Magnetic Hardening in Low-Dimensional Ferromagnets

Abstract:
How “hard” (coercive) a ferromagnet can be has been a puzzle for a century. Seven decades ago, William Fuller Brown offered his famous theorem to correlate coercivity with the magnetocrystalline anisotropy fields in ferromagnetic materials. However, the experimental coercivity values have been far below the calculated levels given by the theorem, which is called Brown’s coercivity paradox. Researchers have attempted to solve the paradox with sustained efforts; however, the paradox remains unsolved, and coercivity still cannot be predicted and calculated quantitatively by modeling.
Progress  has  been  made  in  the  past  20  years  in  understanding  coercivity  mechanisms  in  nanoscale  low-dimensional ferromagnets.  In  fact,  ferromagnetism  is  a  size-dependent  physical  phenomenon,  as  revealed  by  theoretical  studies. However, nanoscale ferromagnetic samples with controllable size and shape have been available only in recent times. By adopting newly developed salt-matrix annealing, surfactant-assisted milling, and improved hydrothermal and chemical solution  techniques,  we  used  a  bottom-up  approach  to  produce  nanostructured  magnets  and  have  successfully synthesized monodisperse ferromagnetic Fe–Pt, Fe–Co, and Sm–Co nanoparticles and Co nanowires with extraordinary properties,  which  are  strongly  size-  and  shape-dependent.  A  study  on  size-dependent  Curie  temperature  of  the L10  ferromagnetic nanoparticles with sizes down to 2 nm has experimentally proved a finite-size effect. A systematic study of nanowires with extremely high coercivity above their magnetocrystalline anisotropy fields has opened a door to the solution of Brown’s paradox.

Bio:
J. Ping Liu (Fellow, IEEE) received the Ph.D. degree in physics from the University of Amsterdam, Amsterdam, The Netherlands, in 1994. For the past four decades, he has worked in research and development of permanent magnets and related magnetic materials in China, Europe, and USA. He is currently a Distinguished University Professor with The University of Texas at Arlington, Arlington, TX, USA. His current research has been focused on hard magnetic nanoparticles, thin films, and bulk nanocomposites, as reported in his more than 320 peer-reviewed journal articles, review articles, and books, including Nanoscale Magnetic Materials and Applications (Springer, 2009), Skyrmions: Topological Structures, Properties, and Applications (CRC Press, 2016), and Permanent Magnets: The History and Future (Science Press, 2020). He has supervised more than 50 graduate students and post-doctoral researchers.  Dr. Liu is an elected fellow of the American Physical Society. He received the Outstanding Achievement Award at the 25th International Workshop on Rare-Earth and Future Permanent Magnets and Their Applications in 2018.