Effect uses less energy than state-of-the-art devices by orders of magnitude.

FINDINGS
We all have experienced that our smartphones or computers can become very warm after operating for a while. This is because of the heat generated by electric current in the smartphone or computer memory and logic elements. This heat is a waste of energy. One way to minimize the waste heat is to use a revolutionary alternative– namely, electric field nonvolatile spintronics – to power memory and logic devices. A research team from UCLA Henry Samueli School of Engineering and Applied Science is the first to demonstrate that an electric field can induce magnetization switching in magnetic topological insulator materials, paving the way to far greater energy efficiency in devices that perform memory and logic functions without standby power dissipation.

A topological insulator is a new class of materials that acts as both an insulator and a metallic conductor. The interior of the topological insulator prevents the flow of electric current while its surface allows the propagation of spin-polarized electrons with very little resistance. When a topological insulator is infused (by a method called doping) with chromium, a magnetic element, the material becomes a magnetic topological insulator, and the spin-polarized surface current is powerful enough to switch the magnetic polarity of the chromium atoms from up (or one) to down (or zero). This switching is what enables the devices to write memory.

Last year, the same research team from UCLA Engineering demonstrated that electric current could switch the magnetic polarity of a magnetic topological insulator. While this technique uses 1,000 times less electric current than comparable memory structures, the breakthrough reported this month uses far less energy.

Unlike electric current, in which electrons move like water flowing in a river, an electric field works on the reservoir of electrons on top of a mountain. The magnetic polarity of the magnetic topological insulator is turned upside down by moving the water (electrons) in the reservoir from a high voltage (or a high potential energy) down to a low voltage (or low potential energy). This voltage change requires extremely small (nearly zero) electric current.

This month, in the new research published in Nature Nanotechnology (DOI: 10.1038/NNANO.2015.294), the researchers, led by Kang L. Wang, UCLA’s Raytheon Chair Professor of Electrical Engineering, demonstrate the concept that they can switch the magnetic polarity of magnetic topological insulators by the electric field generated from gate voltage. In this new research, the UCLA Engineering team achieves two orders of magnitude less energy for switching than comparable gate-controlled memory devices.

IMPACT
Together with the previous work, these two breakthroughs demonstrate that topological insulator is a very promising material candidate for ultrahigh-performance, ultralow-power, and energy-efficient computers and smartphones in this era of big data.

AUTHORS
The lead authors on the research were UCLA electrical engineering Ph.D. candidate Yabin Fan, and Xufeng Kou and Pramey Upadhyaya, both recent UCLA Ph.D. graduates in electrical engineering. The principal investigator on the research is Kang L. Wang, UCLA’s Raytheon Chair Professor of Electrical Engineering. Other authors include: Qiming Shao, Lei Pan, Murong Lang, Xiaoyu Che, Jianshi Tang, Mohammad Montazeri, Koichi Murata, Li-Te Chang, Mustafa Akyol, Guoqiang Yu, Tianxiao Nie, and Kin L. Wong from UCLA’s Electrical Engineering Department; Jun Liu and Yong Wang from Zhejiang University, China; and Yaraslov Tserkovnyak, UCLA Professor from Department of Physics and Astronomy.

FUNDING
The research was funded by the U.S. Department of Energy and the U.S. Army Research Office. The researchers were also supported by the Center for Function Accelerated nanoMaterial Engineering (FAME), a multi-institution research center based at UCLA and funded by DARPA and a consortium of semiconductor industry companies.

JOURNAL:
The research has been published online in the journal Nature Nanotechnology.
DOI: 10.1038/NNANO.2015.294

Image: Schematic of magnetization reversal induced by electric field in a magnetic topological insulator.