R&D: Discovery Could Improve Brain-Like Memory and Computing

Source: University of Minnesota College of Science and Engineering

From various magnetic tapes, floppy disks and computer HDD drives, magnetic materials have been storing our electronic information along with our valuable knowledge and memories for well over half of a century.

The schematic figure illustrates the concept and behavior of magnetoresistance.
The spins are generated in topological insulators.
Those at the interface between ferromagnet and topological insulators
interact with the ferromagnet and result in either high or low
resistance of the device, depending on the relative
directions of magnetization and spins.
(Credit: University of Minnesota)

In more recent years, the new types phenomena known as magnetoresistance, which is the tendency of a material to change its electrical resistance when an externally-applied magnetic field or its own magnetization is changed, has found its success in HDD drive read heads, magnetic field sensors and the rising star in the memory technologies, the magnetoresistive random access memory.

A new discovery, led by researchers at the University of Minnesota, demonstrates the existence of a new kind of magnetoresistance involving topological insulators that could result in improvements in future computing and computer storage. The details of their research are published in the most recent issue of the scientific journal Nature Communications.

“Our discovery is one missing piece of the puzzle to improve the future of low-power computing and memory for the semiconductor industry, including brain-like computing and chips for robots and 3D magnetic memory,” said Robert F. Hartmann, professor of electrical and computer engineering, University of Minnesota, and Jian-Ping Wang, director, Center for Spintronic Materials, Interfaces, and Novel Structures (C-SPIN) based at the University of Minnesota and co-author of the study.

Emerging technology using topological insulators
While magnetic recording still dominates storage applications, the magnetoresistive random access memory is gradually finding its place in the field of computing memory. From the outside, they are unlike the HDD drives which have mechanically spinning disks and swinging heads-they are more like any other type of memory. They are chips (solid state) which you’d find being soldered on circuit boards in a computer or mobile device.

Recently, a group of materials called topological insulators has been found to further improve the writing energy efficiency of magnetoresistive random access memory cells in electronics. However, the new device geometry demands a new magnetoresistance phenomenon to accomplish the read function of the memory cell in 3D system and network.

Following the recent discovery of the unidirectional spin Hall magnetoresistance in a conventional metal bilayer material systems, researchers at the University of Minnesota collaborated with colleagues at Pennsylvania State University and demonstrated for the first time the existence of such magnetoresistance in the topological insulator-ferromagnet bilayers.

The study confirms the existence of such unidirectional magnetoresistance and reveals that the adoption of topological insulators, compared to heavy metals, doubles the magnetoresistance performance at 150° Kelvin (-123.15° Celsius). From an application perspective, this work provides the missing piece of the puzzle to create a proposed 3D and cross-bar type computing and memory device involving topological insulators by adding the previously missing or very inconvenient read functionality.

In addition to Wang, researchers involved in this study include Yang Lv, Delin Zhang and Mahdi Jamali from department of electrical and computer engineering, University of Minnesota, and James Kally, Joon Sue Lee and Nitin Samarth, department of physics, Pennsylvania State University.

This research was funded by the Center for Spintronic Materials, Interfaces and Novel Architectures (C-SPIN), University of Minnesota, a Semiconductor Research Corporation program sponsored by the Microelectronics Advanced Research Corp. (MARCO) and the Defense Advanced Research Projects Agency (DARPA).

Article:Unidirectional spin-Hall and Rashba−Edelstein magnetoresistance in topological insulator-ferromagnet layer heterostructures

Nature Communications has published an article written by Yang Lv, Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, 55455, USA, James Kally, Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA, Delin Zhang, Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, 55455, USA, Joon Sue Lee, Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA, Mahdi Jamali, Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, 55455, USA, Nitin Samarth, Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA, and Jian-Ping Wang, Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, 55455, USA.

Illustration of USRMR in TI/FM bilayer.
Spin accumulation is generated at the interface and in the bulk
when a charge current is applied. The relative direction of the spin polarization
to the magnetization of either a parallel and b anti-parallel results
in different resistance states.

Abstract: “The large spin−orbit coupling in topological insulators results in helical spin-textured Dirac surface states that are attractive for topological spintronics. These states generate an efficient spin−orbit torque on proximal magnetic moments. However, memory or logic spin devices based upon such switching require a non-optimal three-terminal geometry, with two terminals for the writing current and one for reading the state of the device. An alternative two-terminal device geometry is now possible by exploiting the recent discovery of the unidirectional spin Hall magnetoresistance in heavy metal/ferromagnet bilayers and unidirectional magnetoresistance in magnetic topological insulators. Here, we report the observation of such unidirectional magnetoresistance in a technologically relevant device geometry that combines a topological insulator with a conventional ferromagnetic metal. Our devices show a figure of merit (magnetoresistance per current density per total resistance) that is more than twice as large as the highest reported values in all-metal Ta/Co bilayers.”