Physicists at The University of Texas at Arlington have utilized nanostructures to make a breakthrough in understanding the properties of magnetic materials, which are important in a variety of industries and in numerous products used by people worldwide every day.
J. Ping Liu, UTA distinguished professor of physics, and members of his lab recently solved a decades-old dilemma involving coercivity, which is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. They published their findings in an article titled “Extraordinary Magnetic Hardening in Nanowire Assemblies: The Geometry and Proximity Effects” in the journal Advanced Functional Materials.
Jeotikanta Mohapatra, assistant professor of research in Liu’s lab, was lead author of the study. Co-authors include Meiying Xing, Jacob Elkins, and Julian Beatty, former graduate students in the lab.
“In this article, we disclose a significant breakthrough in our understanding of coercivity, which will be used in the materials design of future high-performance permanent magnets for advanced applications,” Mohapatra said.
Magnetic materials with high coercivity, called permanent magnets, are important components in energy generation and conversion, information storage and processing, and other advanced technologies. They are found in computers, cars, and windmills, among many other products. For a century, physicists have sought to predict the coercivity of permanent magnets with no success. In that time, all efforts to enhance coercivity have been based on empirical approaches.
In the 1940s a United States physicist, William Fuller Brown Jr., gave a theorem which stated that coercivity is scaled with the anisotropy field of ferromagnetic materials. An anisotropy field represents the hypothetical field that would be able to align the magnetization perpendicular to the easy direction, the c-axis. Ferromagnetism is a kind of magnetism that is associated with the elements iron, cobalt, and nickel. The coercivity values in experiments with these ferromagnetic materials have been much lower than the predicted level, which is known as Brown’s Paradox.
Liu and his team have conducted systematic investigations on the properties of magnetic materials, including coercivity, for more than three decades. They recently, for the first time, achieved coercivity in cobalt nanostructures with coercivity values as predicted by Brown’s theorem, which would be the first successful solution of Brown’s Paradox. Cobalt nanoparticles possess magnetic properties, which leads to applications in imaging, sensors, and many other areas.
“We have tirelessly worked for decades to understand several major fundamental problems in the physics of magnetic materials, including the pending Brown’s Paradox that has been outstanding for about 70 years,” Liu said. “This study enables a precise prediction and calculation of the coercivity in the nanostructured magnets, which can be applied for material designs of future high-performance permanent magnets for advanced applications.”
In the past 20 years Liu has supervised more than three postdoctoral researchers and four graduate students working on this topic. In 2014, he and his lab first reported the result that magnetic coercivity is higher than the magnetocrystalline anisotropy field in nanowires, as the Brown’s theorem predicted, which implies a key step to resolving the paradox.
“After Dr. Mohapatra joined our group seven years ago, we carried out more systematic investigations to reproduce the results and further understand the mechanism,” Liu said. “We also have had collaborations with colleagues in the international community that helped a lot for us to achieve the updated results with high impact.”
Liu and Mohapatra also recently joined with colleagues from Brown University to publish a review article on magnetic nanoparticles in Anisotropic Nanomaterials, a special edition of the American Chemical Society journal Chemical Reviews. The article is titled “Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications” and it summarizes the development of magnetic nanoparticle research related to biomedicine, magnetic recording, magneto-transport, permanent magnets, and green energy generations, Liu said.
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