Performance of synthetic antiferromagnetic racetrack memory: domain wall versus skyrmion Item Type Article Authors Tomasello, R; Puliafito, V; Martinez, E; Manchon, Aurelien; Ricci, M; Carpentieri, M; Finocchio, G Citation Tomasello R, Puliafito V, Martinez E, Manchon A, Ricci M, et al. (2017) Performance of synthetic antiferromagnetic racetrack memory: domain wall versus skyrmion. Journal of Physics D: Applied Physics 50: 325302. Available: http:// dx.doi.org/10.1088/1361-6463/aa7a98. Eprint version Post-print DOI 10.1088/1361-6463/aa7a98 Publisher IOP Publishing Journal Journal of Physics D: Applied Physics Rights This is an author-created, un-copyedited version of an article accepted for publication/published in Journal of Physics D: Applied Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://doi.org/10.1088/1361-6463/aa7a98 Download date 26/09/2021 17:55:59 Link to Item http://hdl.handle.net/10754/625626 IOP Journal of Physics D: Applied Physics Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. J. Phys. D: Appl. Phys. 50 (2017) 325302 (11pp) https://doi.org/10.1088/1361-6463/aa7a98 50 Performance of synthetic antiferromagnetic 2017 racetrack memory: domain wall © 2017 IOP Publishing Ltd versus skyrmion JPAPBE R Tomasello1 , V Puliafito2 , E Martinez3, A Manchon4 , M Ricci5, 6 7,8 325302 M Carpentieri and G Finocchio 1 Department of Engineering, Polo Scientifico e Didattico di Terni, University of Perugia, strada di R Tomasello et al Pentima Bassa, I-05100 Terni, Italy 2 Department of Engineering, University of Messina, C.da di Dio, I-98166, Messina, Italy 3 Department of Fisica Aplicada, Universidad de Salamanca, Plaza de los Caidos s/n, E-38008, Salamanca, Spain 4 Physical Science and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia Printed in the UK 5 Department of Computer Science, Modelling, Electronics and System Science, University of Calabria, via P. Bucci, I-87036 Rende (CS), Italy 6 Department of Electrical and Information Engineering, Politecnico di Bari, via E. Orabona 4, I-70125 JPD Bari, Italy 7 Department of Mathematical and Computer Sciences, Physical Sciences and Earth Sciences, University of Messina, V.le F. D alcontres, 31, I-98166, Messina, Italy 10.1088/1361-6463/aa7a98 ’ E-mail: [email protected] Paper Received 20 April 2017, revised 6 June 2017 Accepted for publication 20 June 2017 Published 18 July 2017 1361-6463 Abstract A storage scheme based on racetrack memory, where the information can be coded in a 32 domain or a skyrmion, seems to be an alternative to conventional hard disk drive for high density storage. Here, we perform a full micromagnetic study of the performance of synthetic antiferromagnetic (SAF) racetrack memory in terms of velocity and sensitivity to defects by using experimental parameters. We find that, to stabilize a SAF skyrmion, the Dzyaloshinskii– Moriya interaction in the top and the bottom ferromagnet should have an opposite sign. The velocity of SAF skyrmions and SAF Néel domain walls are of the same order and can reach 1 values larger than 1200 m s− if a spin–orbit torque from the spin-Hall effect with opposite sign is applied to both ferromagnets. The presence of disordered anisotropy in the form of randomly distributed grains introduces a threshold current for both SAF skyrmions and SAF domain walls motions. Keywords: micromagnetism, spintronics, racetrack memory, domain wall, skyrmion, Dzyaloshinskii–Moriya interaction, spin-Hall effect S Supplementary material for this article is available online (Some figures may appear in colour only in the online journal) 8 Author to whom any correspondence should be addressed. 1361-6463/17/325302+11$33.00 1 © 2017 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 50 (2017) 325302 R Tomasello et al 1. Introduction coupling, tunnelling etc). Firstly, we benchmark our computa- tions with the experimental data on DWs, and then we predict The advent of a scenario where ‘cloud computing’ and ‘internet the performance of SAF skyrmion based racetrack by consid- of things’ are merged together is pushing research efforts to ering realistic parameters, including edge roughness and bulk find a technology that can go beyond the complementary defects in the form of disordered grains with random uniaxial metal-oxide semiconductor (CMOS), especially in terms of perpendicular anisotropy. Our key finding is that skyrmions storage and computing [1]. Spintronics, with its subfields, is a in a SAF racetrack have velocities approaching the ones of promising candidate for that, owed to the possibility to use the DWs. We also show that a skyrmion based SAF racetrack is degree of freedom of the spin angular momentum in addition insensitive to edge roughness (an unavoidable type of defects to the charge of the electrons. Concerning the storage, two in narrow nanotracks). On the other hand, the presence of a different categories of devices are promising: spin-transfer- disordered anisotropy, considering different size of the grains, torque-MRAMs (STT-MRAMs) [1–5] and racetrack memo- introduces a pinning of both SAF DW and SAF skyrmion, ries [6–10]. While STT-MRAMs (already on the market) can which is stronger when the average size of the grains is larger serve as ‘universal memory’ [1] (low writing energy, high read than the skyrmion diameter. In particular, this result shows speed, and ideally infinite endurance) [2, 5], racetrack memo- that, when the grain size (GS) is larger than the skyrmion ries can be used as worthy alternative to hard disk drives in diameter, the pinning of the skyrmion originates only at the storage memory, with the advantage that no mechanical parts interface between two adjacent grains. are necessary [9]. Since the first domain wall based racetrack memory pre- sented in [6], performance improvements have been achieved 2. Micromagnetic model by using materials with perpendicular anisotropy [11, 12], spin–orbit interactions, such as spin-Hall effect (SHE) [13, 14] We perform micromagnetic simulations of multilayered and interfacial Dzyaloshinskii–Moriya interaction (IDMI)) nanowires similar to the ones proposed in [18]. They are [7, 8, 15, 16], and interlayer exchange coupling (IEC) [17, composed of a 3 nm thick Platinum heavy metal (HM) (lower 18]. All previous ingredients gave rise to a racetrack memory HM) with on top two perpendicular CoNi ferromagnetic layer where the perpendicular anisotropy and the IEC, together with (FMs) separated by a thin Ruthenium (Ru), layer designed the IDMI, stabilize DWs in synthetic antiferromagnets (SAFs) to provide an antiferromagnetic exchange coupling [38] (see while SHE drives the DW motion at a velocity near 800 figure 1(a)), and a second HM on top of the whole stack m s−1 [18]. (upper HM). Recently, an alternative scheme of racetrack memory based The nanowire is 100 nm wide and 1000 nm long and the on skyrmions has been proposed [19–23]. Skyrmions are top- thickness of both ferromagnets and Ru layer is 0.8 nm. The ologically protected magnetic configurations that can be used physical parameters of CoNi layers taken from [17, 18], and for different applications [24–28]. In particular, in racetrack equal for both ferromagnets, are: saturation magnetization −1 −1 memories they can directly code the information, i.e. the pres- Ms = 600 kA m , exchange constant A = 20 pJ m , uni- −3 ence/absence of the skyrmion represents the bit ‘1’/‘0’ [19]. axial perpendicular anisotropy constant ku = 0.6 MJ m , and ex Skyrmion motion in ultrathin ferromagnetic materials has damping parameter αG = 0.1. The IEC constant A is fixed been demonstrated experimentally in extended Ta/CoFeB/ to −5.0 × 10−4 J m−2 [17, 18]. We use a discretization cell TaO multilayers [29], and in Pt/CoFeB/MgO [30], where a of 4 × 4 × 0.8 nm3 (the results are robust against the discre- skyrmion velocity near 120 m s−1 has been measured. The tization cell size, see appendix C), and introduce a Cartesian main technological challenge to be overcome is the exper- coordinate system with the x-, y- and z-axes lying along the imental control of a single skyrmion nucleation by an elec- length, the width and the thickness of the wire, respectively trical current and its electrical detection [31, 32]. On the other (see figure 1(a)). The numerical study is carried out by means hand, the main fundamental limitations of the skyrmion based of a self-implemented micromagnetic solver (it includes the racetrack memories are (i) the skyrmion Hall effect (two SHE, IDMI and IEC) and post-processing tools [39–41]. velocity components, parallel and perpendicular to the elec- The total micromagnetic energy density of the system trical cur rent direction) [22, 33, 34] (ii) a transient breathing under investigation is (the superscripts L and U refer to lower mode [22] and (iii) velocities below the ones of DWs for a and upper FMs): fixed set of physical and geometrical parameters [19]. Lately, εL,U = A( m)2 + εex it has been shown that these issues could be solved by using tot ∇· a SAF skyrmion [35], opening a path for a more competitive + DL,U mL,U mL,U mL,U mL,U z ∇· − ·∇ z skyrmion based racetrack memory. In that work [35], the tun- L,U 2 1 L,U L,U (1) ku m ˆz 2 µ0M sm H m nelling interlayer coupling was used to stabilize the SAF skyr- − · − · mion [36, 37]. where mx, my and mz are the x-, y-, and z-components of the Here, we consider the same experimental framework by normalized magnetization m, respectively. The interlayer ex Yang et al [18] where the IEC is mediated by itinerant elec- exchange energy is given by εex = A mL mU (same − tRU · trons via Ruderman Kittel Kasuya Yosida (RKKY) field contribution for both FMs), where t is the thickness of – – – RU [38], resulting in an antiferromagnetic alignment of the two the Ru layer [18].