New Material Structure Enables Magnetic Memory Without Magnetic Fields

The quickly growing demand for data-intensive computing tasks, from artificial intelligence in the cloud to automotive systems and blockchain technologies, is driving demand for new types of emerging random-access memory (RAM).

An emerging class of RAM, referred to as magnetic RAM (MRAM), is a candidate for this purpose, and is gaining increased traction within the semiconductor industry. One type of MRAM, however, called spin-orbit torque (SOT) MRAM, requires an electric current to pass through a material, as well as an external magnetic field to operate, which is hard to implement on a semiconductor chip.

Researchers from Northwestern Engineering, in collaboration with Beihang University in China, University of Messina and Politecnico di Bari in Italy, and the Université Paris-Saclay in France, have developed a new material structure that solves this problem. Once implemented into SOT-MRAM on computer chips, it can enable faster and more energy-efficient data processing in applications such as machine learning, which rely on big amounts of data for their artificial neural networks.

Pedram Khalili, associate professor of electrical and computer engineering, and his colleagues presented the findings in the paper “Field-free Spin-orbit Torque-induced Switching of Perpendicular Magnetization in a Ferrimagnetic Layer With a Vertical Composition Gradient,” published July 27 in the academic journal Nature Communications.

The material is based on a multi-layered stack of ultrathin films of CoTb, an alloy of the elements cobalt and terbium. In addition to demonstrating that SOT switching in this layered material does not require any external magnetic fields, the authors showed that it is also one of the most efficient SOT materials reported to date.

The work also shows reproducibility of the switching method across large semiconductor wafers, which is an important prerequisite for adoption in manufacturing. Finally, the study reveals new fundamental insights about the dynamics of switching by SOT in materials with chiral symmetry breaking (where rotation of spins in one direction is more favorable than in the opposite direction), which may also find applications in areas beyond memory devices.

The work at Northwestern was supported by a grant from the National Science Foundation (NSF), and by the NSF Materials Research Science and Engineering Center (MRSEC) at Northwestern.