Dr. Priyamvada Jadaun and associates at the The University of Texas at Austin's Microelectronics Research Center have been researching SHE, the Spin Hall Effect. First predicted by Dyakonov and Perel, SHE occurs when an electric current is passed through certain materials and a spin current is produced which consists of electrons with opposite spins flowing in opposite directions. It was later shown that this spin current can be utilized to flip the magnetization direction of a magnet say between ‘up’ and ‘down’ states that represent the bit ‘zero’ and ‘one’ respectively. The paper was recently published in PNAS, the Proceedings of the National Academy of Sciences of the United States of America.
As our digital world consumes and produces expansive amounts of data (labelled Datageddon by HBO’s ‘Silicon Valley’), technologies for storing data need to rapidly advance. A leading contender for next-generation memories is a spintronic device that stores data in a small magnet controlled with an electric current or field via the spin Hall effect (SHE). SHE is a mechanism by which materials convert an input electric current into an output spin current. This spin current can be further used to write bits of data onto the nanomagnet, leading to electric current control of the nano-magnet.
With the right spin Hall materials, SHE based memories would be highly attractive for the next-generation as these memories are fast, compact and energy-efficient. In this work, we discover a new class of materials that demonstrate giant spin Hall efficiency, i.e., it can generate large amounts of output spin current per input charge current. Such materials make SHE based memories highly energy efficient.
Apart from its significant impact on technology, SHE involves important scientific questions that are yet to be answered. Here we answer one such question, i.e., what factors controls the value of SHE in a material? We develop five design principles that determine the value of SHE in a wide variety of materials. Our findings bring deeper insight into the physics driving SHE and could help enhance and externally control SHE values.
We spoke with Dr. Jadaun to learn more about SHE and where the research is going.
What are the key factors that determine the value of SHE?
In this work, we derive five general factors that control the value of SHE in a transition metal oxide, essentially determining if it will demonstrate a large spin Hall efficiency. According to our design principles, oxides with (i) weak crystal fields (ii) distortions in structure (iii) optimal positioning of Fermi level are promising for large SHE. We further discover a large class of oxides called anti-perovskites that have weak crystal fields and are highly promising for giant SHE. We also note that structures of oxide thin films can be distorted using electric fields in a controllable way, thereby allowing for real-time control of SHE in devices. Additionally, we show that (iv) certain materials with mixing of orbitals due to moderate spin orbit coupling and (v) moderate electron correlations can also enhance SHE values.
What materials are used?
An interesting discovery has been the promise of anti-perovskites TM3BO for giant SHE, where TM is a 5d transition metal. Anti-perovskites as the name suggests are oxides with a structure opposite to that of the well-known perovskites. In an anti-perovskite TM3BO, every TM atom is surrounded by only two O atoms (as opposed to six O atoms in a regular perovskite) thereby making the crystal field due to O weak in the anti-perovskite. As we have seen above, weak crystal fields can lead to large SHE, making anti-perovskites very promising.
What is the next step? Where do you go from here?
We are following up on this research by exploring SHE in a variety of anti-perovskite materials. We calculate SHE values using our in-house SHE code (https://github.com/codellama/Spin-Hall-W90) and TACC’s resources and are working to (i) discover new native anti-perovskite materials with large SHE and (ii) design composite alloys to further enhance SHE values.
The resources provided by the Texas Advanced Computing Center (TACC), including access to their supercomputing systems (Lonestar5 and Stampede2), their storage system (Ranch) as well as their expert support staff have been critical to the success of the project. TACC’s leading high performance computational facilities help us push scientific discovery forward.
Dr. Priyamvada Jadaun is a Research Associate in the Dept. of Electrical & Computer Engineering at The University of Texas at Austin. She works on the design of innovative devices and materials for the next generation of technologies such as emerging memories, neuromorphic computing and quantum computing. She obtained her MA in Physics and PhD in ECE from UT, following which she worked as a postdoctoral researcher at Cornell University.