How “hard” (coercive) a ferromagnet can be has been a puzzle for a century. The experimental coercivity values have been far below calculated levels which is called, Brown’s Coercivity Paradox. The paradox remains unsolved, and coercivity still cannot be predicted and calculated quantitatively by modeling.
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. 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.
Prof. J. Ping Liu, PhD, is a Distinguished University Professor at the University of Texas at Arlington. His recent research has been focused on hard magnetic nanoparticles, thin films, and bulk nanocomposites. He is an IEEE 2023 Distinguished Lecturer.