Mössbauer study of Nd₅Fe_77-xCr_xB ₁₈ (x = 0, 3 and 5) nanocomposite magnets

The magnetic properties of amorphous Nd₅Fe_77-xCr _xB₁₈ (x = 0, 3 and 5) after annealing at 953 K for 0.6 ks have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy with particular attention paid to the effect of heating rate on the proportions of the magnetically hard and soft phases. The hard Nd₂Fe ₁₄B phase is absent from Nd₅Fe₇₇B₁₈ regardless of heating rate. On the contrary, ∼10 mol% of Nd ₂Fe₁₄B is confirmed for Nd₅Fe _7.2Cr₅B₁₈ and the molar fraction of Nd ₂Fe₁₄B is almost independent of heating rate. Hence, the intrinsic coercivity of Nd₅Fe₇₇B₁₈ is as low as ∼ 10kA/m while that of Nd₅Fe₇₂Cr₅B ₁₈ shows a high value > 300 kA/m. On the other hand, the proportion of the hard and soft phases for Nd₅Fe₇₄Cr ₃B₁₈ is highly dependent on the heating rate and the fraction of Nd₂Fe₁₄B increases from 0 to 12 mol% with increasing heating rate from 0.042 to 1.33 K/s. This microstructural change results in a dramatic enhancement of the coercivity from 9 to 370 kA/m.