On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.09.2020 - 15:00 (CET)

Beyond Heisenberg Solids: From Multi-Spin Interactions to Novel Chiral Particles

Stefan Blügel, FZ Juelich

 

It became customary to study the stability, lifetime, dynamics, thermodynamics and transport properties of localized nanoscale magnetization particles such as skyrmionics by classical spin-lattice models with pairwise Heisenberg-type exchange interactions. The mapping of fermionic many-body systems onto a classical Heisenberg model is a nontrivial thing and by far not unique. In this presentation I motivate beyond Heisenberg multi-spin interactions [1]. I give examples, where these interactions play a decisive role [2]. I focus on MnGe in the B20- phase, which exhibits a three-dimensional spin-texture. We introduce a novel class of magnetic exchange interactions [3] – the topological-chiral interactions (TCI) rooted in the so- called topological orbital moment, which manifests as a result of finite scalar spin chirality in non-coplanar magnets. The long-wave length limit of the interactions relates to the highly acclaimed Faddeev model demonstrating that the interaction is an origin of 3D magnetization textures all the way down to hopfions.

[1] M. Hoffmann, S. Blügel, PRB 10, 024418 (2019).
[2] A. Krönlein, M. Schmitt, M. Hoffmann, J. Kemmer, N. Seubert, M. Vogt, J. Küspert, M. Böhme, B. Alonazi, J. Kügel, H. A. Albrithen, M. Bode, G. Bihlmayer, and S. Blügel, PRL 120, 207202 (2018).
[3] S. Grytsiuk, J.-P. Hanke, M. Hoffmann, J. Bouaziz, O. Gomonay, G. Bihlmayer, S. Lounis, Y. Mokrousov, S. Blügel, Nat. Commun. 11, 511 (2020).

PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.09.2020 - 15:00 (CET)

Spintronics Nanodevice
- How small can we make it and what else can we use it for -

Hideo Ohno, Tohoku University

Development of spintronics nonvolatile nanodevices and their integration with CMOS circuits has resulted in realizing low-energy, yet high performance integrated circuits suitable for a number of applications such as Internet-of-Things (IoT), high-performance computing and artificial intelligence. Magnetic tunnel junction (MTJ), a spintronics device, plays a central role here, which has been shown to scale down to 20 nm with the perpendicular-easy-axis CoFeB-MgO system [1, 2]. I will first discuss the factors that limit the scalability of such MTJs. Then show how one can extend its scalability to the range of 4-8 nm and below [3, 4] by employing a new (and yet not so new) concept. Current-induced switching of magnetization and high thermal stability of these devices are also shown. I will then describe how one can use less stable MTJs for a novel form of computation, probabilistic computing, to address optimization problems. I show that one can formulate integer factorization as an optimization problem in such a way that the most preferred state in terms of energy gives the factorized result [5]. If I have time I will touch upon proof-of-concept spintronics devices for artificial synapse as well as neuron for neuromorphic applications [6, 7].
Work done in collaboration with S. Fukami and the CSIS team. A portion of the work described here is a result of collaboration with A. Z. Pervaiz, K. Y. Camsari, and S. Datta of Purdue University. Supported in part by the ImPACT Program of CSTI, JST-OPERA JPMJOP1611 and Grant-in-Aid for Specially Promoted Research (17H06093).

References
[1] S. Ikeda, et al. Nature Materials, 9, 721 (2010).
[2] H. Sato, et al. IEDM 2013 and Appl. Phys. Lett. 105, 062403 (2014).
[3] K. Watanabe, et al. Nature Commun. 9, 663 (2018).
[4] B. Jinnai, et al. Appl. Phys. Lett. (Perspective), 116, 160501 (2020).
[5] W. A. Borders, et al. Nature 573, 390-393 (2019).
[6] W. A. Borders et al. Appl. Phys. Express 10, 013007 (2017).
[7] A. Kurenkov, et al. Advanced Materials 31, 1900636 (2019).

PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.09.2020 - 15:00 (CET)

Spintronic devices for artificial neural networks

Saima Siddiqui, University of Illinois

Spintronics promises intriguing device paradigms where electron spin is used as the information token instead of its charge counterpart. In the future cognitive era, nonvolatile magnetic memories hold the key to solve the bottleneck in the computational performance due to data shuttling between the processing and the memory units. The application of spintronic devices for these purposes requires versatile, scalable device design that is adaptable to emerging material physics. We design, model and experimentally demonstrate spin orbit torque induced magnetic domain wall devices as the building blocks (i.e. linear synaptic weight generator and the nonlinear activation function generator) for in-memory computing, in particular for artificial neural networks. Spin orbit torque driven magnetic tunnel junctions show great promise as energy efficient emerging nonvolatile logic and memory devices. In addition to its energy efficiency, we take advantage of the spin orbit torque induced domain wall motion in magnetic nanowires to demonstrate the linear change in resistance of the synaptic devices. Modifying the spin-orbit torque from a heavy metal or utilizing the size dependent magnetoresistance of tunnel junctions, we also demonstrate a nonlinear activation function for thresholding signals (analog or digitized) between layers for deep learning. A complete neuromorphic hardware accelerator using embedded nonvolatile magnetic domain wall devices can revolutionize computer architectures by embedding memory into logic circuits in a fine grained fashion.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.09.2020 - 15:00 (CET)

Long-Range Phonon Spin Transport

Rembert Duine, Utrecht University

One of the goals of spintronics is to achieve dissipationless spin currents. In this talk, I will discuss phonon spin transport in an insulating ferromagnet-nonmagnet-ferromagnet heterostructure. I will discuss how the magnetoelastic interaction between the spins and the phonons leads to nonlocal spin transfer between the magnets. This transfer is mediated by a local phonon spin current and accompanied by a phonon spin accumulation. The spin conductance depends nontrivially on the system size, and decays over millimeter length scales for realistic material parameters, far exceeding the decay lengths of magnonic spin currents.

 

 

 

 

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.08.2020 - 15:00 (CET)

Magnetic Matchmaking: Hybrid Magnon Modes

Axel Hoffmann, University of Illinois

Hybrid dynamic excitations have gained increased interest due to their potential impact on coherent information processing. Towards this end, magnons, the fundamental excitation quant of magnetically ordered systems, are of particular interest, since they can be easily tuned by external magnetic fields and interact with a wide range of other excitations, such as microwave and optical photons, phonons, and other magnons.1 We have explored recently the integration of permalloy (Ni80Fe20) thin film structures into hybrid magnon systems. By combining permalloy structures with high-quality superconducting microwave resonators, we demonstrated strong magnon-photon coupling in co-planar, on-chip geometry, which is readily scalable to more complex devices.2 Furthermore, we demonstrated strong coupling of permalloy magnons to standing magnon modes in yttrium iron garnet films, which revealed the importance of dampin-like torques originating from spin pumping.3 Lastly, we demonstrated how the coupling between magnons in Ni and surface acoustic waves in LiNbO3 can be used to modulate phonon propagation.4
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.

PDF file of the talk available here

References:
1. Y. Li, et al., arXiv:2006.16158.
2. Y. Li, et al., Phys. Rev. Lett. 123, 107701 (2019).
3. Y. Li, et al., Phys. Rev. Lett. 124, 117202 (2020).
4. C. Zhao, et al., Phys. Rev. Appl. 13, 054032 (2020).