Luqiao Liu was the kind of kid who wanted to see how his toys worked rather than playing with them as intended.
Curiosity has been a driving force throughout his life, leading him to MIT, where Liu is a newly appointed associate professor in the Department of Electrical Engineering and Computer Science and a member of the Research Laboratory of Electronics.
Instead of taking things apart, he’s now using new materials and nanoscale fabrication techniques to make next-generation electronics that use dramatically less power than traditional devices. Curiosity is still useful, he says, as he and his collaborators work in the largely uncharted territory of spin electronics, which emerged only in the 1980s.
“There are many challenges we have to overcome in our work. In spin electronics, there is still a gap between what is fundamentally possible and what has been done so far. There’s still a lot to learn about getting better materials and finding new systems so we can reach higher and higher performance,” says Liu, who is also a member of the MIT-IBM Watson AI Lab.
Electrons are subatomic particles with a fundamental quantum property known as spin. One way to visualize this is to think of a spinning top that spins around itself, imparting angular momentum to the top. That angular momentum, which is a product of the spinning top’s mass, radius, and speed, is known as its spin.
Although electrons do not technically spin on an axis like a top, they have a similar spin. Their angular momentum can point “up” or “down”. Instead of using positive and negative electrical charges to represent binary information (1s, 0s) in electronic devices, engineers can use the binary nature of electron spin.
Because less energy is required to change the direction of the electrons’ spin, electron spin can be used to switch transistors in electronic devices with much lower power than conventional electronics. Transistors, the basic building blocks of modern electronics, are used to control electrical signals.
Also, because of their angular momentum, electrons act like tiny magnets. Researchers can use these magnetic properties to represent and store information in computer memory hardware. Liu and his collaborators aim to accelerate this process, removing the speed bottlenecks holding back low-power, high-performance computer memory devices.
attracted to magnetism
Liu’s path to studying computer memory hardware and spin electronics began with refrigerator magnets. As a child he wondered why a magnet stuck to a fridge.
That early curiosity helped spark his interest in science and math. During high school and college, as he learned more about physics, chemistry, and electronics, his curiosity about magnetism and its use in computers grew.
When given the opportunity to earn a PhD at Cornell University and join a research group studying magnetic materials, Liu found the perfect match.
“I spent the next five or six years developing new and more efficient ways to generate electron spin current and use it to write information into magnetic computer memories,” he says.
Fascinated by the world of research, Liu wanted to try an industry career, so he joined IBM’s TJ Watson Research Center after graduation. There, his work focused on developing more efficient magnetic random access memory hardware for computers.
“It was important to finally have something working in a commercially available format, but I didn’t find myself fully engaged in that kind of fine-tuning work. “I wanted to show the viability of very new work — to prove that some new ideas are possible,” says Liu, who joined MIT as an assistant professor in 2015.
Some of Liu’s most recent work at MIT involves making computer memories using nanoscale, antiferromagnetic materials. Antiferromagnetic materials such as manganese contain ions that act as tiny magnets due to their electron spin. The ions align themselves so that the “up” spins and the “down” spins are opposite each other, so the magnetization is eliminated.
Since no magnetic fields are produced, antiferromagnetic materials can adhere to the memory device, leading to higher storage capacity. Their lack of a magnetic field means they can switch spin states “up” and “down” very quickly, so antiferromagnetic materials can switch transistors much faster than conventional materials, Liu explains.
“In the scientific community, it’s been debated whether you can electrically change the spin orientation within these antiferromagnetic materials. With experiments, we showed that you can,” he says.
In his experiments, Liu often uses new materials that were created several years ago, so all their properties are not yet well understood. But he enjoys the challenge of integrating them into devices and testing their functionality. Finding better materials to harness electron spin in computer memories could lead to devices that use less power, store more information and retain that information for longer.
Liu takes advantage of state-of-the-art equipment inside MIT.nano, a 214,000-square-foot nanoscale research facility, to build and test nanoscale devices. He says that having such state-of-the-art facilities at his fingertips is a blessing for his research.
But for Liu, human capital fuels his work.
“Colleagues and students are the most valuable part of MIT. Being able to discuss questions and talk to some of the smartest people in the world is the most enjoyable experience of doing this job,” he says.
He and his students and colleagues are advancing the young field of spin electronics.
In the future, he envisions using antiferromagnetic materials in conjunction with existing technologies to create hybrid computing devices that achieve even better performance. He also plans to delve deeper into the world of quantum technologies. For example, he says, spin electronics can be used to efficiently control the flow of information in quantum circuits.
In quantum computing, signal isolation is critical – information must flow in only one direction from the quantum circuit to the external circuit. He explores the use of a phenomenon known as a spin wave, the excitation of electron spins inside magnetic materials, to ensure that the signal travels in only one direction.
Whether he’s researching quantum computing or investigating the properties of new materials, one thing is true—Liu is driven by insatiable curiosity.
“We are constantly exploring many exciting and challenging new topics towards the goal of building better computing memory or digital logic devices using spin electronics,” he says.