Multiscale examination of strain effects in Nd-Fe-B permanent magnets

TL;DR

This study combines first-principles and micromagnetic simulations to analyze how various strain states affect the magnetic anisotropy and coercivity of Nd-Fe-B magnets, revealing that local strain and interface shape significantly influence magnetic performance.

Contribution

It provides a multiscale analysis showing the impact of strain on magnetic properties, highlighting the importance of local stress states and interface geometry for coercivity enhancement.

Findings

Local 3-4% strain reduces coercivity by ~60%.

Biaxial and triaxial stresses have a greater impact on anisotropy.

Smoothing interface edges can improve coercivity.

Abstract

We have performed a combined first-principles and micromagnetic study on the strain effects in Nd-Fe-B magnets. First-principles calculations on Nd2Fe14B reveal that the magnetocrystalline anisotropy (K) is insensitive to the deformation along c axis and the ab in-plane shrinkage is responsible for the K reduction. The predicted K is more sensitive to the lattice deformation than what the previous phenomenological model suggests. The biaxial and triaxial stress states have a greater impact on K. Negative K occurs in a much wider strain range in the ab biaxial stress state. Micromagnetic simulations of Nd-Fe-B magnets using first-principles results show that a 3-4% local strain in a 2-nm-wide region near the interface around the grain boundaries and triple junctions leads to a negative local K and thus decreases the coercivity by ~60%. The local ab biaxial stress state is more likely to induce a large loss of coercivity. In addition to the local stress states and strain levels themselves, the shape of the interfaces and the intergranular phases also makes a difference in determining the coercivity. Smoothing the edge and reducing the sharp angle of the triple regions in Nd-Fe-B magnets would be favorable for a coercivity enhancement.