Three single elements, iron, cobalt, and nickel, are known to exhibit ferromagnetism at room temperature. Now ruthenium has been added to this exclusive list. According to research recently published in Nature Communications, ruthenium, when applied and manipulated on an ultrathin film, can enter into a ferromagnetic phase at room temperature. The discovery could potentially lead to more efficient magnetoresistive random-access memory technologies, improved sensors, and novel spintronic computing systems.  

“This paper is very exciting because it demonstrates the existence of a new, single-phase ferromagnetic compound in the metastable, body-centered-tetragonal phase of ruthenium,” says Daniel Gopman, a condensed matter physicist at the National Institute of Standards and Technology, who was not involved in the work. “In the present environment—in which strain engineering of magnetic films and devices continues to generate significant attention—the results here showing that you can actually generate a magnetic phase in a nominally non-magnetic material using strain are a significant discovery, indeed.”

According to senior author Jian-Ping Wang, distinguished McKnight University Professor and Robert F. Hartmann Chair in Electrical Engineering at the University of Minnesota, both fundamental curiosity and interest in developing practical applications motivated his and his colleagues’ work. “Magnetism is one of the most interesting behaviors of materials, yet there are still a lot of unanswered questions on the fundamental level,” he says. “We’re also using magnetics every day, in all kinds of motors including motors in electric vehicles, hard disk drives of data centers, and more.”

New magnetism-related discoveries also offer one of the greatest possibilities for pushing beyond Moore’s law, which is Wang’s ultimate goal at the University of Minnesota’s Center for Spintronic Materials, Interfaces, and Novel Architectures (C-SPIN). Moore’s law refers to the yearly doubling of transistors per square inch on an integrated circuit (IC).

While ferromagnetism—strong magnetism that is integral for central processing units (CPU) in computers and other important technologies—can be coaxed out of many compounds at room temperature, Wang and his colleagues were interested in discovering whether the property might exist, unrecognized, in certain single elements. They focused on Ru because past predictions showed that it may be a good candidate for ferromagnetism. Ru is also a material that has been used in both the semiconductor and magnetic industry.

Seed layer engineering—using a template to induce and assist the formation of a body-centered tetragonal crystal structure in a material—was key to their investigation. They began with the time-consuming task of finding the right conditions under which to grow ruthenium with a tetragonal phase, rather than its preferred cubic phase. They started with a standard single crystal aluminum oxide substrate on which they grew a layer of molybdenum. On top of that—and maintaining a vacuum system with a base pressure greater than 8 × 10^-8—they grew the ruthenium. They used the magnetron sputtering process—a standard method used by both academic researchers and industry engineers.

After two years of testing and tweaking, they found a valid formula and conditions for growing the material in a way that would force ferromagnetism in the thin films. The saturation magnetization of the final prepared ruthenium film was 148 emu/cm^-3 at room temperature—about half of the theoretical calculated value.

They performed extensive tests on the ferromagnetic ruthenium’s crystal structure and magnetic properties, and they also cross-validated their findings using complementary experiments to exclude the possibility of magnetic contamination.

“Now that ruthenium can be easily made ferromagnetic as we’ve shown in this process, it gives us great flexibility for making devices for next-gen computing systems,” Wang says. He and his colleagues plan to focus on industry-specific applications next, including the possibility of using an electric field to manipulate ruthenium’s magnetism. On a more fundamental level, Gopman adds, researchers could benefit from studies that more deeply investigate the characteristics of ferromagnetic ruthenium, including its x-ray magnetic circular dichroism and polarized neutron reflectometry.

Wang also hopes that he and his colleagues’ work sparks similar studies. “Before, people weren’t giving much attention to this field of research,” he says. “I hope we have given people confidence for investigating other elements that are predicted to be ferromagnetic.”

Read the article in Nature Communications.