Impact of intergranular phase variations on the anomalous Nernst effect in Nd-Fe-B permanent magnets

Abstract

Improving the anomalous Nernst coefficient (S_ANE) in permanent magnets is essential for increasing the power density in transverse thermoelectric generators, which use permanent magnets to operate the anomalous Nernst effect without relying on an external magnetic field. While recent studies indicate that microstructural engineering can affect S_ANE, the specific relationship between microstructure and S_ANE in permanent magnets remains underexplored. This study investigates S_ANE of hot-pressed, hot-deformed, and RE-Cu (RE = Dy-Nd, Nd, and Pr) grain boundary diffusion-processed Nd-Fe-B magnets. The results show that S_ANE increases by 68%, from -2.6 × 10^-7 VK^-1 in the hot-pressed state to -4.4 × 10^-7 VK^-1 after hot-deformation in which grain growth and crystallographic texture are realized without changing the composition of the magnets. S_ANE further increases to -5.0 × 10^-7 VK^-1 after grain boundary structure and composition change from thin amorphous phase to thick crystalline phase by grain boundary diffusion of Dy-Nd-Cu alloy. The increase in S_ANE is found to be primarily due to the reduction of the opposing transverse electric field caused by the Seebeck-effect-induced carrier flow bent by the anomalous Hall effect. Owing to the crystallographic texture formation after hot-deformation, almost the same transverse thermopower as S_ANE is obtained in the hot-deformed and RE-Cu grain boundary diffusion-processed Nd-Fe-B magnets at a remanence state, i.e., under zero magnetic field. These findings demonstrate that microstructural optimization can effectively enhance the S_ANE in ultra-fine grained Nd-Fe-B magnets, providing a promising avenue for advancing materials in applications of transverse thermoelectrics.

Keywords

Anomalous Nernst effect / hot-deformed magnets / microstructural engineering / Nd-Fe-B / permanent magnets

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