as compared with a conventional device.
  • the thermal stability of magnetization data written in the device may be improved.
  • the first non-magnetic layers 202 and 302 , the second non-magnetic layers 204 and 304 and the third non-magnetic layer 306 may use a metal having a remarkably high electrical conductivity. Additionally, a material having remarkably low electrical conductivity may be used. In this case, a current may be reduced under the same voltage but a magnetic resistance difference according to magnetization rotation by a tunneling effect of electrons may become very great to obtain a high magnetic resistance ratio. Thus, the material having remarkably low electrical conductivity may be used in at least one of the first non-magnetic layer 202 , the second non-magnetic layer 204 and the third non-magnetic layer 206 or in all of the three non-magnetic layers.
  • a structure having a size as small as possible should be realized using a patterning technique in order to obtain a high current density.
  • a section of the device is close to a circle.
  • magnetic shape anisotropy in a plane is the same in any direction such that the high speed rotation of the magnetizations of the first free magnetic layers 203 and 303 and the second free magnetic layers 205 and 305 .
  • the present invention will be described in more detail through preferred embodiments.
  • the present invention is not limited to the embodiments.
  • the present invention is not limited to the following experimental conditions and kinds of materials, etc.
  • the effect of the magnetic memory device according to the present invention was confirmed through micro-magnetic modeling using an equation of motion of magnetization.
  • the justification of this method was efficiently secured through a conventional computer hard disk development and spin-transfer-torque research.
  • “m1” and “m2” denote unit magnetization vectors of the first free magnetic layer 203 and 303 and the second free magnetic layer 205 and 305 , respectively
  • “ ⁇ ” denotes a magnetic rotational constant
  • “H 1 eff ” and “H 2 eff ” denotes entire effective magnetic field vectors of the first free magnetic layer 203 and 303 and the second free magnetic layer 205 and 305 , respectively
  • “ ⁇ ” denotes the Gilbert damping constant
  • “ ⁇ 1 ”, “ ⁇ 2 ” and “ ⁇ 3 ” denote spin polarization efficiency constants of the first free magnetic layer 203 and 303 and the second free magnetic layer 205 and 305 determined by a material and an entire structure of a device
  • “J e ” denotes an applied current
  • P 1 denotes a unit vector showing a spin direction of a spin polarization current inputted from the first fixed magnetic layer 201 and 301 into the first free magnetic layer 203 and 303
  • P 2 denotes a unit vector showing a spin direction of a spin polarization current inputted from the second fixed magnetic layer 307 into the second free magnetic layer 305
  • P 1 and P 2 are unit vectors parallel to a z-axis corresponding to a thickness direction of a layer.
  • FIG. 4A is a graph showing an applied current according to a time.
  • a current pulse having a rise time of 40 ps and a width of 5 ns was applied in order to observe switching behavior of the magnetization.
  • FIG. 4B is a graph showing the magnetization behavior of the first free magnetic layer 203 according to a time.
  • an x-component being a horizontal direction to the plane of the layer oscillates according to the time, and a z-component is changed from +1000 emu/cm 3 to ⁇ 1000 emu/cm 3 at a time (t) of about ins.
  • a magnetization component is switched by the applied current.
  • FIG. 4C is a graph showing magnetization behavior of the second free magnetic layer 205 according to a time.
  • a component in a plane (i.e., an x-axis component) of the magnetization of the second free magnetic layer 205 is very greater than a perpendicular component (i.e., an z-axis component) of the magnetization of the second free magnetic layer 205
  • the second free magnetic layer 205 shows behavior oscillating according to a time with the same period as the magnetization of the first free magnetic layer 203 .
  • the oscillation (i.e., precessional motion) according to the time of the second free magnetic layer 205 occurs by perpendicular directional spin torque spin-polarized by the first free magnetic layer 203 magnetized in a perpendicular direction when the current is applied to the entire structure.
  • FIG. 4D is a graph showing an alternating current (AC) magnetic field generated in the first free magnetic layer 203 by the precessional motion of the second free magnetic layer 205 according to the time.
  • the alternating current (AC) magnetic field is a magnetic field that is generated by the magnetization of the second free magnetic layer 205 in a position of the first free magnetic layer 203 . This occurs because of the precessional motion of the magnetization of the second free magnetic layer 205 according to the time.
  • the alternating current (AC) magnetic field having an x-component and a magnitude of about 200 Oe is autonomously generated in the magnetic memory device structure according to the present invention.
  • the device structure according to the present invention does not require an external additional element generating an alternating current (AC) magnetic field in order to reduce a current density for reversing the magnetization of the first free magnetic layer 203 , unlike a conventional device structure.
  • the x-component of the alternating current (AC) magnetic field induced in the first free magnetic layer is very greater than a z-component of the alternating current (AC) magnetic field.
  • the induced magnetic field reduces anisotropy energy of a magnetization easy axis (z-axis) of the first free magnetic layer 203 such that the magnetization switching of the first free magnetic layer 203 is easy.
  • a sectional area of an entire structure of each of the two structures is 314 nm 2 .
  • the physical property values of the new structure according to the present invention are as follows:
  • perpendicular anisotropy constant (K ⁇ ) 0 erg/cm 3
  • saturation magnetization value (M S2 ) 700 emu/cm 3
  • Gilbert damping constant ( ⁇ ) 0.01
  • spin polarization efficiency constant ( ⁇ 2 ) 1.0”.
  • the fixed magnetic layer, the non-magnetic layer and the free magnetic layer of the conventional structure have the same structures and the same physical property values as the fixed magnetic layer, the first non-magnetic layer and the first free magnetic layer of the new structure according to the present invention.
  • a temperature of the device is 300K, and the experiment was repeated 100 times with respect to each applied current, thereby measuring probability of magnetization switching.
  • FIG. 5A is a graph showing switching probabilities P SW according to applied currents of the new structure ( FIG. 2 ) according to the present invention and the conventional structure ( FIG. 1 ).
  • a switching current is defined as a current having a switching probability P SW of 0.5.
  • the switching current of the new structure was 7.9 ⁇ A and the switching current of the conventional structure was 17.6 ⁇ A. In other words, this means that the switching current is reduced by about 55%.
  • FIG. 5B is a graph showing values obtained by differentiating the switching probabilities shown in FIG. 5A with a current.
  • a Q-factor is a value obtained by dividing an x-axis value of a peak by a width of a distribution functions at a position having a y-axis value of 0.5 (i.e., full width half maximum (FWHM)) in a general probability distribution.
  • the Q-factor is defined as I c / ⁇ I.
  • the Q-factor of the new structure of FIG. 2 according to the present invention is 13.5, and the Q-factor of the conventional structure of FIG. 1 is 3.6.
  • the high Q-factor of the magnetic memory device structure according to the present invention means that the switching probability distribution is small. This means that dispersion of the current applied for changing a magnetization state is small. Thus, the magnetic memory device structure according to the present invention is excellent in commercialization.
  • a temperature of the device is 300K, and switching probability was measured after the experiment was repeated 100 times with respect to each applied current like the experimental example 2.
  • FIG. 6 is a graph showing a switching current with respect to the saturation magnetization value and the spin polarization efficiency of the second free magnetic layer 205 .
  • the switching current is varied according to the saturation magnetization value (M S2 ) of the second free magnetic layer 205 having the horizontal anisotropy.
  • a case having the saturation magnetization value (M S2 ) of 0 emu/cm 3 corresponds to a structure not including the second free magnetic layer, i.e., the conventional magnetic memory device structure of FIG. 1 .
  • the saturation magnetization value (M S2 ) is 300 emu/cm 3 or more
  • the switching current is reduced as compared with the conventional structure regardless of the spin polarization efficiency of the second free magnetic layer 205 .
  • the reduction effect of the switching current is greatest when the saturation magnetization value (M S2 ) is in a range of 300 emu/cm 3 to 500 emu/cm 3 .
  • the induced alternating current (AC) magnetic field of the second free magnetic layer 205 is required in order to reduce the switching current of the first free magnetic layer 203 .
  • the reduction effect of the switching current density is produced when the saturation magnetization value (M S2 ) of the second free magnetic layer 205 is 300 emu/cm 3 or more.
  • the new magnetic memory device structure according to the present invention includes the second free magnetic layer 205 having the saturation magnetization value equal to or greater than a certain value and the horizontal anisotropy such that the switching current is effectively reduced as compared with the conventional structure.
  • a sectional area of an entire structure of each of the two structures is 314 nm 2 .
  • a temperature of the device is 300 k, and the experiment was repeated 100 times with respect to each applied current to measure a probability of the magnetization direction of the free layer (first free layer) switched from an initial direction to an opposite direction.
  • FIG. 7 is a graph showing switching probabilities (P SW ) according to the applied currents of the new structure according to the present invention of FIG. 3 and the conventional structure of FIG. 1 .
  • P SW switching probabilities
  • I opp denotes an applied current density
  • I SW denotes a switching current density
  • ⁇ denotes standard deviation of probability distribution
  • a switching current is defined as a current having a switching probability P SW of 0.5 and is obtained by fitting the equation 4.
  • the switching current is 10.1 ⁇ A in the structure according to the present invention, and the switching current is 18.36 ⁇ A in the conventional structure.
  • the switching current of the magnetic memory device applied with the structure according to the present invention was reduced by about 45%.
  • FIG. 7 is a graph showing values obtained by differentiating switching probabilities P SW according to applied currents of the conventional structure of FIG. 1 and the new structure according to the present invention of FIG. 3 with a current.
  • a Q-factor is a value obtained by dividing an x-axis value of a peak by a width of a distribution functions at a position having a y-axis value of 0.5 (i.e., full width half maximum (FWHM)) in a general probability distribution.
  • the Q-factor is defined as I SW / ⁇ I.
  • the Q-factor of the new structure of FIG. 3 according to the present invention is 7.14 and the Q-factor of the conventional structure of FIG. 1 is 3.43.
  • the standard deviation o is obtained from the equation 4.
  • the standard deviation G of the new structure is 1.21 and the standard deviation o of the conventional structure is 2.26.
  • the magnetic memory device structure according to the present invention has a characteristic of the high Q-factor.
  • the dispersion of the current required to change the magnetization state is small in the new structure. This is an excellent characteristic for commercialization.
  • a sectional area of an entire structure of each of the two structures is 314 nm 2 .
  • the conventional structure of FIG. 1 has the same structure and the same physical property values as considered in the experimental example 1.
  • a temperature of the device is 300 k, and the experiment was repeated 100 times with respect to each applied current to measure a probability of the magnetization direction of the free layer (first free layer) switched from an initial direction to an opposite direction.
  • relative directions of the free magnetic layer (the first free magnetic layer) and the fixed magnetic layer (the first fixed magnetic layer) are anti-parallel to each other before a current is applied, and the magnetization switching of the free magnetic layer (the first free magnetic layer) occurs by an applied current such that the magnetization directions of the free and fixed magnetic layers (the first free and fixed magnetic layers) are arranged to be parallel to each other.
  • FIG. 8A is a graph showing a switching current and a distribution of the magnetic memory device having the conventional structure ( FIG. 1 ) and a switching current according to a saturation magnetization value of the second free magnetic layer 305 in the magnetic memory device having the new structure ( FIG. 3 ).
  • a switching current is varied depending on the saturation magnetization value (M S2 ) of the second free magnetic layer 305 having the horizontal anisotropy.
  • the switching current of the magnetic memory device of the new structure ( FIG. 3 ) according to the present invention is always lower than a value of the conventional structure ( FIG. 1 ) shown by a black full line regardless of the saturation magnetization value of the second free magnetic layer 305 .
  • the reduction effect of the switching current is the greatest when the M S2 is in a range of 300-500 kA/m.
  • FIG. 8B is a graph showing a distribution of a switching current of the magnetic memory device having the conventional structure ( FIG. 1 ) and a distribution of a switching current according to a saturation magnetization value of the second free magnetic layer 305 of the magnetic memory device having the structure ( FIG. 3 ) according to the present invention.
  • the magnetic memory device applied with the new structure according to the present invention has a switching current distribution lower than that of the conventional structure.
  • the switching current distribution is reduced by about 50% when the M S2 is 300 kA/m or more.
  • the magnetic memory device having the new structure according to the present invention effectively reduces the switching current and the distribution by the second free magnetic layer 305 having the horizontal magnetization and the saturation magnetization value equal to or greater than a certain value, as compared with the conventional structure.

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