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Multiple Tuning of Magnetic Biskyrmionsusing in Situ L-TEM In Journal of Physics: Condensed Matter PAPER Multiple tuning of magnetic biskyrmions using in situ L-TEM in centrosymmetric MnNiGa alloy To cite this article: Licong Peng et al 2018 J. Phys.: Condens. Matter 30 065803 View the article online for updates and enhancements. This content was downloaded from IP address 159.226.35.194 on 18/01/2018 at 01:08 IOP Journal of Physics: Condensed Matter Journal of Physics: Condensed Matter J. Phys.: Condens. Matter J. Phys.: Condens. Matter 30 (2018) 065803 (7pp) https://doi.org/10.1088/1361-648X/aaa527 30 Multiple tuning of magnetic biskyrmions 2018 using in situ L-TEM in centrosymmetric © 2018 IOP Publishing Ltd MnNiGa alloy JCOMEL Licong Peng1,2 , Ying Zhang1 , Min He1,2, Bei Ding1,2, Wenhong Wang1 , Jianqi Li1,2, Jianwang Cai1,2, Shouguo Wang3, Guangheng Wu1 065803 and Baogen Shen1,2 1 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy L Peng et al of Sciences, Beijing 100190, People’s Republic of China 2 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China 3 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Printed in the UK E­mail: [email protected] CM Received 17 November 2017, revised 25 December 2017 Accepted for publication 4 January 2018 Published 18 January 2018 10.1088/1361-648X/aaa527 Abstract Magnetic skyrmions are topologically protected spin configurations and have recently received Paper growingly attention in magnetic materials. The existence of biskyrmions within a broad temperature range has been identified in our newly­discovered MnNiGa material, promising for potential application in physics and technological study. Here, the biskyrmion microscopic 1361-648X origination from the spin configuration evolution of stripe ground state is experimentally identified. The biskyrmion manipulations based on the influences of the basic microstructures and external factors such as grain boundary confinement, sample thickness, electric current, 6 magnetic field and temperature have been systematically studied by using real­space Lorentz transmission electron microscopy. These multiple tuning options help to understand the essential properties of MnNiGa and predict a significant step forward for the realization of skyrmion­based spintronic devices. Keywords: magnetic biskyrmion, multiple manipulations, L­TEM, MnNiGa (Some figures may appear in colour only in the online journal) 1. Introduction works [8–10]. Recently, the experimental breakthroughs have been made in the study of skyrmion existence and stability, the Nanometric magnetic skyrmions with topologically nontrivial topologically physical properties [3, 4, 11], and the manipu­ spin configurations, have attracted a rich interest in chiral lation behaviors under external stimulus such as temperature magnetic materials with Dzyaloshinskii–Moriya interaction. [12], magnetic field [11, 13] and electric current [4, 6] in Many fascinating physical features such as emergent electro­ chiral helimagnets. However, the narrow magnetic field (B) magnetic fields [1–3] and ultralow current­driven behavior and temperature (T) range for equilibrium skyrmions in most [4–6], enable the potential applications of skyrmions in chiral materials (below room temperature), limits the appli­ memory device as prime candidates. This vortex­like sky­ cation, thereby proposing desperate need for discovery of rmion configuration, swirling to wrap a 3D spherical surface various skyrmion material systems within broad temperature unit with spins pointing in all directions, was originally pro­ range including room temperature. posed by Skyrme [7] as a model for hadrons, then was pre­ Magnetic skyrmions in dipolar magnets have been studied dicted existing in magnetic materials by many theoretical experimentally in centrosymmetric bulk systems [14–16] and 1361-648X/18/065803+7$33.00 1 © 2018 IOP Publishing Ltd Printed in the UK J. Phys.: Condens. Matter 30 (2018) 065803 L Peng et al theoretically described in Hamiltonian model [17] as a conse­ L­TEM method, the magnetic domain walls can be imaged as quence of mutual competition between different energy terms bright contrast (converging beam) or dark contrast (diverging such as dipolar, anisotropy and exchange interactions. Unlike beam) on the defocused image planes by applying a nominal the fixed spin helicity of chiral­type skyrmions introduced by defocus value of about 500 µm. The high­resolution lateral the crystallographic broken inversion symmetry, skyrmions magnetization maps are reconstructed by using a phase­ in centrosymmetric magnets allow two helicity degrees of retrieval QPt software based on the transport of intensity freedom selected at random [1, 18]. To identify different spin equation [23]. configurations, the topological number is defined to count how many times the spins wrapping the unit sphere [11, 14]. 3. Results and discussions For skyrmions in chiral magnets, the topological number is 1 with only one unique helicity. Recently a new biskyrmion 3.1. Magnetic microstructure analysis of the stripe domain configuration with two oppositely swirling spins possessing topological number 2, is discovered and fully explained From the microscopic point of view, the spin configuration in dipolar magnets such as tetragonal manganite La2−2xSr evolution from stripe ground state to biskyrmions is illustrated 1+2xMn2O7 (x = 0.315) below 60 K [14], Fe/Gd thin film at in centrosymmetric MnNiGa based on the L­TEM results in 300 K [19] and our hexagonal (Mn1−xNix)65Ga35 (x = 0.5) figure 1. The delicate magnetic microstructures along 101 (MnNiGa) alloy over 16 K–338 K [16, 20]. Regardless of the and [0 0 1] directions (identified by the diffraction patterns in various spin configurations, the non­zero topological number figures 1(e) and (j)) are demonstrated respectively in the two is the key factor to drive skyrmions by electric currents, which columns. The difference of the domain wall contrast in fig­ can be presented as nontrivial skyrmions by topological Hall ures 1(a) and (f) is further manifested by their corresponding resistivity [3, 16]. magnetization maps (figures 1(b) and (g)), phase maps (figures The existence of biskyrmion configuration and the reali­ 1(c) and (h)) and the My magnetization profiles (figures 1(d) zation of zero­field biskyrmion lattice in super­wide oper­ and (i)). More information about the magnetic microstruc­ ating temperature via manipulations in our MnNiGa material ture features of the stripe domain are observed along [0 0 1] [16, 20, 21], drive us to fully investigate the microscopic direction, which better reflects the intrinsic domain structure origins, prompting the application in skyrmion­based spin­ since it is along the easy­axis magnetization direction. When tronic devices. In this work, the magnetic microstructures of deviated from [0 0 1] direction, these fine domain structures the stripe domains and biskyrmions have been interpreted by are hidden under the mixed in­plane magnetization projec­ using real­space Lorentz transmission electron microscopy tion including the tilted perpendicular magnetic comp onent. (L­TEM). Multiple influential factors such as confined geom­ Based on the fine domain features, the spin configuration of etry, electric current, magnetic field, thickness and temper­ stripe ground state is derivate to be a periodic canting spin ature, have been extensively analyzed to help in understanding structure as schematically shown in figure 1(k), which is ener­ the essential properties of skyrmion generation in MnNiGa. getically determined by the competition between magnetic This easily fabricated MnNiGa material hosting skyrmions dipolar, anisotropy and exchange interactions in centrosym­ with multiple tuning options in wide temperature range indi­ metric structure [17]. With gradually magnetizing towards cates significant progress toward the realization of skyrmion­ the external magnetic fields, this periodic stripe domains are based spintronic information storage. broken and transformed into biskyrmion configurations with topological number 2, which is energetically stable [24] and 2. Materials and methods consistent with the previous biskyrmions study [14, 16, 19]. With further increasing the field, the biskyrmion state is com­ pletely turned into saturated state. Therefore, the microscopic (Mn1−xNix)65Ga35 (x = 0.5) (MnNiGa) polycrystalline is fab­ ricated by arc melting method as elaborated in [16]. The hex­ origins of biskyrmions are attributed to the spontaneously agonal structured MnNiGa crystal exhibits uniaxial (c­axial) periodic canting configuration of stripe domain. anisotropy due to Mn canting [16, 22] and thus the individual grain is observed along [0 0 1] zone axis to better reflect the 3.2. Grain boundary confinement effects on the generation of morphology of biskyrmion in our TEM study. The magnetic biskyrmions skyrmion evolution at different temperatures is imaged by L­TEM (JEOL 2100F and Tecnai F20) equipped with double­ The confinement effects on biskyrmions were respectively tilt liquid­nitrogen holder and liquid helium holder. In situ revealed in the annealed and non­annealed samples with dif­ current­driven biskyrmion motion is carried out by using an ferent grain sizes. Figure 2(a) shows the randomly scattered electrical TEM holder with the dc current supplied by a cur­ distribution of biskyrmions in the annealed sample, where the
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