Nano-scale Patterned Magnetic Tunnel Junction and Its Device Applications

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1 Celebrating 20 Years of GMR Past, Present, and Future (II) Nano-scale Patterned Magnetic Tunnel Junction and Its Device Applications X. F. Han, Z. C. Wen, Y. Wang, L. Wang and H. X. Wei State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing , China X. F. Han Z. C. Wen Y. Wang L. Wang H. X. Wei Nano-scale patterned techniques, DCpulsed-current-driven magnetization switching and spin-dependent transport properties of magnetic tunnel junctions (MTJs) are presented in this paper. The experiments show that the spin transfer torque (STT) plays a main switching role in the magnetization current switching and the current-induced circular magnetic field plays an assisted-switching role in the nanoring-mtjs. Such nano-scale MTJs can be used to develop the STT-MRAM, nano-oscillator, current-driven magnetic logic, spin torque diode and double barrier MTJ spin-transistor etc. We also summarize recent research on spin-dependent transport properties in MgO(001)-based MTJs using first-principles calculations, including tunneling mechanism, interfacial resonance states in single barrier Fe/MgO/Fe MTJs, and quantum-well resonance in double barrier MTJs. Keywords: Nano-scale MTJ, spin transfer torque effect, MRAM, nano-oscillator, current-driven magnetic logic, spin torque diode, double barrier MTJ spin-transistor. 1. INTRODUCTION Since 1995 the large tunneling magnetoresistance (TMR) effect at room temperature was found in the Al-O barrier-based magnetic-tunnel-junction (MTJ)[1, 2], there has been an increasing motivated intense research because of its technological and potential applications in a series of magnetic devices and promoting the magneto-electronics and spin-electronics developments [3, 4]. Based on the continuous efforts by many researchers and engineers on the MTJ structural and material optimizations, the first successful application of MTJ and TMR was in computer TMR-read-head techniques, which resulted in the magnetic recording density in hard disk drive (HDD) system continuously increasing from Gbit/inch 2 mainly based on amorphous Al-O barrier MTJs [5-7] to present Gbit/inch 2 based on single crystal MgO(001) barrier MTJ [8, 9] combined with longitudinal and perpendicular magnetic recording media development. It has led to the distinct improvements of computer and information technology (IT) industries with each passing day during last decade. The second application of MTJ and TMR was to develop magnetic random access memory (MRAM) devices in last ten years [10-12] because the MRAM has many predicted coexistence advantages including the non-volatility, anti-radiation, unlimited endurance, high speed, high density and low power consumption, and so on. Although the newfangled MRAM devices with the capacity of 16 Mbit and 4 Mbit using the Astroid and Toggle model designs were fabricated respectively [13, 14], an expected MRAM device with the density or capacity more than 256 Mbit/inch 2 and even 1 Gbit/inch 2 have not yet been realized based on either the conventional magnetic field magnetization switching or the elliptic/rectangularshaped MTJ structures in the state of the present MRAM devices. The main cause is that the stray field energy and the shape anisotropy energy of a nano-scale size MTJ increase distinctly with increasing density, resulting in the very large coercivity and switching field and further higher power consumption compared. From this viewpoint, conventional and magnetic field magnetization switching ellipticor rectangular-shaped MTJ bit cell may restrict the present MRAM device development. Therefore, how to reduce power consumption and at the same time increase thermal stability are major challenge in developing ever smaller magnetic elements and higher density for advanced magnetic memory technology [15, 16]. Fortunately, recent theoretical and experimental investigations on spin transfer torque effects both in giant magneto-resistance (GMR) nanopillar and nano-scale MTJs provide us a possible new approach for developing the novel MRAM devices [17-20]. Further investigations on the nano-patterned Al-O and MgO(001)-barrier based MTJs com- 24 AAPPS Bulletin December 2008, Vol. 18, No. 6

2 Nano-scale Patterned Magnetic Tunnel Junction and Its Device Applications bined with spin-polarized current switching suggested the possible MRAM designs with high-density magnetic memory cells, enhanced thermal stability and reduced power consumption [21-26]. The currentdriving MRAM devices with the density or capacity over than 1 Gbit/inch 2 can be expected to come true in next few years. The third kind of important applications for MTJ and TMR is to develop multifarious magneto-sensitive sensors since the high TMR effect was discovered. The micro- and nano-scale patterned MTJs can be used as the key magneto-sensitive elements for/in designing the various digital sensors, such as very weak magnetic field sensors [27, 28], magnetic bio-sensor [29-31], magneto-audiphone & hearing aids, motorcar magnetic sensors, magnetic anti-counterfeit detector and TMR cash detector, location sensors in vernier calipers and machine tools, angle or speed or accelerated-speed sensors for taximeter, tachometer, cyclometer or accelerometer etc. The other sorts of concernful applications for nano-scale patterned MTJs and TMR effect can be expected for developing the magnetic logic devices [32], nano-oscillators [33-36], spin torque diode [37], double-barrier tunneling transistors [38, 39] etc., which can be hopefully carried out some remarkable improvements of information technology (IT) and semiconductor integrate circuit industries in the near future. In this paper we briefly introduce recent investigations on Nano-scale patterned techniques, DC-pulsed-current-driven magnetization switching, spin-dependent transport properties of magnetic tunnel junctions (MTJs) and their possible applications in spin-electronic devices. We also summarize recent research on spin-dependent transport properties in MgO(001)-based MTJs using first-principles calculations, which can be referenced for designing the STT-MRAM, nano-oscillators, magnetic logic, spin-transistor, and other spintronic devices in future. 2. NANO-FABRICATION METH- OD AND NANO-SCALE MTJ PROPERTIES Nowadays, with science and technology developing rapidly, there are many methods to deposit nanoscale thick metallic or nonmetallic films, such as molecular beam epitaxy (MBE), sputtering, electron beam evaporation, chemical vapor deposition (CVD), and so on. MTJ films are mainly comprised of nanoscale magnetic multilayers insulated by a nonmagnetic layer, together with seed layers and capping layers. Magnetron sputtering has been widely applied to deposit MTJ stack layers due to high deposition rate and films quality. For an instance, selecting a substrate Si(100)/SiO 2 and cleaning it with conventional method first, a typical MTJ stack layers with the spin-valve-type structure of Ta(5)/Ir 22 Mn 78 (10)/Co 75 Fe 25 (2)/ Ru(0.75)/ Co 60 Fe 20 B 20 (3)/Al(d nm)-oxide/ Co 60 Fe 20 B 20 (2.5)/Ta(3)/Ru(5) (thickness unit: nm) were then deposited on the substrate using an ULVAC TMR R&D Magnetron Sputtering System (MPS HC7) with a base pressure of Pa. The Al oxide barrier was made by inductively coupled plasma (ICP) oxidizing Al layer with the thickness of d = 0.6 ~ 1.0 nm and an oxidation time of 10 ~ 60 s in a mixture of oxygen and argon at a pressure of 1.0 Pa in a separate vacuum chamber. An in-plane magnetic field of about 100 Oe was applied to induce the uniaxial magnetic anisotropy of the magnetic layers. After deposition, the MTJ films were subsequently annealed at 265 for one hour to achieve optimal TMR ratio. Here the TMR ratio is defined as TMR = (R AP R P )/R P. The nano-fabrication process of nanosized MTJs can be based on electron beam lithography, extreme ultraviolet lithography process, or nano-print technique etc. The MTJ stack films deposited on the Si(100)/SiO 2 substrate were patterned into bottom electrode by ultraviolet optical lithography combined with argon ion beam milling and reactive ion etching (RIE). The nano-size active MTJ area was patterned by electron beam lithography (EBL) with a Raith 150 scanning electron microscope using positive E-beam resist of polymethyl methacrylate (PMMA) and negative E- beam resist of hydrogen silsesquioxane (HSQ) techniques. The nano-sized MTJ pillar including the top resist was then buried by SiO 2 deposition in a magnetron sputtering system. Finally, the resist and SiO 2 on the top of a nano-size MTJ were removed using a lift-off process mentioned above, before the top electrode was patterned in the perpendicular direction. The transport properties was measured using a standard four probe method or a Physical Properties Measurement System (PPMS). For example, Fig. 1 shows scanning electron microscope (SEM) images for arrays of nano-ring MTJs with outer diameters of 100, 300, 500, and 700 nm, respectively, where the ring width is 30 nm. Fig. 2 shows SEM images for an array of nano-size elliptic MTJs with the long and short axis of 40 and 30 nm. Figure 3 shows the tunnel resistance (R) and TMR ratio versus spin-polarized current (I) loops driven by a pulse DC current for a typical spin-valve-type Nanoring-MTJs with the outer diameter of 100 nm and ring-width of 25 nm and barrier thickness of Al(0.60 nm)-oxide. For the NR-MTJ with a size of 5.89 knm 2, its TMR ratio of 18.2%, lower and higher resistant of 285 Ω and 337 Ω, minimum and maximum resistance-area product RA of 1.68 Ωμm 2 and 1.98 Ωμm 2 at RT were observed respectively. The critical value I C of switching current between 500 and 540 μa for the free Co 60 Fe 20 B 20 (2.5 nm) layer was observed at RT. The experiments and further simulations in nano-size ring-shaped MTJs show that the spin transfer torque is a dominant factor for the observed magnetization currentswitching [24, 25]. Furthermore, the pulse DC current induces a circular magnetic field (Oersted field) of smaller than 20 (Oe) at the outer boundary of the 100 nmdiameter ring layer, which can affect the magnetization states as assisted-switching AAPPS Bulletin December 2008, Vol. 18, No. 6 25

3 Celebrating 20 Years of GMR Past, Present, and Future (II) Fig. 3: The tunnel resistance (R) and TMR ratio as a function of the spin-polarized current at RT for a typical spin-valve-type NR-MTJ with an outer diameter of 100 nm and ring-width of 25 nm. The barrier was formed with thickness of Al (0.60 nm) layer and the oxidation time of 10 s. mechanism. Such TMR & R versus I loops driven by a pulse spin-polarized DC current in the nano-scale MTJs show good quadrate shape for a basic device element applications. The critical switching current from antiparallel (parallel) to parallel (antiparallel) states using Co60Fe20B20 (2.5 nm) free layer in such MTJs are smaller than 1.0 ma; this corresponds to the critical switching current density about JC = A/cm2. Such JC value can be expected to further decrease by optimizing FM materials, barrier materials, nano-mtj size and structure to fit the lowest-powerconsumption requirement in the device applications. the spin-polarized current, in which the shape of the magnetic element has been patterned into elliptic or rectangular shape. Undesired properties such as a large shape anisotropy and a strong stray field from each magnetic element would limit the memory cell density increase. Therefore, it is desirable to use nanoring-shaped MTJs whose magnetization directions can be directly controlled by the spin-polarized current and STT effect simultaneously to decrease the power consumption and improve the thermal stability for developing the MRAM as shown in Fig Current Driving Magnetic Logic Nanoscale magnetic materials and structures have been usually used in data storage and sensor applications. However, another potential application based on magnetic phenomena is the logic program device application though there are relatively fewer studies on magnetic logic. Spin transfer torque effect offers superior prospects to realize magnetic logic based on nanoscale elliptic or nanoring-shaped MTJs due to simply architecture of devices and lower power consumption, comparing with previous attempts of magnetic logic elements [32]. This novel concept for STT magnetic 3. PATTERNED MTJ AND ITS logic devices is based on one single napotential DEVICE APPLI- noscale MTJ cell composed of soft ferromagnetic (FM) layer, barrier, and hard CATIONS 3.1. STT and Nano-ring MRAM Devices Fig. 1: SEM images of the typical nano-ring Some experiments demonstrated that the MTJ arrays with outer diameters of 100 (a), 300 (b), 500 (c), and 700 (d) nm. The ring power consumption for switching the width is 30 nm. spin-valve pillars [21] or MTJs [22] can be decreased by using the STT effect from Fig. 2: SEM images of the typical nano-size elliptic MTJ arrays with the long and short axis of 40 and 30 nm. 26 FM layer sandwiched by capping and seed layer with current of logic operations input and output electrodes, and read in/out electrodes. Fig. 5 shows the schematic graph of magnetic logic device based on nanosize elliptic MTJ with spin-polarized Fig. 4: A prototype 2 2 MRAM demo device based on one NRMTJ and one transistor structure with spin polarized current switching. Here the word line, also as the gate line, plays the role of addressing each bit together with the cross bit line. AAPPS Bulletin December 2008, Vol. 18, No. 6

4 Nano-scale Patterned Magnetic Tunnel Junction and Its Device Applications current driving. Fig. 6(a) is one example for logic AND operation. We demonstrate the operation sequences of the logic AND gate. Firstly, we set the initial state R = low (0) of MTJ by inputting current -I (0) from electrodes A, B, and C. The magnetization of the soft FM layer is parallel to the alignment of the hard FM layer. This configuration is low resistance state of MTJ cell defined as logical 0 of output, meanwhile, we define positive (negative) Fig. 5: Schematic graph of magnetic logic device based on nanosize elliptic or nanoring shaped MTJ with spin-polarized current driving. current as shown in Fig. 5 at the electrodes A and B as logical 1 (0) of the inputs. The logic operation sequence as follows: (1) The parallel configuration of magnetization can not be changed by applying 0 (negative current) from electrode A and B which corresponds to an output logical 0. (2) The input electrode A and B are applied 0 and 1 (negative and positive current) individually. The direction of magnetization is unchanged due to lower density of the net current through the MTJ. The same case takes place if 1 is applied at the input electrode A which is corresponding to output 0. (3) As applying the logical 1 (positive current) at both input electrode A and B, the magnetization direction of soft FM layer is switched by the STT or spin polarized current. The logic output 1 is obtained by the antiparallel configuration of magnetization. The logic AND is displayed completely in the truth table. Also, we can get logic function of OR, NOR, NAND, and XOR in similar way whose truth tables are shown in Fig. 6(b). Although there are still some difficulties to realize magnetic logic function, such as technology of nanofabrication and stabilization of devices, the magnetic logic devices based on spin-polarized current open up way for the possibility of realizing all magnetic information processing systems incorporated by both magnetic memory and logic Patterned MTJ and Superconductor-ring Mixed Magneto-sensitive Device Figure 7 shows a typical TMR magnetic sensor using Wheatstone bridge design for the general applications. The upper right and bottom left drawing show one bridge branch with two sensors leg1 and leg2 comprised of two uncovered MTJs. The upper left and bottom right show other bridge branch with one pair of leg3 and leg4 configured of two MTJs which were shielded by the permalloy film. Using such Wheatstone bridge design, the sensitivity of TMR sensor can be increased comparing with a magnetosensitive sensor consist of only one single patterned MTJ element. For increasing the break-down voltage and anti-jamming stability each bridge leg can also consist of series-wound MTJs with same numbers. The only sensors capable of detecting less than ft (10-15 Tesla) magnetic field have been suggested based on low-temperature superconducting loop and GMR element mixed devices, operating at 4.2 K [28]. Here, we show a magnetic field sensor design that combines a superconducting flux-to-field transformer with a low-noise patterned-mtj sensor as shown in Fig. 8. It was designed a flux-to-field transformer formed of a large superconducting loop closed by a micrometer-sized constriction. When an external low-frequency field H a is applied perpendicular to the superconducting loop, a supercurrent is established to maintain the flux through the loop. If this current flows through a micro- or nano-scale constricted area, its high surface current density will lead to a very high coplanar magnetic field above and below the narrow part of the loop. This field can be detected by the TMR sensor, which is sensitive to the in-plane field. Therefore, this superconductor/mtj mixed devices can be expected to detect Fig. 6: (a) Logic AND operation; (b) Truth tables for logic function of OR, NOR, NAND, and XOR. Fig. 7: A typical TMR magnetic sensor using Wheatstone bridge design for the general applications. AAPPS Bulletin December 2008, Vol. 18, No. 6 27

5 Celebrating 20 Years of GMR Past, Present, and Future (II) Fig. 8: A magnetic field sensor design that combines a superconducting flux-to-field transformer with a low-noise patternedmtj sensor. Fig. 9: 2DBZ resolved transmission probability at Fermi energy as a function of kǁ. (a), (b) and (c) for Fe(001)/MgO/Fe; (d), (e) and (f ) for Fe(001)/Co(1)/MgO(8)/Co(1)/Fe. (a) and (d), Majority-spin channel; (b) and (e), Minority-spin channel; (c) and (f ), either majority-to-minority or minority-to-majority spin channels in antiparallel configuration. extremely weak magnetic fields up to ft level. The sensor can be operated up to 77 Kelvin with a high-tc superconductors materials. 4. FIRST-PRINCIPLES CALCULATIONS OF SPIN-DEPENDENT TRANSPORT PROPERTIES IN MGO-BASED MTJS Recently another breakthrough in MTJs is the recognition of the fact that the spin-dependent tunneling is not only determined 28 by the properties of the ferromagnetic electrode, but also depends on the electronic structure of the insulator barrier as well as the electrode/barrier interfaces. In 2001 giant TMR effect was predicted in epitaxial Fe(001)/MgO(001)/Fe(001) MTJs by first-principles calculations [40]. Succedent success [41, 42] in achieving large TMR ratio of about 200% at room temperature in both sputtered and epitaxial MTJs with MgO(001) barrier confirmed this theoretical prediction. Now experi- AAPPS Bulletin December 2008, Vol. 18, No. 6 mental samples [43] using the MgO(001) barrier have achieved the TMR ratio of over 500% at room temperature. These amazing discovery raises both scientific and commercial interest and accelerates the development of new spin-electronics devices. Here first-principles calculations play an significant role for exploiting and understanding deep fundamental physics involved in exciting phenomena including tunneling mechanism [40, 44], quantumwell (QW) resonance [45, 46], interfacial resonance (IR) states [47, 48]. The main results of these calculations are summarized as below Tunneling Mechanism in Single Barrier Fe/MgO/Fe MTJs Butler et al. first predicted that Fe(100)/ MgO(100)/Fe(100) MTJs show huge TMR ratio due to strong filtering effect of majority 1 states [40] in the MgO barrier layer. They used first-principles based calculations of the tunneling conductance and magnetoconductance within the layered Korringa-Kohn-Rostoker (LKKR) implementation [49] of the local spin density approximation (LSDA) of the density functional theory. Comparing with simple spin-dependent tunneling models including the much used Julliere model, the first-principles calculations were able to show the underlying electronic structure of MTJs more realistically and indicated how the electronic structures of both Fe electrodes and MgO barrier determine the TMR. They proved the difference in the decay rates for different symmetries of the Bloch states in majority-spin and minority-spin channels. Majority channel tunneling is dominated by the transmission through 1 states at small values of transverse crystal momentum in the two dimensional Brillouin zone (2DBZ), while minority channel tunneling as well as both spin channels in antiparallel configuration is much smaller and is strongly enhanced for values of kǁ near IR states, also called hot spots, which is be shown in Fig. 9(a), (b) and (c), respectively. A huge TMR effect is therefore expected due to large difference between the conductances in parallel and antiparallel configurations.

6 Nano-scale Patterned Magnetic Tunnel Junction and Its Device Applications 4.2. Effect of Co Interlayers in Fe/MgO/ Fe MTJs Theoretically the interfacial resonance states in Fe/MgO/Fe MTJs can dramatically reduce of the TMR at zero bias [40, 47]. Thus modification of the interface to reduce the IR contribution has been considered [47], for example, by inserting 1 ML Ag as interlayer at the Fe/MgO interfaces. However, Ag is not a good choice to use as an interlayer because it has strong spin-orbit coupling which would generate more spin-flip scattering. A very small mismatch of the lattice constant between experimental bulk bcc Co (2.82 Å) and Fe (2.86 Å), and same majorityspin 1 electron filter effect in MgO(001) for Co electrode as Fe [40, 44], makes Co a very good candidate for use as the interlayer at the interface of Fe/MgO/Fe. For Fe(001)/Co(001)/MgO(001)/ Co(001)/Fe(001) MTJs with Co interlayers, calculated transmission probabilities as a function of k ǁ for 2DBZ are shown in Fig. 9(d), (e) and (f) for majority-spin channel, minority-spin channel and either spin channels in AP configuration, respectively. Because only majority-spin 1 states from Co and Fe/Co can be effectively transmitted through MgO(001) barrier [44], the conductance in majorityspin channel, which is shown in panel (d), is dominated by the states near the Γ point in 2DBZ, almost as same as majority-spin channel of Fe/MgO/Fe shown in panel (a). However, a notable feature of both panels (e) and (f) in Fig. 9 is the strong suppression of the IR state contributions that dominate the conductance channels for Fe/MgO/Fe in panels (b) and (c). The dramatic change caused by 1 ML of Co interlayer is because of the interface electronic structure and Fe-Co hybridization which makes the interface resonant band more dispersive, similar to the result for Fe/Ag(1 ML)/MgO sturcture [47]. We also confirmed that the IR states would appear again in minority-spin channel in P configuration as well as both spin channels for AP configuration by further increasing the Co thickness, and both the intensity of resonance effect and the positions of the Fig. 10: s-resolved partial DOS at the Γ point within the central Fe layer in Fe(001)/MgO/Fe7/MgO/Fe MTJs. Solid line, ma jority spin; dashed line, minority spin. Red and blue lines represent the DOS for the middle atomic layer and the one next to the middle layer in Fe7 film, respectively. n indicates the number of nodes in the wave function. IR states change with the thickness of the Co layer [48]. Also it should be noted that experimental reports of giant TMR in fully epitaxial -Fe(001)/Co/MgO/Fe- and -Fe(001)/Co/ MgO/Co/Fe- junctions with bcc Co(001) electrodes [50, 51] show higher TMR ratios compared to similar epitaxial Fe/MgO/ Fe MTJs. Although theoretically the Co electrodes have been proved to be superior to the Fe electrodes [44], the Co layers of 0.57 nm (about 4 ML) in Refs. [50] and [51] are more likely to act as interlayers instead of electrodes. Thus reduction of the contribution from IR states by Co interlayers should be responsible for the enhancement of TMR effect in such experiments Quantum-well Resonance in Fe/ MgO/Fe/MgO/Fe Double Barrier MTJs Another interesting phenomena is the spindependent resonant tunneling through QW states, which has been studied variously using both single and double barrier MTJs. Earlier experiments of single barrier MTJs [52, 53] observed very small effect that was attributed [45] partly to smearing from the amorphous barrier layers. Recent success [42] in achieving large TMR ratio of over 200% at room temperature in Fe/MgO/Fe(100) epitaxial single barrier MTJ raised the expectation of observing the QW resonance effect in MgO-based MTJs. Because of the importance of the Fe(100) 1 band in MgO-based MTJs with Fe electrodes, and the predominant s-character of the 1 band and its preferential transmission in the MgO barrier layer, this band is also the primary candidate for producing QW resonances [45]. Indeed, Nozaki et al. [54] observed oscillations in di /dv as a function of the bias voltage, although the effect is still too small to significantly affect the TMR. Similar to single barrier MTJs, first-principles theory is also a critical step in understanding the QW resonances in double barrier junctions. In Fig. 10, we show the calculated s-resolved partial DOS at the Γ point within the central Fe layer in Fe(001)/MgO/Fe7/ MgO/Fe with a 7 ML thickness of the Fe layer. We assume that all of the Fe layer moments are aligned parallel in this system. In Fig. 10(a) several sharp spikes in the DOS indicate the existence of majority-spin QW states derived from the 1 band of the Fe middle layer, because the Γ point the 1 band is primarily s-character (angular momentum l = 0). Although the coupling through the MgO barrier layers to the electrodes makes the width of these peaks finite, the exponential decay of the wave function is fast in the barrier region, AAPPS Bulletin December 2008, Vol. 18, No. 6 29

7 Celebrating 20 Years of GMR Past, Present, and Future (II) Fig. 11: Illustration of possible structures for spin transistors based on double barrier MTJs with ferromagnetic (FM), non-magnetic metal (NM) or magnetic semiconductor (SM) middle-layers. which sufficiently confines these states. The wave functions of the majority spin QW states extend throughout all seven atomic layers of the middle Fe film. The states with n = 0 (n is the number of nodes in the wave function) are not shown in Fig. 10(a) clearly, due to its greatly reduced height near the edge of the 1 band. The minority-spin QW states, derived from the 1 band of the minority Fe, are about 1.5 ev above the Fermi energy. Thus the minority-spin QW states are away from the usual transport energy window in experiments, which is consistent with the absence of oscillatory conductance for the antiparallel configuration in Ref. [54]. By matching these QW states to the oscillations found in the experimental measurement [54], we confirmed that these oscillations indeed were caused by the QW states at the Γ point with the 1 symmetry in the middle Fe layer [46]. The shifts in the QW resonances due to the Coulomb blockade (CB) effect were used to estimate the size of the possibly discontinuous middle Fe layer. The CB effect was shown to play an important role in terms of determining the position of a resonance as well as its sharpness. Thus fabricating continuous middle Fe layer to minimize the CB energy is the key for realizing QW resonant tunneling in experiments. Such first-principles calculations can help us to seek the potential structures for developing spin transistors based on double barrier MTJs with ferromagnetic (FM), non-magnetic metal (NM) or magnetic semiconductor (SM) middle free layers as shown in Fig CONCLUSION In conclusion, the nano-fabrication techniques, spin-dependent transport and magnetization current switching properties of the nano-scale MTJs were summarized. The critical current density of the order of A/cm 2 is sufficient to realize the magnetization switching in nano-scale Al-O barrier-based MTJs with Co 60 Fe 20 B 20 (2.5 nm) free layer. We also presented recent research on spin-dependent transport in MgO-based MTJs using first-principles calculations, including tunneling mechanism, interfacial resonance states in Fe/ MgO/Fe MTJs, and quantum-well resonance in double barrier MTJs. Combining with the experiments, first-principles calculations play an significant role for predicting and indicating new phenomena, as well as clarifying and understanding physical mechanisms in experiments. These basic investigations on the Al-O and MgO(001) barrier-based MTJ materials and physics offers a possible new approach for developing the MRAM devices with high-density, enhanced thermal stability but low power consumption. Such nano-scale MTJs together with current switching mechanism can also be used to develop the nano-oscillators, magnetic logic devices, spin torque diode, and spin transistors in the near future. ACKNOWLEDGEMENTS The project was supported by the State Key Project of Fundamental Research of Ministry of Science and Technology (MOST, No. 2006CB932200), National Natural Science Foundation (NSFC, No , , ), and the Knowledge Innovation Program project of Chinese Academy of Science. X. F. 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