In this review, we discuss current-induced domain wall motion under the action of different spin torques in a wide range of antiferromagnetically coupled materials. New approaches and prospective applications of the antiferromagnetic structurebased devices are also discussed.
Journal of Science: Advanced Materials and Devices (2018) 389e398 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Review Article Current-induced domain wall motion in antiferromagnetically coupled structures: Fundamentals and applications Do Bang a, *, Pham Van Thach a, b, **, Hiroyuki Awano b a b Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468- 511, Japan a r t i c l e i n f o a b s t r a c t Article history: Received June 2018 Received in revised form September 2018 Accepted September 2018 Available online 17 September 2018 The current-induced spin-torques provide an efficient means of manipulating magnetization, introducing the way for next generation spintronic devices During the past decades, studies have mainly focused on ferromagnetic materials In recent years, antiferromagnetically coupled structures have been found with more efficient spin-toques and robust to magnetic noises This is because they have the strongly exchange-coupled magnetic sublattice structures, and the antiferromagnetic order parameter dynamics are different from those of the ferromagnetic ones As a result, the antiferromagnetically coupled structures offer a novel approach for reducing energy consumption as well as the immunity against external magnetic perturbation in spintronic devices In this review, we discuss current-induced domain wall motion under the action of different spin torques in a wide range of antiferromagnetically coupled materials New approaches and prospective applications of the antiferromagnetic structurebased devices are also discussed © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Antiferromagnet Ferrimagnet Spin-transfer torques Spineorbit torques Magnetization switching Domain wall motion Perpendicular magnetic anisotropy Introduction Magnetic materials are a central component in many electronic devices, such as data storage, sensors, motors etc After the discovery of giant magneto-resistance (GMR) by Albert Fert and Peter Grünberg in 1988 [1,2] offered with the Nobel prize in physics in 2007, the research on applications in spintronics has speeded up quickly [3e12] Then the spin-polarized current excites the magnetization state and might eventually lead to the magnetization reversal in multilayered structures of a nonmagnetic (NM) layer sandwiched between two ferromagnetic (FM) layers (Fig la) [13e17] and magnetic wires (Fig 1b) [18] Therefore, continuous efforts in the spintronics field, which use the spins of conduction electrons to further reduce the size as well as energy consumption of the magnetic devices, are important for such applications * Corresponding author ** Corresponding author Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam E-mail addresses: dobang.tti@gmail.com (D Bang), phamthach@toyota-ti.ac.jp (P Van Thach) Peer review under responsibility of Vietnam National University, Hanoi As shown in Fig 1a, a multilayered structure consists of FM1/ NM/FM2 where the in-plane FM1 is the pinned layer and supposed to have much higher coercivity than that of the FM2 one The electrons passing FM1 become spin-polarized along the direction of FM1 magnetization The misalignment between the magnetization directions of FM1 and FM2 will cause the conduction electrons to lose the transverse component of their spin momentum This momentum is transferred to the local FM2 magnetization because of total angular momentum conservation [13,14] It means that the magnetization of the FM2 can be excited or even reversed by the injecting current without any applied external magnetic fields Besides that, one of the most interesting consequences is the possibility of manipulating domain walls (DWs) position in nanowires solely by an electric current: current-induced DW motion (CIDM) effect (Fig 1b) [19e24] This idea had been proposed for a long time [25], but the nanotechnology had limited its approaches such as micro-fabricating technique and effective methods for observing magnetization The advantage of electric current with respect to the effect of magnetic field is that it drives all domain walls in the direction of electron flow, whereas the magnetic field tends to expand or shrink domains of opposite magnetizations [25] The majority of studies have been carried out for single NiFe nanowires exhibiting in-plane magnetic anisotropy (IMA) [23,26e28] One of the highest DW velocities in NiFe exceeded https://doi.org/10.1016/j.jsamd.2018.09.003 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 390 D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 Fig Schematic representation of (a) the effect of Rashba field (HR) and (b) Spin Hall current (JSHE) induced DW motion by injecting a dc (IDC) current into the bilayer structure of a FM/HM wire Fig Schematic of an injecting current induces (a) in-plane magnetization exciting and magnetization switching in a multilayered structure of FM1/NM/FM2 and (b) domain wall (DW) motion in an in-plane magnetic wire 100 m/s in zero field [29,30] This is convenient for designing magnetic storage devices based on a shift register [31,33] The DW velocity can be enhanced with increasing temperature due to a decrease of magnetization and pining strength [33] On the other hand, the DW velocity can be enhanced or suppressed depending on the mutual magnitude and orientation of the field and current [34,35] The velocity enhancement is also dependent on the field range However, the current density required for inducing magnetization switching and DW motion is of the order of 1012 A/ m2, which justifies the need for nanometer sizes with as small cross-sections as possible, to minimize the injected current Growing interest has recently been devoted to the systems with perpendicular magnetic anisotropy (PMA) [36e44] The magnetic wires contained very narrow Bloch walls, which contributed to high spin transfer efficiency, as the DWs were displaced with a current density of one order smaller than that of the in-plane magnetic one Some systems with PMA, like Pt/Co/AlOx [36], exhibited larger DW velocities compared to NiFe, up to 400 m/s [37] Recently, a lot of papers have reported on current-induced DW motion in PMA systems composed of different materials such as FePt [38], Co/Ni [39e41], and Co/Pt [42,44] However, the high current density is still needed to drive DWs in the PMA wires, thus limiting its practical applications Very low critical current densities of the order of l09 A/m2 have been reported for magnetic semiconductors, such as GaMnAs [45,46] This is attributed to the low spontaneous magnetization and high-carrier spin polarization of this material [47] But this approach is only achieved at low temperatures due to the low Curie temperature of the semiconductor In the last seven years, an asymmetric (ASY) bilayer of a perpendicular anisotropy FM and heavy metal (HM) with strong spineorbit coupling has attracted considerable interest for their potential utility in spintronic elements In these systems, currentinduced spineorbit torques (SOTs) [48,49] on the magnetization can be induced by the Rashba effect at the interface (Fig 2a) [37,50] and by spin currents injected from the heavy metal due to the spin Hall effect (SHE) (Fig 2b) [49,51e56] SOTs have been identified and quantified by a variety of techniques, including spin-torque ferromagnetic resonance [53], quasistatic magnetization tilting probed through harmonic voltage measurements [54], and current- induced hysteresis loop shift measurements [57] In the bilayer wires, DWs can be moved with high speed up to a few hundreds m/ s under current densities of the order of 1011 A/m2 in the opposite direction to the direction of DWs motion in a single layer under the effect of conventional spin-transfer torques (STTs) Unlike the FM system, in antiferromagnetically (AF) coupled structures, their angular momentum is not associated with the order parameter so that spin dynamics in these structures is intrinsically much faster than in FMs Up to now, a maximum DW velocity of up to 750 m/s has been reported for Co/Ni/Co/Ru (t)/Co/Ni/Co wires with a synthetic antiferromagnetic (SAF) structure Such a high DW velocity can be achieved owing to the SAF structure that gives rise to a novel torque associated with the antiferromagnetic exchange coupling field [58] Current efforts are to optimize material structures for realizing a high-speed and low-power magnetic memory based on el order field as the CIDM Recently, current-driven relativistic Ne well as DW motion in AFs where magnetic atoms have a local environment with broken inversion symmetry have been predicted theoretically and experimentally [59e65] It has been demonstrated that the antiferromagnetic DW can be moved in these AFs with very high velocities which are orders of magnitude greater than those in FMs due to the efficiency of the staggered spineorbit fields to couple to the order parameter and the exchange-enhanced phenomena in AF texture dynamics Furthermore, the absence of a Walker breakdown limit can keep the velocity of the antiferromagnetic DW up to values of few km/s [61] Besides that, rare-earth (RE) transition-metal (TM) alloys named ferrimagnetic alloys in which the moments of RE sublattices are anti-ferromagnetically coupled with those of TM sublattices are potential candidates for realizing such high speed devices [66e73] In ferrimagnetic alloys, a negative exchange interaction between the RE and TM sublattices can induce much faster DW motion at low current densities [74] and magnetic fields [75] Therefore, the AFs have led to the recent development of spintronic-based devices with the ultimate speed of magnetic dynamics [59,76e79] Current induced domain wall motion in wires based on antiferromagnetically coupled structures 2.1 Synthetic antiferromagnetic bilayer wires A synthetic antiferromagnetic structure composes of two magnetic sublayers of upper magnetic (UM) and lower magnetic (LM) layers which are exchange-coupled via an ultrathin spacer layer D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 [80] Fig 3a shows a perpendicularly magnetized bilayer nanowire with the SAF coupled to the HM layer In the SAF structure, two domain walls in the upper and lower layers mutually interact with each other due to the exchange coupling and magnetostatic coupling Recently, it has been predicted theoretically [62] and confirmed experimentally [58] that in SAF-HM systems with the presence of interfacial DzyaloshinskiieMoriya interaction (DMI) the antiferromagnetic DW velocity can reach a few kms per second, which is much larger than that of a ferromagnetic DW In particular, Yang et al showed that DWs can be moved with high speeds up to 750 m/s in a SAF racetracks formed from two PMA Co/Ni multilayers separated by an ultrathin Ru layer in AF coupling regime on a HM layer of Pt (Fig 3b) [58] When the Co/Ni multilayer is deposited el structure due to a on the Pt underlayer, the DWs have a chiral Ne DMI derived from the strong spineorbit coupling and the proximity-induced moment in Pt [81e84] When J ¼ 0, the DW with the magnetizations of MU and ML in the centre of the SAF el structure structure respectively, exhibits an anticlockwise Ne When J s 0, MU and ML are rotated towards the spin accumulation direction denoted by the magenta arrows (Fig 3b) and are subjected to longitudinal fields HUlg and Hlg L , respectively, which are composed of the corresponding DMI fields and Hx, and exchangeex coupling fields Hex U and HL , respectively (Fig 3c and d) Each of Fig Schematic illustration of (a) DWs in a SAF wire and (b) Spin Hall current from an HM layer induced DWs motion in the SAF wire (c, d) Directions of fields and torques in the SAF bilayer, upper and lower panels correspond to the UF and LF, respectively Replotted from Ref 58 (Copyright 2015 Macmillan Publisher Limited) 391 these fields gives rise to a corresponding torque, namely tUlg and tlg L, ex el DW and tex U and tL As a result, the SAF structure stabilizes the Ne structure, thereby allowing for the larger spineorbit torques for the same current density, and therefore the higher DW velocities To observe the CIDM in a magnetic wire one can use a polar Kerr microscopy (Fig 4a) Then the DW velocity is estimated by the DW displacement within duration time of a current pulse One also can estimate the DW velocity by measuring typical anomalous Hall effect (AHE) signals as shown in Fig 4b The DW is driven by a current pulse Then the DW motion is monitored at each Hall bar in real time The two AHE signal data set gives us the time (t) taken passing each Hall cross of length l Thus the DW velocity is calculated as v ¼ l/t (m/s) Typically, the DW velocity strongly depends on external magnetic fields (Fig 5a), injected current density (Fig 5b), temperature, sizes and materials used Furthermore, the strength of magnetic coupling of bilayers also can be attributed to a change in the DW velocity in a SAF wire by changing the thickness of the spacer (Fig 5c) For the antiferromagnetically coupled wires with Ru thicknesses of about and Å, the DW velocities are much higher than those of the ferromagnetically coupled ones As mentioned above, theoretical estimations suggest the possibility to substantially reduce the critical currents and tailor the domain structure of antiferromagnetically -coupled wires at critical currents above 109 A/m2, which are about one order of magnitude below the highest value reported for the ferromagnetic structures However, most recent studies on the SAF wire have been focused on only Co/Ni system [85,86] due to its strong coupling in the ultrathin regime To optimize effectively the current-induced DW motion in AFs, one must control the thickness and magnetization by changing their compositions As such, antiferromagnetic compounds such as Mn2Au and CuMnAs [61] and ferrimagnetic alloys such as the RE-TM alloys offer additional advantages Fig (a) Estimation of velocities for current-induced DW motion in a SAF wire using real-time polar Kerr microscopy images after applying each current pulse Replotted from Ref 56 (Copyright 2015 Macmillan Publisher Limited) (b) Schematic of the DW velocity measurement based on Anomalous Hall Effect signal Replotted from ref 85 (Copyright 2018 AIP Publishing) 392 D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 Fig DW velocity as functions of (a) longitudinal field (Left and right panels: measured and calculated result), (b) current pulse densities for different thicknesses and (c) Ru spacer thickness (the orange and blue shaded regions correspond to ferromagnetic and antiferromagnetic couplings) Replotted from ref 56 (Copyright 2015 Macmillan Publisher Limited) 2.2 Current-induced DW motion in ferrimagnetic alloys In the RE-TM alloys, where RE element (e.g., Tb, Gd, etc.) and TM element (e.g., Co, Fe, etc.) sublattices are antiferromagnetically coupled, the net magnetic moment can be tuned easily by varying the RE-TM composition as shown in Fig RE-TM alloy such as TbeCo films can be simply fabricated from a single target of TbCo alloys or co-sputtering of two Co and Tb targets by using RF or DC magnetron sputtering at room temperature without any post- Fig (a) Schematic of RE and TM moments in the ferrimagnetic order of TM-rich, compensated and RE-rich compositions (b) Coercivity (blue lines) and magnetization (red lines) of RE-TM alloys as a function of RE content annealing processes At room temperature, pure Tb and Co are paramagnetic and in-plane anisotropy ferromagnetic, respectively In form of an alloy or multilayers with alternating thin layers of Tb and Co, however, the Tb and Co sublattices show a ferrimagnetic order (Fig 6a) [87,88] This ferrimagnetic order between the Tb and Co sublattices may be confirmed by observation of the compensation points (Fig 6b) It has been reported that the critical current density to drive domain wall motion strongly depends on the layered structure as well as its composition [68] The lowest critical current density of about 1.5 Â 1011 A/m2 and the highest slope of domain wall velocity curve are obtained for the wire having thin Co sublayers and more inner Tb/Co interfaces, while the largest critical current density of 2.6 Â 1011 A/m2 is required to drive domain walls in the TbeCo alloy magnetic wire (Fig 7) An enhancement of the antidamping torques by extrinsic spin Hall effect due to Tb rare-earth impurity-induced skew scattering is suggested to explain the high efficiency of current-induced domain wall motion in the Co/Tb multilayer with more number of inner interfaces (n) This study indicates an efficient way to reduce the critical current density for DW motion through inner interface engineering As discussed above, the effective SAF films are mostly limited in ultrathin sublayers of a few Å and total film thickness of few nanometers due to the dominance of interfacial effects Recently, ultrafast field-induced DW motion has been observed in GdFeCobased ferrimagnets at the angular momentum compensation g-factors betemperature [75,89,90] Because of different Lande tween the RE and TM elements, below the Curie temperature these ferrimagnets have two special temperatures of the magnetization compensation temperature (TM), at which the two magnetic moments cancel each other, and the angular momentum compensation temperature (TA), at which the net angular momentum vanishes [91,92] As a result, the nature of the dynamics of the ferrimagnets changes from ferromagnetic to antiferromagnetic on D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 393 Fig (a) Schematic illustration of TbeCo based films with different layered structures (b) Current density dependence of DW velocity for different magnetic wires (1.1-mm width) of A-, B-, and C-stack structures Replotted from Ref 68 (Copyright 2016 American Physical Society) approaching the TA Furthermore, the net magnetic moment of ferrimagnets is nonzero at TA and can thus couple to an external magnetic field as well as the efficiencies of field-induced DW motion As shown in Fig 8, the DW velocity can reach up to over km/s under around 100 mT field at temperature around TA However, it will be rapidly reduced when the temperature is far from the TA In the following, a theory for the field-driven DW dynamics in ferrimagnets is discussed briefly The low-energy dynamics of a DW in quasi-one-dimensional magnets is generally described by Fig DW velocity in a GdFeCo ferrimagnetic wire as a function of temperature under different applied in-plane fields Replotted from ref 75 (Copyright 2017 Macmillan Publisher Limited) Fig DW velocity and critical current density (Jc) induced DW motion in magnetic wires with different DW types and symmetric (SYM) or asymmetric (ASYM) layered structures as functions of year 394 D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 two collective coordinates, its position (X) and angle (4), which capture the translational and spin-rotational degrees of freedom of the DW, respectively In ferromagnets, the force on X induces the dynamics of and vice versa Due to this coupling, the motion of a DW is limited by the Walker breakdown [93] In contrast, in antiferromagnets the dynamics of X and are independent due to vanishing of the net spin density The DW dynamics can be fast and be free from the Walker breakdown In RE-TM alloys, the RE and TM moments can be easily controlled by varying the composition or temperature, so that the TA can be designed for appropriate applications To date, the effect of angular momentum compensation point on magnetization dynamics has attracted to studies of ferrimagnetic systems [67,94,95] The lower threshold current density of one order of magnitude, smaller power consumption, and higher speed than those of FM systems would be a key to realizing spintronic devices Finally, The improvement of DW velocity and reduction of critical current density (Jc) by optimizing magnetic materials and layered structures as functions of year are shown in Fig It clearly shows that the DW velocity is rapidly increased, while the Jc is reduced by a few orders of magnitude at room temperature without applying any external fields 2.3 Prospective applications of the current-induced DW motion based spintronic devices Recently, magnetic nanostructures have been at the heart of a multitude of devices ranging from applications of sensors to data storage Probably the best known concept device is the racetrack memory, which was proposed by IBM [31,32,96] based on the CIDM effect As shown in Fig 10, the racetrack memory can be designed along lines in two and three dimensions or even in circular loops One of the key problems of the conventional hard drive is the mechanical motion of the media, which can lead to failure in the case of a mechanical shock In the case of the racetrack memory, positions of domains as well as coded information can be moved without using any mechanical motors On the other hand, the racetrack only comprises one write or read element which can Fig 10 Schematic of the racetrack memory designed along lines in two dimensions (a) and three dimensions (b) or in circular loops (c) Replotted from ref 32 and 96 (Copyright 2008 and 2016 Macmillan Publisher Limited) D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 make its low cost Furthermore, it can be more compact as well as has higher storage densities To increase the bit density of the DW-based race-track memory, on the other hand, Kurokawa et al [97] has proposed a new novel technique using multi-level magnetic wires which have asymmetric layered structure of HM1/FM1/HM2/FM2/HM1 as shown in Fig 11a The author found that the anomalous Hall effect signal of the multi-level wire has four stable levels under a zero magnetic field Furthermore, DWs in the both TbeCo FM1 and FM2 layers can be controlled by a single current as shown in Fig 11b Recently, current-driven spin logics (AND, OR, NOT) have been proposed and successfully demonstrated with TbFeCo nanowires (Fig 12a) by Awano et al [98] As shown in Fig 12 for the AND logics, types of input signals are set at each input gate and then are moved to the confluence point under injecting of a current Due to the symmetry of the Y-shape, the two domains from inputs arrive at the confluence point at same time and then are combined each other to continue moving to the output gate as a signal “l” for the case of input (1, 1) For the case of input (1, 0) or (0, l), a single domain will be trapped at the confluence points due to the different widths of the branches of inputs and output, so that the output signal is “0” By using this concept device, memory and logic can be 395 prepared in the same area and directly connected with each other without data bus Therefore, the device is expected to operate with high speed and low power consumption To realize applications of these two proposed structures, a combination with magnetic tunnel junction (MTJ) nanopillar is highly recommended, due to its high responding speed However, a current density needed to move magnetic DWs is still relatively high This is a challenging task in designing integration circuits and architectures for devices based on the CIDM effect A novel integration has been proposed for magnetic materials with high resistivity Recently, a huge step towards the spin transistor has been done for the magnon transistor, which consists of an yttrium ion garnet (YIG, a magnetic insulator) thin film and platinum electrodes [99] This new magnon transistor can be well integrated to the CIDM-based devices for both data storage (often using a magnetic process) and data processing (electronic transistors) It means that these devices can potentially work faster with higher energyefficiency Additionally, other solutions such as magnetic field and heat can be utilized to assist the CIDM for reducing the Jc Therefore, it is believed that the new structures can be very promising to build up future memory and logic devices performing ultra-high density and speed Fig 11 (a) Schematic of DW configurations in a multi-level magnetic wire (b) Experimental observation of DWs motion in a multi-level magnetic wire of Pt/TbeCo/Ta/TbeCo/Pt layered structure by injecting a pulse current in forward (left panel) and backward direction (right panel) Replotted from ref 99 (Copyright 2018 AIP Publication) 396 D Bang et al / Journal of Science: Advanced Materials and Devices (2018) 389e398 Fig 12 (a) Schematic of current driven AND type spin logic with different input signals (b) SEM image of AND spin logic using the TbFeCo wire and experimental setup Replotted from ref 100 (Copyright 2015 Published by Elsevier) Conclusion We have shown the key distinctions in current-induced dynamics in ferromagnetic, antiferromagnetic and ferrimagnetic structures As compared to the FM-ordered materials, the AF-ordered ones have more efficient spin-toques, as well as high DW motion speeds of up to few hundreds m/s and are robust to magnetic noises This is because the AF materials have a strongly exchange-coupled magnetic sublattice structure, and the antiferromagnetic order parameter dynamics are different from those of the ferromagnetic ones However, these effective SAF films are mostly limited to the ultrathin sublayers of a few Å and the total film thickness of a few nanometers due to the dominance of interfacial effects such as Rashba effect and DMI Alternatively, the RE-TM ferrimagnetic alloys are not limited in these thin thickness regions The DW velocity can reach over km/s under around 100 mT field at temperature around the angular compensated temperature The RE and TM moments can be easily controlled by varying composition or temperature, so that the angular compensated temperature can be designed for appropriate applications The RETM ferrimagnetic alloys are therefore expected to provide a new approach for reducing energy consumption as well as the immunity against external magnetic perturbation in spintronic devices, such as racetrack memories and spin logic gates Acknowledgments This research was partially supported by the Ministry of Education, Culture, Sport, Science and 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