Revealing high coercivity in Nd-Fe-B with gradient rare earth-rich phase sizes

The conventional approaches for boosting the coercivity of permanent magnets via microstructural engineering rely heavily on rare/precious elements, causing high costs, limited maximum energy products and difficult recycling. Herein, a directional magnetization reversal with the reversed domains nucleating and propagating in the direction of gradient increasing rare-earth-rich phase (RERP) sizes is discovered in (Nd_0.7Pr_0.3)₃₁Fe_ballM₁B_0.95 (M = Cu, Co, Al, Ga, wt%) sintered magnets. As a result, a considerable coercivity of up to 14.91 kOe, 11.60% higher than that of the control group, is attained without sacrificing remanence (14.21 kGs). The gradient RERP sizes are achieved by introducing a top-down gradient stress from compressive to tensile inside the magnet, driving the liquefied RERPs at high temperatures to migrate along the grain boundaries. Micromagnetic simulations coupled with the magneto-optical Kerr effect demonstrate that the directional magnetization reversal, in stark contrast to the random behavior observed in stress-free magnets, originates from gradient RERP size-induced gradient demagnetizing fields. This study highlights the importance of understanding and regulating the RERPs to further improve the coercivity in high-performance Nd-Fe-B magnets.