Defects-aggregated cell boundaries induced domain wall curvature change in Fe-rich Sm–Co–Fe–Cu–Zr permanent magnets

Grain Boundary Evolution of Cellular Nanostructured Sm-Co Permanent Magnets

“…Due to elemental redistribution, the Cuenriched 1:5H precipitates will further transform into the Sm n+1 Co 5n−1 phase and the 1:3R precipitates will further thicken the Sm n+1 Co 5n−1 primary precipitates because 1:3R is one of the series of Sm n+1 Co 5n−1 compounds. The transformation from 1:5H to Sm n+1 Co 5n−1 at the grain boundary is supported by our recent aging temperature-dependent and aging time-dependent studies where the grain boundary Sm n+1 Co 5n−1 precipitates become thicker either after aging at higher temperatures or for a longer time [21,24]. For instance, after aging for over 30 h, abnormal cell growth occurred close to the grain boundaries, which could even form micron-sized 2:17R cells.…”

“…However, the magnets usually contain various types of microstructural deficiencies, which weaken the pinning force against DW motions locally and lead to an inhomogeneous demagnetization process as well as a poor squareness factor. These microstructural deficiencies include: (i) the intersections between 1:5H precipitates and the co-existing 1:3R precipitates [14], (ii) the cell edges that usually contain stacking faults (SFs) or the 2:17R' intermediate phase [20], (iii) the defect-aggregated cell boundaries (DACBs) free of the 1:5H phase [21], and (iv) the grain boundaries with sparser 1:5H cell boundary precipitates (i.e., larger cells) than the grain interiors and phase mixture of Zr-stabilized Sm n+1 Co 5n−1 (n = 2, 1:3R; n = 3, 2:7R; n = 4, 5:19R) precipitates that are magnetically softer than the 1:5H precipitates [15,[22][23][24][25][26][27]. Among them, the grain boundaries have been thought to be the primary demagnetization sites [5,28].…”

Local Phase Segregation Induced by Ion Milling in 2:17-Type Sm-Co Based Magnets

“…The coercivity mainly arises from the domain wall energy density between the 1:5H phase and 2:17R phase [ 7 , 8 , 9 ]. In addition, the nano-sized structural defects generated during the phase decomposition process also play an important role in the magnetic properties of the 2:17-type Sm-Co based magnets, especially the remanent stacking faults at the cell edge [ 10 , 11 , 12 ]. Since these crystalline phases and defects are very small in size (e.g., the thickness of the 1:5H phase is usually smaller than 10 nm) and the structures of the coherent 1:5H and 2:17R phases are similar and their lattice parameters are approximately equal [ 13 , 14 ], it is difficult to perform the specific microstructure by scanning electron microscopy or conventional X-ray diffraction technologies.…”

“…Since these crystalline phases and defects are very small in size (e.g., the thickness of the 1:5H phase is usually smaller than 10 nm) and the structures of the coherent 1:5H and 2:17R phases are similar and their lattice parameters are approximately equal [ 13 , 14 ], it is difficult to perform the specific microstructure by scanning electron microscopy or conventional X-ray diffraction technologies. Thus, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) are indispensable to reveal the true structure of the 2:17-type Sm-Co based magnets [ 3 , 4 , 5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 15 ].…”

“…However, the TEM (or HR-TEM) is not risk-free, in terms of introducing microstructural artifacts, especially during the TEM specimen preparation process. Generally, the thin film specimens for TEM examinations of the 2:17-type Sm-Co based magnets are fabricated by ion milling or focused ion beam, since these magnets are too brittle to suffer electropolishing [ 4 , 5 , 7 , 8 , 10 , 11 , 12 , 13 ]. Although the energy used for ion milling or focused ion beam is relatively low (2–6 keV), the possible structure damage can be easily introduced into metallic materials since the temperature rising of the specimen surface is unavoidable during the ion milling process.…”