Spin Orbit Torque (SOT) induced multilevel magnetization switching shows potential applications in the field of high-density data storage1 and neuromorphic computing2. However, the difficulty to locally pattern magnetic bits with a well-defined set of several different controlled SOT switching current values limits the practical applicability of this technology.
We investigate the current induced switching properties of Pt/Co/W heterostructures with perpendicular magnetic anisotropy (PMA) by patterning into Hall bars and performing focused He+ beam assisted mask-less irradiation. The critical current required to switch the magnetic state depends on the saturation magnetization and magnetic anisotropy, which are interface dependent and can be tuned by the He+ irradiation dose 3. By means of partial irradiation of the Hall junction with several different doses, we design a device whose intermediate multi-domains states can be deterministically accessed using an electric current, resulting in a SOT induced multi-level switching.
We do the in-situ monitoring of the Hall voltage under ion irradiation of different exposed areas (Fig. 1a) which not only allow us to determine the critical dose but also confirms the Hall bar’s spatial sensitivity towards partial junction irradiation. By irradiating the Hall cross distinct areas with two different doses, we achieve a 4-level switching. To better understand the sample behavior and to optimize parameters, in operando magneto-optical Kerr effect (MOKE) imaging is used during SOT measurements. A thorough investigation of irradiation dose and pattern design make possible up to 8-level switching as shown in Fig. 1b.
We demonstrate a promising and practical approach for improved data storage and high-performance computation in SOT-based spintronic devices which opens the door to preferential current driven magnetisation switching of predetermined areas of the sample, defined down the nm resolution of ion beam microscopy.
1 K.-F. Huang, D.-S. Wang, M.-H. Tsai, H.-H. Lin, and C.-H. Lai, “Initialization-Free Multilevel States Driven by Spin–Orbit Torque Switching,” Adv. Mater., vol. 29, no. 8, p. 1601575, 2017, doi: https://doi.org/10.1002/adma.201601575. 2 Y. Cao, A. Rushforth, Y. Sheng, H. Zheng, and K. Wang, “Tuning a Binary Ferromagnet into a Multistate Synapse with Spin–Orbit-Torque-Induced Plasticity,” Adv. Funct. Mater., vol. 29, no. 25, p. 1808104, 2019, doi: 10.1002/adfm.201808104. 3 C. Chappert et al., “Planar Patterned Magnetic Media Obtained by Ion Irradiation,” Science, vol. 280, no. 5371, pp. 1919–1922, Jun. 1998, doi: 10.1126/science.280.5371.1919.