Results and discussion
3.3 Spin-Valve Like Perpendicular Magnetic Tunnel Junctions without pinned layer in Nano-Dimensioned without pinned layer in Nano-Dimensioned
3.3.1
Introduction
Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy materials have been extensively studied and they have a potential for realizing next generation high density non-volatile memory and logic chips with high thermal stability and low current[1-3]. To achieve the perpendicular anisotropy, various material system have been studied including rare earth/transition metal alloys[4], L10-ordered CoPt and FePt[5,6] and Co/Pt[7] multilayer structure. However, only CoFeB/MgO system has high thermal stability at nano dimensional, low current current-induced magnetization switching and high tunnel magnetoresistance ratio[8].
In process, in order to achieve the nano-scale devices, the lithography etching technique should be not omitted. However, this complex synthesis cost the time and reduced the good rate of devices. The method of electrodeposition has been demonstrated that can synthesize the nano materials[9,10] but it is only limited in metal or semiconductor materials, the insulator as MgO or Al2O3 are impossible. In this work, we explore a simple method that is based on electrochemical[10] and physical deposition[11] to synthesize the nano-scale perpendicular MTJs.
Combination with advantage of synthesis permalloy nanowires by using electrochemical deposition, low cost, timeless and easy process and sputter technique of nano dimensional insulator as MgO and CoFeB with magnetic perpendicular
44 NW/MgO/CoFeB/Ta was prepared by combining techniques of electrochemical plating and magnetron sputter deposition. The magnetic nanowires structure was fabricated by electrochemical deposition into the anodic aluminum oxide template (AAO) and then sputtered the thin films of insulator as MgO, magnetic film as CoFeB and top electrode as Ta. The AAO templates with a diameter of 50 nm and length of 6
m were prepared using a two-step anodizing process. The thin films of MgO, CoFeB and Ta prepared by sputtering were capped with nanowires and during the depositions the base pressure was kept less than 3.0 10−7 Torr. Highly purified Ar gas was kept at a pressure as low as approximately 5.0 10−7 Torr during sputtering in order to obtain a flat surface morphology.
X-ray powder diffraction (XRD, BL-01C2 NSRRC, Taiwan) was performed to elucidate the crystallographic structures. Transmission electron microscopy (TEM, Hitachi H-7100) was performed to detect the shape of nanowires and thickness of multilayer film. The magnetic hysteresis loops were measured using a commercial vibrating sample magnetometer (VSM) with fields of up to 5.0 T at room temperature.
Measurements were made on a silica micro-slide that was held perpendicular and parallel to the magnetic field. The electric characteristic with interaction of magnetic
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field was measured by superconducting quantum interference device (SQUID) under low temperature at 10K.
3.3.3
Results and discussion
A diagram of the device is shown in Fig. 14 and the MTJ structure consist of the magnetic layers with perpendicular anisotropy as like Ni3Fe1 (permalloy) and CoFeB alloy and non-magnetic layer of insulator as like MgO. The Ni3Fe1 with diameter of 50 nm nanowires were electrodeposited into the AAO templates and with the length 6
m. The aspect ratio (height/diameter) of the nanowires is maintained at approximately 120. Due to the magnetic shape anisotropy, the nanowires have the perpendicular anisotropy along the c-axis of the wires. The ends of nanowires were exposed to the templates and slightly to be capped with the MgO layer certainly. The CoFeB layer with magnetic perpendicular anisotropy was deposited capping on the MgO layer and the perpendicular anisotropy is due to magnetocrystalline anisotropy.
The thickness of MgO and CoFeB were modulated to having the largest perpendicular TMR ratio. The spin interactions under the magnetic field at low temperature were studied to understand the spin dynamics on the nano-scale.
The crystal structure and morphology of perpendicular MTJs with removing the bottom electrode gold film were characterized by XRD with a wavelength in 0.77 nm and TEM. Figure 15(a) presents the XRD pattern of perpendicular MTJs in the 2θ region between 15° and 40° and compares it with the standard diffraction peaks of the corresponding constituent materials (JCPDS nos. 65-3244 and 88-2338). Reflection
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peaks are observed at 21.85°, 25.26° and 36.02°, corresponding to the (111), (200) and (220) facets of Ni3Fe1, respectively, and at 18.30° corresponding to the (111) facet of Ta, respectively. No other impurities can be detected from this pattern. These results reveal that the crystal structures of perpendicular MTJs has simple cubic of Ni3Fe1, simple cubic of Ta and no obvious peaks of MgO and CoFeB were obtained.
It indicates that no crystallization structure for MgO and CoFeB layers. The prepared perpendicular MTJs structures were further characterized by using TEM. Figure 15(b) and (c) present the morphology of multilayer films and nanowires. As shown in figure 15(b), the top layer is Ta, the medium layer is CoFeB and the bottom layer is MgO.
The thickness of MgO layer should be exactly dominated to 0.9 nm as same with previous report to get higher TMR ratio. The CoFeB layer was modified to various layers in thickness as 1.5 nm and 1.0 nm. The local elemental composition of the CoFeB layer was studied by EDX microanalysis and the elements ratio of Co, Fe and B is 1:1:1 same with the ratio of target. The function of Ta layer is protection to avoid the oxidation in devices and also as be top electrode for electric characterization. The high resolution lattice image display that the crystallization of MgO and CoFeB are amorphous and match with the result of XRD. The diameter of nanowires can be clearly observed in figure 15(c), equal to around 50 nm and match with the pore sizes of AAO template. The product is composed of a large quantity of wire-like structures with high aspect ratios and length up to few micrometers.
Magnetic properties of the thin film of CoFeB with different thickness and perpendicular MTJs devices are studied by using a commercial vibrating sample magnetometer. Figure 16 are the magnetic hysteresis loops of CoFeB film on MgO/silicon substrates and perpendicular MTJs devices measured at room
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temperature. The hysteresis loops of different thickness of 1.0 nm and 1.5 nm for CoFeB are shown in Fig. 16(a) and (b). The magnetic field was applied along with various angles of 0, 45 and 90 and the range was from 5T to -5T. The thinner film of CoFeB presents the property of paramagnetism like as shown in Fig. 16(a) and the hysteresis loops along various directions are similar with no obviously change. The reason is that the thickness is too thin to make the magnetic spins display the long range order arrangement and the spins will be more attracted by external field. The thicker film of CoFeB presents the property of magnetic perpendicular anisotropy as shown in Fig. 16(b) and the squarness (Mr/Ms) along magnetic field normal with film is larger than along magnetic field parallel with film, where the Mr means the residual magnetization and Ms means the saturation magnetization. Although the crystal structure of thicker CoFeB film is still amorphous, the thicker film offers the enough space to make the spins arrange in long rang order and so the magnetic perpendicular anisotropy can be observed clearly. Figure 16(c) plots the hysteresis loops obtained on the perpendicular MTJs including permalloy nanowires and thin films of MgO and CoFeB in an applied field with different direction. The squarness reveals the change in the spin orientation of the perpendicular MTJs from perpendicular to the wire axis to parallel to it (from 90∘to 0∘as shown in insert Fig. of Fig. 16(c)). The results reveal that the magnetic easy axis tends rapidly to become parallel to the axis of the nanowires as the direction of magnetic field change, and the magnetic perpendicular anisotropy can be modulated by increasing the length of nanowires or the thickness of the CoFeB film. The spins dynamics of CoFeB thin film, permalloy and perpendicular MTJs can be explained and the property of magnetic perpendicular anisotropy have demonstrated for CoFeB film and permalloy nanowires due to the intrinsic magnetocrystalline anisotropy and shape anisotropy individually.
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Figure 17 plots the IV curves of the CoFeB ultra-thin film (1.5 nm), permalloy nanowires and nanometer MTJs. The CoFeB and permalloy display the metallic properties. The nanometer MTJs display the semiconductor properties due to insert the insulator MgO film between the CoFeB film and permalloy nanowires. Figure 18 plots the magnetoresistive loop of perpendicular MTJs and the top electrode, an Au pad, was evaporated on the Ta film. The magnetic field was applied parallel to the nanowires to get high perpendicular magnetoresistance. The measurement conditions were as follows; the source current was 1 μA; the temperature was 10 K, and the magnet field was varied 5k Oe to -5k Oe. The resistance of perpendicular MTJs under zero magnetic field is around 0.016 and the resistance decrease as the spins direction rotation with applying magnetic field. Figure 18(a) displays the magnetoresistance curve of perpendicular MTJs with thinner CoFeB film (1.0 nm). At 3000 Oe the MR ratio was 110% at 10K and the difference between the resistance in the antiparallel and parallel states is around 0.008 . In the positive magnetic field area, the spins are in the high resistance state and it means that spins direction of permalloy nanowires and CoFeB film are opposite. In the negative magnetic field area, the spins are in the low resistance state and it means that spins direction of permalloy nanowires and CoFeB film are the same with magnetic field. Under the positive magnetic field, the magnetoresistance were not changed and still keep around 0.016 and under negative magnetic field, the magnetoresistance was decreased rapidly and the hysteresis effect also can be observed clearly between direction changing of magnetic field. The spins of CoFeB were limited by exchange force between permalloy nanowires and CoFeB film and the exchange force could resist the applied magnetic field to keep the spins distribution of CoFeB film. The result can be explained that the thinner CoFeB film will not display the long range order spin array
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and the quantum limitation of spins also exist which due to the exchange force between permalloy nanowires and CoFeB film, so the spins direction of CoFeB film were fixed. For the thicker CoFeB film (1.5 nm), the magnetoresistance curve is symmetrical and the MR ratio is around 104% at 10K. The long range order spins distribution exist in the thicker CoFeB film and reduce the effect of quantum limitation. The spins direction under the positive and negative magnetic field will be influenced and can observed spins transfer from high resistance state to low resistance state under the positive and negative field area. The quantum limitation in the nano-magnetism was studied and the high TMR ratio for nano-scale perpendicular MTJs was measured.
3.3.4
Conclusions
In summary, the nano-scale perpendicular MTJs synthesized by electrochemical and physical deposition has been demonstrated. The device consists of ferromagnetic nanowires and film with perpendicular anisotropy due to the shape and magnetocrystalline anisotropy individually. A TMR ratio of up to 110% at 10K is achieved in response to magnetic field parallel to the wire axis. The quantum limitation of nano-magnetism also can be observed and as the thickness of CoFeB film increased, the exchange force will be reduced to eliminate the quantum limitation.
The nano perpendicular MTJs have potential applications as key components in the next generation of nanoscale perpendicular magneto-electronic devices, including perpendicular magnetoresistive random access memory devices.
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Herein table 4 is shown in comparing the results with other group of Prof. Ohno.
Fig. 1. Typical top-view SEM image of AAO templates prepared by two-step anodization.
Fig. 2. (a) Current response and corresponding thickness of cobalt and copper metal during electrodeposition of Co/Cu multilayer nanowires and (b) Transmission electron microscopic image of 50 nm diameter nanowire with 40 nm-thick Co layers and 20 nm-thick layers.
Fig. 3. X-ray diffraction patterns of 50 nm-diameter Co/Cu multilayer nanowires with crystalline structure in Co segment with mixed hcp and fcc phases and Cu segment with fcc phase.
Fig. 4. Magnetic hysteresis loops of electrodeposited Co/Cu multilayer nanowires embedded in AAO templates at 300 K for various directions of applied field relative to wire axis. Crystalline structures of Co segment with single fcc phase and fcc and hcp mixed phase (a) applied field parallel to wire axis, (b) applied field perpendicular to wire axis in multilayer nanowires. Aspect ratios of multilayer nanowires with Co/Cu ratios of 0.1, 1 and 2 with (c) applied field parallel to wire axis, (d) applied field perpendicular to wire axis.
Fig. 5. Mechanism of transition of perpendicular magnetic anisotropy: local structure, magnetic domain and net effective anisotropy.
Fig. 6. Magnetoresistive hysteresis curve of 1 μm-long Co (40 nm)/Cu (4 nm) multilayer nanowires electrodeposited into AAO templates at room temperature.
Fig. 7. Typical top-view SEM image of AAO template. Template with pore diameter of 50 nm is prepared by two-step anodization and the thickness equal to 35 m is shown in the inserting figure.
Fig. 8. Thickness of Ni3Fe1 and Cu as a function of time during electrodeposition.
Solid line and dotted line represent thicknesses of Ni3Fe1 and Cu at deposition rates of 1.3nm/s and 0.1 nm/s, respectively.
Fig. 9. X-ray diffraction patterns of as-prepared Ni3Fe1(5 nm)/Cu(4 nm) multilayer nanowires with diameters of 50 nm.
Fig. 10. Transmission electron microscopic image of 50 nm-diameter multilayered Ni3Fe1/Cu nanowires and SAED patterns of layers of Ni3Fe1 and Cu in multilayered nanowires. (a). Ni3Fe/Cu: 50 nm/4 nm, (b) Ni3Fe/Cu: 50 nm/10 nm, (c) Ni3Fe/Cu: 25 nm/25 nm, (d). Ni3Fe/Cu: 25 nm/4 nm.
Ni3Fe1
Cu
Fig. 11. Magnetic hysteresis loops of electrodeposited Ni3Fe1/Cu multilayered nanowires embedded in AAO templates at 300 K with various aspect ratios - 0.1, 1 and 3 – and a pore diameter of 50 nm; field is applied (a) parallel and (b) perpendicular to wire axis.
Fig. 12. Coercivity and squareness (Mr/Ms) of hysteresis loops of nanowires parallel and perpendicular to magnetic field.
Fig. 13. Magnetoresistive hysteresis curve of Ni3Fe1 (150 nm)/Cu (4 nm) multilayer nanowires electrodeposited into AAO templates; magnetoresistance ratio is 26%.
Insert figure is the configuration of magnetoresistance measurement.
Fig. 14. Schematic of a nano scale perpendicular MTJs device for TMR.
Fig. 15. X-ray diffraction patterns and transmission electron microscopic image of perpendicular MTJs. (a) crystal structure of Ni3Fe1 NW/MgO/CoFeB/Ta, (b) image of MgO/CoFeB/Ta and (c) image of Ni3Fe1 NW.
Fig. 16. Magnetic hysteresis loops of CoFeB ultra-thin film with various thickness on MgO/Si substrates and perpendicular MTJs. (a) thickness of CoFeB is 1.0 nm, (b) thickness of CoFeB is 1.5 nm and (c) nano-scale MTJs.
Fig. 17. IV curve of CoFeB film, permalloy nanowires and MTJs
Fig. 18. Magnetoresistive hysteresis curve of Ni3Fe1 NW (6 m)/MgO (0.9 nm)/CoFeB/Ta (10 nm) with different thickness of CoFeB film. (a) thickness of CoFeB is 1.0 nm and (b) thickness of CoFeB is 1.5 nm.
T = 10K
T = 10K
Table 1. The value of coercivity, saturation magnetization, residual magnetization, and squarness of hysteresis loops for diameters of 50 nm Co(fcc)/Cu(fcc) and Co(fcc+hcp)/Cu(fcc) nanowires.
Sampl Hc(Oe) Mr Ms Mr/Ms
H ⊥Co(fcc)/Cu 324 6.25×10-4 3.13×10-3 0.20 H //Co(fcc)/Cu 551 5.61×10-4 2.97×10-3 0.19 H ⊥Co(fcc、hcp)/Cu 304 1.71×10-4 1.59×10-3 0.11 H //Co(fcc、hcp)/Cu 694 6.88×10-4 2.21×10-3 0.31
Table 2. The value of coercivity, saturation magnetization, residual magnetization, and squarness of hysteresis loops for diameters of 50 nm Co(5, 50, 100 nm)/Cu(4 nm) nanowires.
Sampl Hc(Oe) Mr Ms Mr/Ms
H ⊥Co(a. r.=0.1)/Cu 368 3.21×10-3 1.62×10-2 0.20 H //Co(a. r.=0.1)/Cu 489 2.49×10-3 9.54×10-3 0.26 H ⊥Co(a. r.=1.0)/Cu 232 5.20×10-4 3.55×10-3 0.15 H //Co(a. r.=1.0)/Cu 572 1.23×10-3 3.24×10-3 0.38 H ⊥Co(a. r.=2.0)/Cu 109 5.65×10-5 4.14×10-4 0.14 H //Co(a. r.=2.0)/Cu 621 2.43×10-4 5.68×10-4 0.43
Table 3. Coercivity, saturation magnetization, residual magnetization, and squareness of hysteresis loops of Ni3Fe1(5, 50, 150 nm)/Cu(4 nm) nanowires with a diameter of 50 nm.
Sample Hc(Oe) Mr(emu) Ms(emu) Mr/Ms
H ⊥Ni3Fe1 (a. r.=0.1)/Cu 1196.0 2.12×10-5 1.03×10-4 0.210.011
H // Ni3Fe1 (a. r.=0.1)/Cu 1025.1 1.21×10-5 8.27×10-5 0.150.008 H ⊥Ni3Fe1 (a. r.=1.0)/Cu 834.1 3.60×10-5 2.43×10-4 0.150.008
H // Ni3Fe1 (a. r.=1.0)/Cu 1527.6 3.82×10-5 1.81×10-4 0.210.011 H ⊥Ni3Fe1 (a. r.=3.0)/Cu 804.0 2.76×10-4 2.45×10-3 0.110.006
H // Ni3Fe1 (a. r.=3.0)/Cu 25212.6 8.15×10-4 2.28×10-3 0.360.018
Table 4. Comparing the results including MTJs structure, synthesis method, magnetic property and TMR ratio with Prof. Ohno’s group.
Character This study Ohno group
MTJs structure
Our perpendicular MTJs is combined with permalloy nanowires and CoFeB thin film to be the magnetic layers and the MgO thin film is the insulator layer. The permalloy nanowires are 50 nm in diameter and 6 m size of 50 nm and then deposited the MgO, CoFeB and Ta films on the exposed nanowires.
MTJs devices were fabricated by sputtering.
Magnetic shape anisotropy of permalloy nanowires, the MTJs have the axis with out-of-plane saturation field. The sample with tCoFeB = 1.3 nm shows a clear perpendicular easy axis with in-plane saturation field 0HK = 340mT and out-of-plane coercivity 0HC = 1.5mT.
TMR ratio TMR ratio of MTJs with 1.0 nm thick CoFeB layer are 110% at 10 K and with 1.5 nm thick CoFeB
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