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Electric current induced dynamics of bubble domains in a ferrimagnetic Tb/Co multilayer wire below and above the magnetic compensation point Electric current induced dynamics of bubble domains in a fe[.]
Electric-current-induced dynamics of bubble domains in a ferrimagnetic Tb/Co multilayer wire below and above the magnetic compensation point Masaaki Tanaka, Sho Sumitomo, Noriko Adachi, Syuta Honda, Hiroyuki Awano, and Ko Mibu Citation: AIP Advances 7, 055916 (2017); doi: 10.1063/1.4974067 View online: http://dx.doi.org/10.1063/1.4974067 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics Articles you may be interested in Enhancement of spin orbit torques in a Tb-Co alloy magnetic wire by controlling its Tb composition AIP Advances 7, 055917055917 (2017); 10.1063/1.4974280 Electrical switching of antiferromagnets via strongly spin-orbit coupled materials AIP Advances 121, 023907023907 (2017); 10.1063/1.4974027 Magnetic domain-wall creep driven by field and current in Ta/CoFeB/MgO AIP Advances 7, 055918055918 (2017); 10.1063/1.4974889 Robust spin-current injection in lateral spin valves with two-terminal Co2FeSi spin injectors AIP Advances 7, 055808055808 (2016); 10.1063/1.4972852 AIP ADVANCES 7, 055916 (2017) Electric-current-induced dynamics of bubble domains in a ferrimagnetic Tb/Co multilayer wire below and above the magnetic compensation point Masaaki Tanaka,1 Sho Sumitomo,1 Noriko Adachi,1 Syuta Honda,2 Hiroyuki Awano,3 and Ko Mibu1 Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan Department of Pure and Applied Physics, Kansai University, Suita 564-8680, Japan Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya 468-8511, Japan (Presented November 2016; received 23 September 2016; accepted 26 October 2016; published online 11 January 2017) We investigated the electric-current-induced dynamics of bubble domains in a perpendicularly magnetized ferrimagnetic {Tb/Co}7 multilayer wire with a heavy-metal Pt cap layer The {Tb/Co}7 wire with the transition-metal-dominant and rare-earthdominant magnetizations was obtained by changing temperature We found that the bubble domains moved to the electric current direction with growing in oblique angles when electric current pulses were applied The oblique directions of the bubbledomain’s growth in the {Tb/Co}7 wire with the transition-metal-dominant and rareearth-dominant magnetizations were opposite with each other The micromagnetic simulations imply that these oblique growths are accounted by the spin injection from the Pt layer via the spin Hall effect © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4974067] INTRODUCTION Electric-current-induced movements of domain walls in magnetic wires are paid attention because of their potential as new spintronic devices.1–3 Among them, the movements in perpendicular magnetized wires are intensively studied recently because of the higher domain wall velocity.4–8 Some groups have reported that the magnetic domain walls did not simply move in the electron direction in ferromagnetic/nonmagnetic wires.9–15 To account for these behaviors, the spin injection from spin Hall effect (SHE) of the nonmagnetic layer were proposed.12–14 In our previous study, the electric-current-induced movements of bubble domains (BDs) in ferrimagnetic amorphous TbFeCo wires with perpendicular magnetic anisotropy with Pt cap layers were studied.16 The effective torque which was induced by the injected spins from the Pt layers was stronger than the spin transfer torque (STT) from the electric current flowing in the TbFeCo layers Because the BDs, whose domain walls not reach the wire edges, are expected to be driven without the effect of the edge roughness, the current induced dynamics of BDs would allow us to distinguish the influence of such torques clearly.17 In this study, we investigated the current-induced dynamics of BDs in a ferrimagnetic {Tb/Co}7 multilayer wire with a heavy-metal Pt cap layer We also performed micromagnetic simulations using our homemade code based on Landau-Lifshitz equation, in which the spin injection from the Pt cap layer was considered In the ferrimagnetic {Tb/Co}7 multilayer, the magnetic moments of the rare earth Tb take a random cone arrangement with the net magnetization opposite in direction to that of the transition metal Co Because of this opposition, {Tb/Co}7 multilayer wires with the rare earth (RE) and transition metal (TM) dominant magnetization can be prepared by changing the Tb and Co thicknesses In addition, because the magnetic moment of RE layers decreases more than that of the TM in a TM/RE multilayer with the increase of temperature, the magnetically dominant layer can be changed by changing temperature.18 2158-3226/2017/7(5)/055916/5 7, 055916-1 © Author(s) 2017 055916-2 Tanaka et al AIP Advances 7, 055916 (2017) FIG Magneto-optical Kerr loops of the {Tb/Co}7 multilayer sample at (a) 25 ◦ C and (b) 60◦ C These results indicated that the sample at 25 ◦ C and 60◦ C have RE and TM dominant magnetization, respectively EXPERIMENTAL A 10- µm- wide and 40-µm- long {Tb(0.60 nm)/Co(0.31 nm)}7 /Pt (2 nm) multilayer wire with electric pads on both sides was fabricated on thermally oxidized Si substrates using electron-beam lithography, sputtering and a lift-off method The cap Pt layer was used not only for protecting the {Tb/Co}7 multilayer wire from oxidization but for injecting spins through the SHE effect The net saturation magnetization of the multilayer and the magnetization dominant layer were determined by an alternative gradient field magnetometer and a polar Kerr microscope using a mercury lamp, respectively We prepared the {Tb/Co}7 multilayer wire with RE dominant magnetization at 25◦ C (M S = 10 emu/cm3 ) Figure shows the polar-magneto-optical Kerr effect (MOKE) loops of the multilayer sample at 25◦ C and 60◦ C Because polar Kerr signals using the mercury lamp reflect the magnetic moment of Co layers, the magnetic moment direction of Co layers at 25◦ C and 60◦ C was found antiparallel and parallel direction to the external magnetic field, respectively These results indicated that the {Tb/Co}7 multilayer wire had RE and TM dominant magnetization at 25◦ C and 60◦ C, respectively The current-induced BD dynamics was directly observed using polar-MOKE microscopy in differential imaging modes Figure shows a MOKE microscope image of the BD in the {Tb/Co}7 multilayer wire and a schematic illustration for the current-induced measurements The BDs were generated in the wire as follows.16,19 A magnetic field of more than kOe was first applied perpendicular to the substrate plane upward or downward in order to align the magnetization of the wire in one direction The center of the {Tb/Co}7 wire was then heated for 0.5 second using FIG Polar Kerr image of a BD in a {Tb/Co}7 multilayer wire and illustration of the experimental setup for the current induced dynamics measurements 055916-3 Tanaka et al AIP Advances 7, 055916 (2017) a 407-nm laser of mW with a magnetic field of about kOe applied perpendicular to the substrate plane downward or upward A BD was obtained at the center of the {Tb/Co}7 multilayer wire Subsequently, the diameter of the BDs was adjusted to about µm using a perpendicular magnetic field without heating No permanent damage in the wire due to the laser heating was found by the MOKE microscope observation The current induced BD dynamics was observed after applying current pulses of 100 ns repeatedly The electric current flow direction was the +x direction in Fig RESULTS AND DISCUSSION Figures 3(a) and (b) show MOKE microscope images of the downward (–z) magnetized and upward (+z) magnetized BDs dynamics of the {Tb/Co}7 multilayer wire at 25o C (RE dominant magnetization) after a single current pulse (1.8×107 A/cm2 ) was applied under a perpendicular magnetic field of –1900 Oe and +1500 Oe, respectively The –z and +z magnetized BD’s centers moved to the (+x, –y) and (+x, +y) directions, respectively If only the STT from the electron flow was considered, the BD’s center should move to the -x direction The experimental result means that the spin injection from the Pt cap layer affected the BD dynamics stronger than the STT from the electric current flow in the {Tb/Co}7 part.12–14 On the other hand, as shown in Figs 3(c) and (d), the –z and +z magnetized BD’s centers in the {Tb/Co}7 multilayer wire at 60o C (TM dominant magnetization) moved in the (+x, +y) and (+x, –y) directions in the current density of 1.8×107 A/cm2 under a perpendicular magnetic field of –900 Oe and +1000 Oe, respectively The oblique growth direction of the –z magnetized BDs in the wire with the TM (Fig 3(a)) and RE (Fig 3(c)) dominant magnetizations was opposite with each other These results indicated that the BD’s growth direction was not determined by the net magnetization direction but the magnetic moment of RE or TM layers in the RE/TM multilayer The difference in directions of oblique magnetic domain wall movement as a function of the magnetization direction has been reported by some groups,15,20–23 whereas the difference in directions of the BD movement between the wire with the RE dominant magnetization and that with the TM FIG Magneto-optical Kerr micrographs of the current-induced effect on the BDs in the {Tb/Co}7 multilayer wire (a) The –z magnetized and (b) +z magnetized BDs at 25 ◦ C (the RE dominant magnetization) (c)The –z magnetized and (d) +z magnetized BDs at 60 ◦ C (the TM dominant magnetization) The images in the initial states and after the application of one current pulse of BDs are shown by solid and dashed lines, respectively 055916-4 Tanaka et al AIP Advances 7, 055916 (2017) dominant magnetization has not been reported In order to investigate the BD dynamics in a magnetic wire through the spin injection from the Pt layer, we performed micromagnetic calculations of the BD dynamics in a simple ferromagnetic nanowire with perpendicular magnetic anisotropy based on the Landau–Lifshitz equation with spin-transfer torque term, as follows:24 ∂m (m × (m × uP)) , = − |γ| m × Heff − α |γ| (m × (m × Heff )) − ∂t MS (1) where m is the unit vector of the magnetic moment, t is the simulation time, γ is the gyro-magnetic ratio of –1.76×107 rad/(s Oe), α is the damping constant of 0.4, H eff is the effective field composed of the contribution from the long-range magnetic dipole-dipole interactions, the short-range exchange interactions between the neighboring computational cells, the perpendicular magnetic anisotropy, and the DMI effective field,25 M S is the saturation magnetization, u is a parameter which depends on the amount of the injected spins and has the dimension of velocity, and P is the spin-polarized vector of the injected spins We assumed the following parameters in order to stabilize magnetic bubbles: M S of 300 emu/cm3 , a DMI parameter D of 0.1 erg/cm2 , an exchange stiffness constant of 1.0×1.0 7 erg/cm,26 and an anisotropy constant K u of 1.0×106 erg/cm3 26 For the sake of ease, the STT from the electric current flowing in the magnetic wire was neglected Instead, the spin-current of +y polarized spins via SHE of the Pt layer were assumed in the calculation, where the spin-polarized vector of the injected spin P was (0,1,0) An injected velocity, which was the measure of the spin current, was taken to be u = 1000 cm/s This value was equivalent to the spin current of 6.6×106 A/cm2 with spin-polarization of 1.0 The spin-diffusion length of the injected spin was assumed to be nm The width, length, and thickness of the wire were 100 nm, 250 nm, and nm, respectively For our simulations, each simulated wire was divided into small cuboid cells (dimensions: 2.5 nm×2.5 nm×5 nm) The domain walls of BDs were N´eel walls The domain-wall-magnetic moments of –z and +z magnetized BDs respectively point toward and outward from the BD centers, which was determined by the current induced BD dynamics under the in-plane magnetic fields.14 Figure shows the micromagnetic calculations of (a) –z and (b) +z magnetized BD dynamics after the spin injection from the upper nonmagnetic layer The –z and +z magnetized BDs moved in the (+x, +y) and (+x, –y) direction, respectively These calculation results were similar to the experimental results of the current induced BD dynamics of the {Tb/Co}7 wire of the TM dominant magnetization The mechanism which causes the oblique FIG Micromagnetic calculations of BDs injected the +y polarized spins from the upper nonmagnetic metallic layer The solid lines show initial shapes of BDs The domain walls of the initial states are N´eel walls (a) The –z magnetized BDs moved in (+x, +y) direction (b) The +z magnetized BDs moved in (+x, –y) direction 055916-5 Tanaka et al AIP Advances 7, 055916 (2017) BD movement cannot be explained at the present stage More experimental demonstrations of the current induced dynamics of BDs are needed for the 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wire with the transition-metal-dominant and rare-earthdominant magnetizations was obtained by changing temperature We found that the bubble. .. second using FIG Polar Kerr image of a BD in a {Tb/Co}7 multilayer wire and illustration of the experimental setup for the current induced dynamics measurements 055916-3 Tanaka et al AIP Advances