Exchange Bias in Bulk α-Fe/γ-Fe₇₀Mn₃₀ Nanocomposites for Permanent Magnet Applications

Here we report on the microstructural factors influencing the formation of the interfacial exchange bias effect in three-dimensional transition-metal-based nanocomposite systems, with relevance to permanent magnet applications. Bulk phase-separated nanocomposites consisting of the ferromagnetic α-Fe and metastable antiferromagnetic γ-Fe70Mn30 phases exhibit a notable low-temperature exchange bias and substantial coercivity (Hex = 24.6 kA/m, HC = 95.7 kA/m) as well as a near room-temperature blocking temperature. Structural investigation by synchrotron X-ray diffraction, neutron scattering, and transmission electron microscopy confirm that the ferromagnetic α-Fe phase nucleates as small precipitates (d ≈ 50 nm) at the grain boundaries of the antiferromagnetic γ-Fe70Mn30 grains (d = 360−740 nm) and grows anisotropically upon heat treatment, resulting in an elliptical geometry. These results indicate that optimization of the exchange bias effect in bulk nanocomposite systems may be achieved through maximizing the surface-to-volume ratio of ferromagnetic precipitates in an antiferromagnetic matrix, enhancing the magnetocrystalline anisotropy of the antiferromagnetic phase to facilitate interfacial pinning and ensuring a balanced distribution of the ferromagnetic and antiferromagnetic phases. This work further clarifies critical factors influencing the formation of an exchange bias in an inexpensive transition-metal-based bulk nanocomposite system with potential for scalable production.