Johan Åkerman of University of Gothenburg

Mutually synchronized spin torque nano-oscillators (STNOs) are one of the promising platforms for bioinspired computing and microwave signal generation [1,2]. Using STNOs one can achieve 90% recognition rate in spoken vowels [3]. However, in order to do more complex tasks, larger scale synchronized oscillators are needed, something that is not easily done with the STNOs demonstrated so far.

In my talk, I will describe a different type of spin current driven device called spin Hall nano-oscillators (SHNOs), which can generate microwave frequencies over a very wide frequency range [4]. The SHNOs are based on 50 – 120 nm wide nano-constrictions in Pt(5)/Hf(0.5)/NiFe(3) trilayers (all numbers in nm).  When multiple nano-constrictions are fabricated close to each other (300 – 1200 nm separation) they can mutually synchronize and chains of up to nine nano-constrictions have been demonstrated to exhibit complete synchronization [5]. For the first time, we can now also synchronize two-dimensional SHNO arrays with as many as 8 x 8 = 64 SHNOs [6]. The mutual synchronization is observed both electrically and using scanning micro-BLS microscopy. Both the output power and linewidth of the microwave signal improves substantially with increasing number of mutually synchronized SHNOs, such that quality factors of about 170,000 can be reached. Following the approach of Romera et al [3], we also demonstrate neuromorphic computing using a 4 x 4 SHNO array with two injected microwave signals as inputs. Given their high operating frequency (~10 GHz), easy of fabrication, and highly robust synchronization properties, nano-constriction SHNO arrays are likely the most promising candidates for neuromorphic computing based on oscillator networks.

[1]   J. Grollier, D. Querlioz, and M. D. Stiles, Proc. IEEE 104, 2024 (2016)

[2]   J. Torrejon et al, Nature 547, 428 (2017)

[3]   M. Romera et al, Nature 563, 230–234 (2018)

[4]   T. Chen, R. K. Dumas, A. Eklund, P. K. Muduli, A. Houshang, A. A. Awad, P. Dürrenfeld, B. G. Malm, A. Rusu, and J. Åkerman, Proc. IEEE 104, 1919 (2016)

[5]   A. A. Awad, P. Dürrenfeld, A. Houshang, M. Dvornik, E. Iacocca, R. K. Dumas, and J. Åkerman, Nature Physics 13, 292–299  (2017)

[6]   M. Zahedinejad, et al. arXiv:1812.09630 (2018)

Biography:

Prof. Johan Åkerman received his Ph.D. in Materials Physics from KTH Royal Institute of Technology in 2000. After a post-doc at University of California, San Diego, he joined Motorola, for four years, to be responsible for MRAM reliability. The MRAM technology he helped to launch remains the most commercially successfully MRAM to date. In 2005, he returned to Sweden to start his own research group at the Department of Materials and Nanophysics at KTH Royal Institute of Technology. In 2008 he was recruited as Full Professor to the Physics Department at University of Gothenburg, while remaining a Guest Professor at KTH. Prof. Åkerman has been working with spintronic technology for the last 20 years and has authored over 240 scientific papers, cited about 9000 times, holds 10 patents, and has given over 100 invited talks. He is also the founder and CEO of two start-up companies, NanOsc AB, commercializing spintronic devices, and NanOsc Instruments AB, designing and manufacturing spectrometers for ferromagnetic resonance measurements at cryogenic and room temperatures. His main projects are related to spin torque and spin Hall nano-oscillators, with particular focus on mutual synchronization, magnetodynamical solitons, and oscillator networks for neuromorphic computing.

Email:

Address:Department of Physics, Origovägen 6 B, Göteborg, Vastra Gotalands lan, Sweden, 41296