Abstract
Hard magnetic Nd2Fe14B systems are dominating the high performance permanent magnet market due to their highest energy product (BH)max. The rare-earth (RE) supply crisis of ten years ago, which led to a drastic increase in prices, revealed the dependency on critical RE elements for the production of high performance permanent magnets. As a consequence, a great effort has been made in the search for new material systems which present magnetic properties similar or superior to those of benchmark Nd2Fe14B, but which contain less critical elements One of such alternatives are Nd- and Sm-based intermetallic compounds with a tetragonal ThMn12-type crystal structure, exhibiting intrinsic magnetic properties comparable to Nd2Fe14B. In this study, we carried out detailed microstructural and magnetic characterization of twins in single crystal and polycrystalline bulk samples of composition SmFe11Ti. The twins were observed at micro- and nano-length scales, and it was found that the twin orientation is approximately 58±2◦ for each grain. The twin boundary plane corresponds to a {011}-plane family in the unit cell of Sm(Fe,Ti)12. High resolution electron microscopy and atom probe tomography (APT) studies on polycrystalline bulk samples show an enrichment of Sm and a depletion of Ti at the twin boundary. The composition of the twin boundary is estimated to be Sm9.6Fe84.9Ti5.5, matching the composition of the Sm3(Fe,Ti)29 phase, which with a lower anisotropy field. The effect of twins on magnetization reversal was investigated by means of magneto-optical Kerr effect (MOKE) microscopy. It was shown that initial magnetization starts at the twin boundary, indicative of strong magnetic coupling between the neighboring variants across the twin boundary. Micromagnetic simulations were carried out with a microstructure similar to that observed in the bulk samples, and the results show that the formation of twins reduces the estimated coercivity values to 38% of the expected switching field. The simulation results account for the observation of low coercivity values in twinned SmFe11Ti systems.
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