Current-induced domain wall motion in antiferromagnetically coupled structures: Fundamentals and applications

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.

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a p o Quo -tor ene mat found with more efficient spin-toques and robust to magnetic noises. This is because they have the dynamics are different from those of the ferromagnetic ones. As a result, the antiferromagnetically l com e, sen istance physics in 2007, the research on applications in spintronics has lose the transverse component of their spin momentum. This mo- ization because of It means that the n reversed by the l magnetic fields. uences is the pos- ition in nanowires W motion (CIDM) sed for a long time 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 * 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: (D. Bang), (P. Van Thach). Contents lists availab Journal of Science: Advance journal homepage: www.el Journal of Science: Advanced Materials and Devices 3 (2018) 389e398Peer review under responsibility of Vietnam National University, Hanoi.such applications. [25], but the nanotechnology had limited its approaches such asspeeded 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 nonmag- netic (NM) layer sandwiched between two ferromagnetic (FM) layers (Fig. la) [13e17] and magnetic wires (Fig. 1b) [18]. There- fore, 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 mentum is transferred to the local FM2 magnet total angular momentum conservation [13,14]. magnetization of the FM2 can be excited or eve injecting current without any applied externa Besides that, one of the most interesting conseq sibility of manipulating domain walls (DWs) pos solely by an electric current: current-induced D effect (Fig. 1b) [19e24]. This idea had been propoand Peter Grünberg in 1988 [1,2] offered with the Nobel prize in directions of FM1 and FM2 will cause the conduction electrons toKeywords: Antiferromagnet Ferrimagnet Spin-transfer torques Spineorbit torques Magnetization switching Domain wall motion Perpendicular magnetic anisotropy 1. Introduction Magnetic materials are a centra tronic devices, such as data storag the discovery of giant magneto-res 2468-2179/© 2018 The Authors. Publishing services b ( 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 structure- based 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 ( ponent in many elec- sors, motors etc. After (GMR) by Albert Fert 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 magnetizationAvailable online 17 September 2018coupled structures offer a novel approach for reducing energy consumption as well as the immunityAccepted 9 September 2018 strongly exchange-coupled magnetic sublattice structures, and the antiferromagnetic order parameterReview Article Current-induced domain wall motion in coupled structures: Fundamentals and a Do Bang a, *, Pham Van Thach a, b, **, Hiroyuki Awan a Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang b Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468- 511, Japan a r t i c l e i n f o Article history: Received 1 June 2018 Received in revised form 5 September 2018 a b s t r a c t The current-induced spin ducing the way for next g focused on ferromagneticy Elsevier B.V. on behalf of Vietnamntiferromagnetically plications b c Viet, Cau Giay, Hanoi, Vietnam ques provide an efficient means of manipulating magnetization, intro- ration spintronic devices. During the past decades, studies have mainly erials. In recent years, antiferromagnetically coupled structures have been le at ScienceDirect d Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license ed M100 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 Fig. 1. 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. D. Bang et al. / Journal of Science: Advanc390perpendicular 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 semi- conductors, 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 tem- peratures 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, current- induced 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 ferro- magnetic 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 movedwith 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 syn- thetic 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 the CIDM. Recently, current-driven relativistic Neel order field as 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 demon- strated that the antiferromagnetic DW can be moved in these AFs with very high velocities which are 2 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 Fig. 2. 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.aterials and Devices 3 (2018) 389e398phenomena in AF texture dynamics. Furthermore, the absence of a Walker breakdown limit can keep the velocity of the antiferro- magnetic DWup 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]. 2. 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 mag- netic sublayers of upper magnetic (UM) and lower magnetic (LM) layers which are exchange-coupled via an ultrathin spacer layer [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 antiferro- magnetic 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 multi- layers 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 on the Pt underlayer, the DWs have a chiral Neel structure due to a 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 structure respectively, exhibits an anticlockwise Neel structure. When Js 0,MU andML are rotated towards the spin accumulation direction denoted by the magenta arrows (Fig. 3b) and are sub- jected to longitudinal fields HUlg and HLlg, respectively, which are composed of the corresponding DMI fields and Hx, and exchange- coupling fields HUex and HLex, respectively (Fig. 3c and d). Each of these fields gives rise to a corresponding torque, namely tUlg and tLlg, and tUex and tLex. As a result, the SAF structure stabilizes the Neel DW 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 DWmotion 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 calcu- lated as v¼ l/t (m/s). Typically, the DWvelocity strongly depends on external magnetic fields (Fig. 5a), injected current density (Fig. 5b), temperature, sizes andmaterials 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 4 and 8 Å, the DW velocities are much higher than those of the ferromagnetically coupled ones. As mentioned above, theoretical estimations suggest the possi- bility 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 D. Bang et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 389e398 391Fig. 3. Schematic illustration of (a) DWs in a SAF wire and (b) Spin Hall current from an HM layer induced DWsmotion 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).alloys offer additional advantages. Fig. 4. (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). and mag D. Bang et al. / Journal of Science: Advanced M3922.2. 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 Fig. 5. DW velocity as functions of (a) longitudinal field (Left and right panels: measured thickness (the orange and blue shaded regions correspond to ferromagnetic and antiferrocoupled, the net magnetic moment can be tuned easily by varying the RE-TM composition as shown in Fig. 6. 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. 6. (a) Schematic of RE and TM moments in the ferrimagnetic order of TM-rich, compensated and RE-rich compositions. (b) Coercivity (blue lines) and magnetiza- tion (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 calculated result), (b) current pulse densities for different thicknesses and (c) Ru spacer netic couplings). Replotted from ref. 56 (Copyright 2015 Macmillan Publisher Limited). aterials and Devices 3 (2018) 389e398order (Fig. 6a) [87,88]. This ferrimagnetic order between the Tb and Co sublattices may be confirmed by observation of the compensa- tion 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 1011 A/m2 and the highest slope of domainwall 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 1011 A/m2 is required to drive domainwalls 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 GdFeCo- based ferrimagnets at the angular momentum compensation temperature [75,89,90]. Because of different Lande g-factors be- 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 mo- ments cancel each other, and the angular momentum compensa- tion 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 3 (2018) 389e398 393approaching 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 mo- tion. As shown in Fig. 8, the DW velocity can reach up to over 1 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. Fig. 7. (a) Schematic illustration of TbeCo based films with different layered structures. (b) C of A-, B-, and C-stack structures. Replotted from Ref. 68 (Copyright 2016 American Physica Fig. 8. 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).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 urrent density dependence of DW velocity for different magnetic wires (1.1-mmwidth) l Society). Fig. 9. 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. two collective coordinates, its position (X) and
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