Liu's Research on Magnetics at AAMU Shared Worldwide

Alabama A&M University Associate Professor of Mathematics, Dr. Congxiao Liu, has published a paper on theoretical magnetics that is being shared in peer-reviewed journals around the world.

Liu’s paper, “Classification of hysteresis loops for exchange biased F/NM/F trilayer with antiferromagnetic interlayer coupling,” was first published in the July 2023 issue of the Journal of Applied Physics, an influential journal publishing new and significant experimental and theoretical results of applied physics research.

“The problem I studied is a key part of the so called giant magnetoresistive sensor, or GMR sensor, an effect discovered in late 80s of the last century and won the Noble Prize in 2007," said Liu.

GMR sensors are used extensively in the high-tech industry and in our daily life, such as magnetic cloud storage system, biosensors, microelectromechanical systems, magnetoresistive and random-access memory. The paper reveals the underling physics of a key component of these sensors, which is important not only theoretically but also critical in the manufacturing of those sensors.

“The paper tries to understand the complicated magnetization behaviors in nanomagnetic sensor devices and build the theoretical grounding for extracting materials parameters from magnetic testing of those devices,” he said. 

Originally from China, Liu attended the University of Alabama as a graduate student where he studied magnetic materials in the physics department. After obtaining his Ph.D. in physics in 2001, he worked as a senior engineer in the R&D division of a magnetic recording high-tech company in Silicon Valley. At that time, Liu was always puzzled by different types of hysteresis loops of GMR sensors and started to tackle this problem theoretically.

Liu then decided to return to the University of Alabama to study mathematics in 2002. After earning his second Ph.D., he came to AAMU in 2008 where he never gave up solving his theory, and recently, finally solved it.

“The problem came from real devices in magnetic nanostructures," he said. "The fact is, magnetization behavior in this device was quite subtle and was much more complicated than I expected. In principle, each type of measured hysteresis loop corresponds to specific materials parameters of the device. Understanding this relationship in depth was important but not easy and that was the problem I faced."

"I realized that the key was to classify the hysteresis loops theoretically according to the magnetization process. So I started from the system energy and analyzed magnetization behaviors of the system at each stage of the hysteresis cycle. To include all scenarios, the study was carried out in the whole range of materials parameters, and that ended up with a rich variety of magnetic phases,” says Liu.

“After some tedious analysis, eventually I had a complete picture of the underlying physics. When it was resolved, that really gave me a lot of happiness as it puzzled me for so many years. I hope engineers in the production line can use my discovery to establish the root causes of failure for the GMR sensors. From the measured hysteresis loop of a GMR sensor, they can determine the parameters of the materials based on my work, and that will help them to pinpoint the specific failed part in the sensor and the reason for failure. This is especially important in the optimization of device design and processing.”

Soon after its publication in the Journal of Applied Physics, NewsRX shared Liu’s work in its Physics Week titled, “Alabama A&M University Researcher Details New Studies and Findings in the Area of Applied Physics.” It's also been shared on ResearchGate, the Harvard NASA Astrophysics Data System, and the Japanese Journal of Applied Physics.

Liu says researchers can email him for details, and some already have. He also believes AAMU and his students can benefit from more research in magnetics.

“Magnetism is a broad area and is very important in modern nanotechnology. This importance is somewhat overlooked in HBCUs. I hope to establish a nano-magnetism research center at AAMU where faculty and students can conduct cutting edge research. The center will open a window of modern nanotechnology for students and motivate their curiosity and creativity in exploring the unknown world with great interest. It will enhance the institution’s research and education capabilities and the quality of the University’s STEM programs by preparing a high-tech workforce from underrepresented groups for national security agencies and private sectors.”