Abstract and Presentation Notice-Spin-Orbit Technologies: From Magnetic Memory to Terahertz Generation

Prof. Hyunsoo Yang

https://www.chalmers.se/en/departments/mc2/calendar/Pages/Hyunsoo-Yang.aspx

Spintronic devices utilize an electric current to alter the state of a magnetic material and thus find great applications in magnetic memory. Over the last decade, spintronic research has focused largely on techniques based on spin-orbit coupling, such as spin-orbit torques (SOTs), to alter the magnetic state. The phenomenon of spin-orbit coupling in magnetic heterostructures was also recently used to generate terahertz emission and thus bridge the gap between spintronics and optoelectronics research. I will introduce the basic concepts of SOTs, such as their physical origin, the effect of SOTs on a magnetic material, and how to quantitatively measure this effect [1,2]. 

Next, I will discuss the latest trends in SOT research, such as the exploration of novel material systems like topological insulators and two-dimensional materials to improve the operation efficiency [2,3]. Following this, some of the technical challenges in SOT-based magnetic memory will be highlighted [3]. Moving forward, I will introduce the process of terahertz generation in magnetic heterostructures [4], where the spin-orbit coupling phenomenon plays a dominant role. I will discuss the details of how this terahertz emission process can be extended to novel material systems such as ferrimagnets [5] and topological materials [6]. The final section will focus on how the terahertz generation process can be used to measure SOTs in magnetic heterostructures, thus highlighting the interrelation between terahertz generation and the SOTs, which are linked by the underlying spin-orbit coupling. 

 

[1] X. Qiu et al., “Characterization and manipulation of spin orbit torque in magnetic heterostructures,” Adv. Mater., 30, 1705699 (2018). 

[2] Y. Wang et al., “FMR-related phenomena in spintronic devices” J. Phys. D: Appl. Phys., 51, 273002 (2018).

[3] R. Ramaswamy et al., “Recent advances in spin-orbit torques: Moving towards device applications” Appl. Phys. Rev., 5, 031107 (2018). 

[4] Y. Wu et al., “High-performance THz emitters based on ferromagnetic/nonmagnetic heterostructures” Adv. Mater., 29, 1603031 (2017). 

[5] M. Chen, et al., “Terahertz emission from compensated magnetic heterostructures,” Adv. Opt. Mater., 6, 1800430 (2018).

[6]   X. Wang, et al., “Ultrafast spin-to-charge conversion at the surface of topological insulator thin films” Adv. Mater. 30, 1802356 (2018).

Hyunsoo Yang obtained the bachelor’s degree from Seoul National University and the PhD degree from Stanford University. He worked at C&S Technology, Seoul; LG Electronics, San Jose, CA; and Intelligent Fiber Optic Systems, Sunnyvale, CA, USA. From 2004 to 2007, he was at the IBM-Stanford Spintronic Science and Applications Center, IBM Almaden Research Center. He is currently a GlobalFoundries chaired associate professor in the Department of Electrical and Computer Engineering, National University of Singapore, working on various magnetic materials and devices for spintronics applications. He has authored 170 journal articles, given 100 invited presentations, and holds 15 patents. Prof. Yang was a recipient of the Outstanding Dissertation Award for 2006 from the American Physical Society’s Topical Group on Magnetism and Its Applications and the IEEE Magnetic Society Distinguished Lecturer for 2019.  ​

If you want to meet and discuss with Prof. Hyunsoo Yang​ please contact Saroj Dash, saroj.dash@chalmers.se​

Category Lecture; Seminar

Location:  Kollektorn, lecture room, Kemivägen 9, MC2-huset

Starts: 13 June, 2019, 11:00

Ends: 13 June, 2019, 12:00

Antiferromagnetic memories allow higher data volume and faster write speed.

http://www.smart2zero.com/news/writing-speed-terahertz

With a frequency of several terabits per second, data rushes through the fiber optic cables. Once they arrive at the PC or television, data processing only continues at the speed of the electronic components - currently just several hundred gigabits per second. Researchers at the University of Mainz (JGU) have developed a technology that can increase data processing speed a hundredfold and close the gap between transport and processing speed. 
In the future, bandwidths that are too low could be a thing of the past: Researchers from the Academy of Sciences of the Czech Republic, together with their colleagues from JGU, have discovered a way to drastically increase the speed of data processing. To be more precise: a hundred times, on a terahertz.

To understand the background, first an excursion to the principle of magnetic storage. Usually magnetic data storage devices are based on ferromagnetic materials. However, these have limits in two respects: On the one hand, the data cannot be packed as tightly as you like, and the capacity of the memory reaches a natural limit. The data is stored in a kind of tiny bar magnet, which symbolizes a zero or a one, depending on the orientation. However, if these "rod magnets" are too close to each other, they influence each other. On the other hand, the speed at which these data memories can be written is limited. It doesn't get any faster than in the gigahertz range - otherwise the energy consumption will be immense.

The situation is different with antiferromagnetic storage devices. They can be described much more densely - because the "rod magnets" are always alternately aligned here and thus do not influence each other. This means that considerably more data can be stored on it. On the other hand, they solve the problem of limited writing speed.

"If data is to be sent, light is used which is sent via optical fibre cables," explains

professor Jairo Sinova, head of the "Interdisciplinary Spintronics Research" group at JGU. "This is extremely fast with frequencies in the terahertz range. At present, this speed must be reduced for processing in computers or televisions, where data is processed and stored electrically at speeds of several hundred gigahertz. The antiferromagnetic memories are now able to work directly with the data in the terahertz range for the first time". With this technology, the signal on the device no longer needs to be slowed down, but can also be processed on the computer or TV at terahertz speed.

The first research was carried out in 2014, when the scientists sent an electric current through the antiferromagnets to align the small storage units. They used a copper cable - a slow connection method. Instead, they use a short laser pulse to induce an electric current. This current aligns the'rod magnets', i.e. the spins. In this way, the researchers were able to drastically increase the speed. 

Abstract-Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface

Wei Zhang, Wei He, Xiang-Qun Zhang, Zhao-Hua Cheng, Jiao Teng, and Manfred Fähnle

https://journals.aps.org/prb/accepted/c2075Y25Ia810e5b542200726ae86a14d2dd92127

The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time, \begin{figure}[htbp] } \label{fig1} \end{figure} , and intrinsic Gilbert damping, \begin{figure}[htbp] } \label{fig2} \end{figure} . In previous attempts to establish the relationship between \tauM and \alphaintr , the rare-earth doping of a permalloy film with two different demagnetization mechanism is not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between \begin{figure}[htbp] } \label{fig3} \end{figure} and \begin{figure}[htbp] } \label{fig4} \end{figure} by means of time-resolved magneto-optical Kerr effect (TRMOKE) via adjusting the thickness of the Ni layers, and obtain an approximately proportional relation between these two parameters. The remarkable agreement between TRMOKE experiment and the prediction of breathing Fermi-surface model confirms that a large Elliott-Yafet spin-mixing parameter b2 is relevant to the strong spin-orbital coupling at the Co/Ni interface. More importantly, a proportional relation between \tauM and \alpha \mbox{intr} in such metallic films or heterostructures with electronic relaxation near Fermi surface suggests the local spin-flip scattering domains the mechanism of ultrafast demagnetization, otherwise the spin-current mechanism domains. It is an effective method to distinguish the dominant contributions to ultrafast magnetic quenching in metallic heterostructures by investigating both the ultrafast demagnetization time and Gilbert damping simultaneously. Our work can open a novel avenue to manipulate the magnitude and efficiency of Terahertz emission in metallic heterostructures such as the perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers, and then it has an immediate implication of the design of high frequency spintronic devices.https://journals.aps.org/prb/accepted/c2075Y25Ia810e5b542200726ae86a14d2dd92127