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IEEE Magnetics Society Distinguished Lecture
“Magnetic Hardening in Low-Dimensional Ferromagnets”
“Additive Manufacturing of Composite RE-TM based Permanent Magnets by LPBF
J. Ping Liu
Magnetic Hardening in Low-Dimensional Ferromagnets
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.
Paulo A.P. Wendhausen
Additive Manufacturing of Composite RE-TM based Permanent Magnets by LPBF
Rare earth-transition metal permanent magnets are essential concerning renewable energy and electro mobility applications [1]. In the recent years, the Additive Manufacturing (AM) of functional magnetic materials, including permanent magnets, has attracted much attention due to the possibility of exploring main attributes of AM to produce components with tailored properties, gathering both magnetic and geometrical features [2]. The Laser Powder Bed Fusion (LPBF) process has gained momentum and it is one of the most important AM techniques for composite magnets fabrication. Composite magnets based either on the Nd-Fe-B [3] or Sm-Fe-N [4] systems are particularly interesting for these purposes, mainly due to their similarity in terms of magnetic properties and powder preparation. Nevertheless, the state-of-art contemplates composite magnets with limited performance, due mainly to high porosity values and the absence of magnetic texture. Powder preparation techniques and adequation of the feedstock composition are the main strategies to circumvent the porosity issue [3,4]. A major disadvantage for Sm-Fe-N based systems is the scarcity of powders with suitable technological properties for LBPF uses. To overcome this limitation, the Hydrogen-Disproportion-Desorption-Recombination (HDDR) process was proposed on our research [4]. Concerning magnetic texture development approaches, the reported strategies based on the use of a magnetic torque have few disadvantages, such as the porosity increase during texturization or geometrical distortion of the as-printed magnets [5,6]. A method for magnetic texturization based on the mechanical orientation of anisotropic particles during the LPBF for texture development was recently developed [4]. End-of-life magnets may also be processed to prepare adequate feedstocks for LPBF, enabling the fabrication of recycled composite magnets. The HDDR process, for instance, may be directly applied in scrap magnets allowing for the obtention of a ready-to-use powder in AM technologies. A polymeric coating can be further applied on the magnetic powder surface to enhance technological properties and assist sintering via laser. Here we will address each of the presented challenges, focusing on the solutions developed by our research team.
References
[1] Coey, J.M.D., Engineering, 6 (2020) 119
[2] Huber et al., Sci. Rep., 7(2017)9419
[3] Schaefferet al., J. Magn. Magn. Mater., 583 (2023) 171064
[4] Röhrig, M.R.M et al., J. Magn. Magn. Mater, 565 (2022) 170273
[5] Mapley et al., Rapid Prototyping Journal, 27 (2021) 735
[6] Gandha, K et al., Scr. Mater., 183, 91-95 (2020)
