Stress, strain and technical magnetic properties in `exchange-spring' Nd₂Fe₁₄B + α-Fe nanocomposite magnets

A modified Williamson-Hall analysis was applied to synchrotron transmission x-ray diffraction data obtained for a series of three melt-spun magnetic nanocomposite `exchange-spring' materials comprised of Nd 2 Fe 14 B and varying amounts of α-Fe ranging from 0-32 vol%. Analysis of the data demonstrates that there is no measureable contribution from microstrains or other lattice distortions in the Nd 2 Fe 14 B phase. This result indicates that the Nd 2 Fe 14 B Bragg peak broadening arises from particle size only (on the order of 440 Å) and that the shape of the melt-quenched Nd 2 Fe 14 B grains is approximately spherical. In contrast, the α-Fe phase exhibits Bragg peak broadening that may be attributed to a combination of both anisotropic strain and particle-size broadening effects. The average particle size of the α-Fe phase is 360 Å, significantly larger than that determined from the Scherrer formula, and the average stress, deduced to be tensile, of the α-Fe particles is approximately 1 GPa, which translates to an rms strain = ⟨ e 2 ⟩ 1/2 of approximately 0.1%. These results provide a physical basis to explain both the marginal exchange-derived remanence enhancement and the anomalous elevated-temperature coercivities found in these alloys.