Presentation Information
[O13-3]Magnetic Hardening in Low-Dimensional Magnets
*J.Ping Liu1 (1. University of Texas at Arlington (United States of America))
Keywords:
Coercivity,nanoparticles
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, leading to so-called Brown’s coercivity paradox which remains unsolved even though researchers have made sustained efforts to understand it and to resolve it. Coercivity still cannot be predicted and calculated quantitatively by modelling. Progress has been made in the past 20 years in understanding coercivity mechanisms in nanoscale low-dimensional ferromagnets. As it is well known that ferromagnetism is a size-dependent physical phenomenon. 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 have successfully synthesized monodisperse ferromagnetic Fe–Pt, Fe–Co, and Sm–Co nanoparticles and Co nanowires with their properties 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 extraordinary magnetic hardening (coercivity above the magnetocrystalline anisotropy field) has opened a door to the solution of Brown’s paradox.