Phase transformations and coercivity in rare-earth permanent magnets

Local crystalline structure of Sm-Co alloys: Results of XAFS-analysis

“…Observed effects may be a consequence of the transformation to the Sm enriched phase SmCo 5−x accompanied by creation of disordered stacking faults: shift by a Burgers vector t = 1/3a 110 and removal of the interstitial Co layer. Obtained results agree with the XRD data and confirm the hypothesis of "high coercive state, induced by phase transformations, changing the state of the grain surface in basic phase" [5]. Examination of the near edge structure of L 3 -Sm absorption revealed that enrichment of the parent phase with samarium leads to the raise of Sm 5d -band filling.…”

“…Such HT increases H ci of sintered magnets from 0.1 to more than 3.5 T. To explain this phenomenon there was proposed the "perfect lattice" hypothesis [3], but it appears to contradict several experimental facts. So the new mechanism of phase transformation-induced coercivity in sintered SmCo 5−y magnets was developed based on X-ray diffraction (XRD) data in [4,5]. According to these data the changes of lattice parameters of SmCo 5 phase after ageing at 1200-800 • C correspond to the growth of Sm content in SmCo 5 at lower ageing temperature [6,7].…”

Effect of Heat Treatment on the Microstructure and Magnetic Properties of As-Cast SmCo₅-Based Alloys

“…8-9). The rearrangement of the SmCo 5 crystal structure leading to the formation of shear stacking fault was described in [13].…”

“…According to the "perfect lattice hypothesis" [3][4][5], the coercivity increase due to the HT is related to the elimination of equilibrium lattice defects from the high temperature SmCo 5 matrix phase after heat treatment performed at the lower temperature. The "phase transformation-induced coercivity mechanism" [6,7] suggests that the formation of Sm-rich phase on the surface of SmCo 5 grains during slow cooling from 1120 o C smoothes the grains surface, decreases the amount of reversed domains nuclei and, thus, increases H ci . HRTEM investigations of the sintered magnets showed that, along with SmCo 5 grains with a low density of defects, grains with closely packed parallel stacking faults are observed perpendicular to the hexagonal c-axis [8].…”

New preparation and recycling procedure to recover rare earth elements from magnets by using a closed loop treatment

“…This leads to better mechanical properties, a change of the coercive field strength, an optimum corrosion protection or to an elevated Curie temperature to increase the operating temperature. To prevent the oxidation of the magnets, they are covered by nickel or tin/zinc coatings as sacrificial anode or a simple plastic surrounding to shield against oxygen (Menushenkov 2005; Menushenkov and Savchenko 2006; Wang, Li and Li 2007; Ueberschaar and Rotter 2012; BVI Magnet GmbH 2015).…”