Tuning magnetic properties for domain wall pinning via localized metal diffusion

www.nature.com/scientificreports OPEN Received: 7 February 2017 Accepted: 12 November 2017 Published: xx xx xxxx Tuning magnetic properties for domain wall pinning via localized metal diffusion T. L. Jin1,2, M. Ranjbar Piramanayagam 1 1 , S. K. He2, W. C. Law 1 , T. J. Zhou2, W. S. Lew1, X. X. Liu 3 & S. N. Precise control of domain wall displacement in nanowires is essential for application in domain wall based memory and logic devices. Currently, domain walls are pinned by creating topographical notches fabricated by lithography. In this paper, we propose localized diffusion of non-magnetic metal into ferromagnetic nanowires by annealing induced mixing as a non-topographical approach to form pinning sites. As a first step to prove this new approach, magnetodynamic properties of permalloy (Ni80Fe20) films coated with different capping layers such as Ta, Cr, Cu and Ru were investigated. Ferromagnetic resonance (FMR), and anisotropy magnetoresistance (AMR) measurements were carried out after annealing the samples at different temperatures (Tan). The saturation magnetization of Ni80Fe20 film decreased, and damping constant increased with Tan. X-Ray photoelectron spectroscopy results confirmed increased diffusion of Cr into the middle of Ni80Fe20 layers with Tan. The resistance vs magnetic field measurements on nanowires showed intriguing results. Domain wall based devices such as racetrack memory have been proposed as promising candidates for high-density, non-volatile information storage with a low energy consumption1–9. Such devices are also considered as 3-terminal memory devices at a higher level of memory-storage hierarchy. Information in these devices are stored in the directions of domain magnetization and read and written by moving domain walls. Since the domain wall motion is based on spintronics principles, the reading and writing process do not require mechanical rotation as in hard disk drives8–10. Figure 1(a) shows the simplified schematic diagram of domain wall memory that stores information in nanowire based on domain orientation and domain wall movement by pulse current for reading (writing) information. Domain wall motion along the nanowire, driven by in-plane current, has been investigated tremendously9,11–13. The theory for spin-transfer torque was reported by Berger and Slonczewski in 1996. Independently, they pointed out that a spin polarized current is generated when an electric current that goes through the ferromagnetic layer transfers the spin angular momentum to local magnetic moment via electron exchange interaction. This exerts a torque on the local magnetization, resulting in domain wall motion14–18. In earlier research work, the torque exerted by in-plane current named as spin transfer torque (STT), drove domain wall motion in the opposite direction of electric current flow, and is often considered as an effective field19,20. In those cases, the velocity of domain wall was just 100 m/s19,20. In the recent years, faster domain wall motion up to 400 m/s, has been observed in perpendicular magnetic anisotropy (PMA) material using pure spin polarized current12,21,22. In such structures, the pure spin polarized current originates from the electric current in heavy metal due to strong spin orbit coupling and Rashba effect, which is named as spin orbit torque (SOT)23,24. The high speed of domain wall motion also relate to the chiral domain wall structure in PMA material that is formed by interfacial Dzyaloshinskii-Doriya interaction (DMI)25. Recently, the speed of domain wall 750 m/s was reported in synthetic anti-ferromagnetic structure driven by SOT26. Spin polarized current gives a high speed performance of domain wall devices. However, for reliable operation of domain wall devices, the other key issue is the control of domain wall motion by pinning domain walls uniformly9,27–29. In the absence of pinning sites, the domain walls may be swept rapidly without precise control. In general cases, the pinning sites for domain wall devices are fabricated by complicated lithography process, like 1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore. 2Data Storage Institute, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-01 Innovis, Singapore, 138634, Singapore. 3Department of Electrical and Computer Engineering, Shinshu University, Wakasato 4-17-1, Nagano, 380-8553, Japan. Correspondence and requests for materials should be addressed to S.N.P. (email: ) ScIENTIfIc REPorTs | 7: 16208 | DOI:10.1038/s41598-017-16335-z 1 www.nature.com/scientificreports/ Figure 1. Illustration of domain wall device concept and the control of domain wall propagation using different structures. (a) Information stored in nanowire based on domain orientation and an illustration of domain wall movement by pulse current for reading (writing) information. (b) Illustration of controlling domain wall propagation using notches. (c) Schematic representation of the proposed method to control domain wall motion using metal diffusion. creation of notches and zigzag patterns in the ferromagnetic nanowire6,29–31. Moreover, the pinning effect could be non-uniform due to the shape and size variations, as shown in Fig. 1(b). Tuning the properties of nanowire locally to pin domain wall at precise position is more efficient for domain wall devices. Exchange bias has been proposed as one such method32. Several other methods to tune the properties of ferromagnetic, like focused Ga+ ion irradiation on film, non-magnetic metal doped in ferromagnetic film by ion implantation or co-deposition have also been reported33–37. Here, we propose an alternative method for controlling domain wall displacement by tuning the composition of ferromagnetic material locally with annealing to stabilize domain walls, as shown in Fig. 1(c). Experimental investigations were carried out to understand the effect of composition on the properties of Ni80Fe20 film. Diffusion of various metals including Ta, Cr, Cu and Ru, which are widely used in memory industry, was achieved at the interface through annealing under certain temperature. The Ni80Fe20 devices with cross pinning sites have been fabricated to show that this method could be useful in domain wall pinning. Experiments In order to find out if composition in magnetic films can be controlled by annealing induced mixing and if the properties can be tailored, 10 nm thick Ni80Fe20 thin films were sputtered with different metallic capping layers. The film stacks of the type Si/(SiO2)/Ni80Fe20 (10 nm)/X (5 nm), were deposited by dc magnetron sputtering. Here, X refers to the capping metallic layers such Ta, Cr, Cu and Ru. Permalloy film without capping layer was also prepared as reference. After deposition, Ni80Fe20 films with capping layer were annealed for 1 hour at different annealing temperatures, Tan, from 100 °C to 400 °C (...truncated)