Experimental and modeling studies on phase stability of nanocrystalline magnetic Sm2Co7

• Low-Co Sm–Co alloy as new candidate for permanent magnets was proposed.
• Relationship between the phase stability and grain size was quantified for Sm2Co7.
• The work contributes to the development of nanostructured Sm–Co permanent magnetic materials.

In contrast to the conventional polycrystalline low-Co Sm–Co alloys that have very weak permanent magnetic properties, the Sm2Co7 alloy has been found to have fairly promising permanent magnetic performance when its grain size is reduced to the nanoscale. It was discovered that the crystal structure of the nanocrystalline Sm2Co7 has a strong nanograin-size-dependent stability. The rhombohedral structure of Sm2Co7 phase which is metastable at temperatures lower than 1435 K in conventional polycrystalline system can exist stably at room temperature in the nanocrystalline system. To understand the phase stability of the nanocrystalline Sm2Co7, the experimental and nanothermodynamic analyses were combined to describe quantitatively the phase transformation behavior of Sm2Co7 on the nanoscale. The results are important for the development of nanostructured Sm–Co permanent magnets.

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