Unveiling the anomalous atomic stacking and formation mechanism of 1:3R Z-plates in Sm₂Co₁₇-type magnets

Sm₂Co₁₇-type permanent magnets are the favorable choice for applications above 200 °C. Their performance can be enhanced by Zr alloying, mainly due to the formation of Zr-rich 1:3R Z-plates. Herein, the atomic-scale structures of the Z-plates in an 1.7 at% Zr alloyed Sm₂Co₁₇-type magnet are unveiled using aberration-corrected STEM. Zr is observed to preferably replace the dumbbell Sm to form (Sm_1/3Zr_2/3)Co₃ Z-plates, which have a number density of 37.6 µm⁻¹ along c-axis. The Z-plate with thickness of the reported one-unit cell is rarely seen, instead several anomalously stacked types are detected. Most of the Z-plates have a two-third stacking (2/3-stacked) of one-unit cell, some of them are lacking one Sm+Co layer while others have additional Sm+Co layers. Totally four types of single 2/3-stacked Z-plates are identified, acting as the basic units for thick Z-plates. A few Z-plates (number density 2.73 µm⁻¹) have thickness of more than double 2/3-stacking, which deteriorate the oxidation-resistance of the magnet. Further, by analyzing the end regions of Z-plates, the diffusion-controlled mechanisms for the phase transformation from the 2:17R matrix to the 1:3R Z-plates are unraveled.