New Magnetic Semiconductor Material Holds Promise for ‘Spintronics’

Researchers at North Carolina State University have created a new compound that can be integrated into silicon chips and is a dilute magnetic semiconductor – meaning that it could be used to make “spintronic” devices, which rely on magnetic force to operate, rather than electrical currents.

The researchers synthesized the new compound, strontium tin oxide (Sr3SnO), as an epitaxial thin film on a silicon chip. Epitaxial means the material is a single crystal. Because Sr3SnO is a dilute magnetic semiconductor, it could be used to create transistors that operate at room temperature based on magnetic fields, rather than electrical current.

“We’re talking about cool transistors for use in spintronics,” says Dr. Jay Narayan, John C. Fan Distinguished Professor of Materials Science and Engineering at NC State and senior author of a paper describing the work. “Spintronics” refers to technologies used in solid-state devices that take advantage of the inherent “spin” in electrons and their related magnetic momentum.

“There are other materials that are dilute magnetic semiconductors, but researchers have struggled to integrate those materials on a silicon substrate, which is essential for their use in multifunctional, smart devices,” Narayan says. “We were able to synthesize this material as a single crystal on a silicon chip.”

“This moves us closer to developing spin-based devices, or spintronics,” says Dr. Justin Schwartz, co-author of the paper, Kobe Steel Distinguished Professor and Department Head of the Materials Science and Engineering Department at NC State. “And learning that this material has magnetic semiconductor properties was a happy surprise.”

The researchers had set out to create a material that would be a topological insulator. In topological insulators the bulk of the material serves as an electrical insulator, but the surface can act as a highly conductive material – and these properties are not easily affected or destroyed by defects in the material. In effect, that means that a topological insulator material can be a conductor and its own insulator at the same time.

Two materials are known to be topological insulators – bismuth telluride and bismuth selenide. But theorists predicted that other materials may also have topological insulator properties. Sr3SnO is one of those theoretical materials, which is why the researchers synthesized it. However, while early tests are promising, the researchers are still testing the Sr3SnO to confirm whether it has all the characteristics of a topological insulator.

The paper, “Epitaxial integration of dilute magnetic semiconductor Sr3SnO with Si (001),” was published online Sept. 9 in Applied Physics Letters. Lead author of the paper is Y. F. Lee, a Ph.D. student at NC State. Co-authors include F. Wu and R. Kumar, both Ph.D. students at NC State, and Dr. Frank Hunte, an assistant professor at NC State. The work was supported, in part, by the National Science Foundation.

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Note to Editors: The study abstract follows.

“Epitaxial integration of dilute magnetic semiconductor Sr3SnO with Si (001)”

Authors: Y.F. Lee, F. Wu, R. Kumar, F. Hunte, J. Schwartz, and J. Narayan, North Carolina State University

Published: online Sept. 9, Applied Physics Letters

DOI: 10.1063/1.4820770

Abstract: Epitaxial thin films heterostructures of topological insulator candidate Sr3SnO (SSO) are grown on a cubic yttria-stabilized zirconia (c-YSZ)/Si (001) platform by pulsed laser deposition. X-ray and electron diffraction patterns confirm the epitaxial nature of the layers with cube-on-cube orientation relationship: (001)[100]SSOk(001)[100]c-YSZk(001)[100]Si. The temperature dependent electrical resistivity shows semiconductor behavior with a transport mechanism following the variable-range-hopping model. The SSO films show room-temperature ferromagnetism with a high saturation magnetization, and a finite non-zero coercivity persisting up to room temperature. These results indicate that SSO is a potential dilute magnetic semiconductor, presumably obtained by controlled introduction of intrinsic defects.