Magnetic Hardening in Low-Dimensional Ferromagnets
J. Ping Liu (IEEE Fellow, Univ. Texas at Arlington, USA)
The research platform MMM @ U.Wien, jointly with the Christian Doppler Lab. @ U.Krems, together with the Wolfgang Pauli Institut kindly invite you to the talk of Ping Liu (Fellow IEEE)
Time: Wednesday, 13. Sep 2023, 15:15 – 16:15
Place: Hörsaal 13, 2nd floor, Oskar-Morgenstern-Platz 1, 1090 Wien
1) 14h55 – 15h15 : Coffee & Cake
2) 15h15 – 15h20 : Introduction : Thomas Schrefl (DUKrems)
3) 15.20 – 16.10 Uhr : J. Ping Liu (IEEE Fellow, Univ. Texas at Arlington, USA) “Magnetic Hardening in Low-Dimensional Ferromagnets"
4) 16h10 – 17h00 : Drink & Sandwich
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.
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