Breaking the performance trade-off in Sm-Co/FeCo nanocomposite magnets by forming gradient multiphase structure

The nanocomposite magnet based on the exchange coupling effect is a candidate for the new generation of permanent magnets. While the mismatch between the soft and hard magnetic phases introduces a remanence (M_r)–coercivity (H_C) trade-off, it prevents a high performance. Here, a gradient multiphase (GMP) heterogeneous structure was constructed to break the M_r–H_C trade-off in nanocomposite magnets. GMP Sm-Co/FeCo magnet, which comprises multiple Sm-Co hard magnetic phases (1:3, 2:7, 1:5, 1:7, and 2:17) and FeCo soft magnetic phase, achieves both a high H_C (6.4 kOe) and a high M_r (119 emu/g), yielding a remanent–saturation magnetization ratio (M_r/M_S) of 0.87 and high energy product ((BH)_max) of 25.7 MGOe. In situ magnetic domain observations and micromagnetic simulations revealed that the pinning of multiple hard magnetic phases in the GMP structure forms quasi-magnetic domains composed of multiple magnetic phases. The hard magnetic phases at the edges of the quasi-domains form pinning sites, which provide full pinning action and impede the expansion of the demagnetized domains centered on the soft magnetic phases, leading to superior performance. Further micromagnetic simulations verified that the exchange coupling in GMP Sm-Co/FeCo magnets is improved compared to biphasic (BP) nanocomposite magnets. The strategy of GMP structure can be applied to circumvent performance trade-offs in other heterogeneous functional materials.