Abstract
In this work, it was made a study on structural and magnetic properties of nanocrystalline Nd 4.5Fe 72Co 2Cr 3Al 1B 17.5 ribbons obtained from crystallisation of amorphous precursors. A complex two-step crystallisation process, was found by differential scanning calorimetry (DSC): a first crystallisation peak at around T=570 °C exhibiting the precipitation of α-Fe and t-Fe 3B phases and a diffusive stage ending in a second exothermic peak. Also, isothermal annealings of 10 min duration were performed at 605, 635, 650 and 685 °C. Hysteresis curves of isothermally annealed samples showed magnetic hardening, with coercive fields above 2 kOe and an optimised M R/ M S ratio of around 0.6. Except for the highest treatment temperature used in this work, the demagnetizing curves of annealed ribbons exhibited a step near zero field. This fact suggests a low exchange coupling between hard and soft phases, which can be attributed to the grain border phases. Otherwise, Mössbauer effect spectroscopy allowed us to determine that the absence of step in the demagnetizing curve of ribbons annealed at 685 °C, may be attributed to the formation of a α-(Fe, Co) solid solution (with hyperfine parameters B HF=35.5 T and δ=−0.11 mm/s). The formation of this solid solution also explains the diffusive process in the second crystallisation stage observed by DSC experiment.
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