L10 rare-earth-free permanent magnets: The effects of twinning versus dislocations in Mn-Al magnets

Defects of various kinds strongly affect local magnetic anisotropy and thus play a crucial role in coercivity in high-performance permanent magnets. In particular, in rare-earth-free permanent magnetic alloys with L1(0) structure microstructural defects deserve special attention. In this work, we report on the negative effect of twin structure, and the positive effect of dislocations on the coercivity is clarified in a systematic experimental study of L1(0)-MnAl alloys. We find that the nucleation of magnetization reversal is preferentially activated along the twin boundaries and grows into the twin stripes. This suggests that twin structure reduces the domain wall nucleation field, so that the coercivity decreases by approximately 50% according to the direct comparison of twin-free and twinned magnets. In contrast, dislocations dramatically enhance the coercivity by acting as the pinning center, and a high density of dislocations can modify the dominant coercivity mechanism from nucleation to pinning in severely deformed MnAl magnets. With a decreasing dislocation density, the pinning field remains as a constant while the coercivity reduces monotonously, indicative of the strong pinning effect of dislocations on the magnetic domain wall, generating a positive correlation between coercivity and density of dislocation. High coercivity from 424 to 328 kA/m is obtained in deformed and annealed magnets with different densities of dislocations. Thus, the combination of eliminating twin structure and introducing high-density dislocations could overcome the present bottleneck in magnetic performance. This work may inspire avenues for the development of L1(0) rare-earth-free permanent magnetic alloys, and set up a pathway for accelerating the application process.