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 maximizin...