Enhancement of exchange coupling and hard magnetic properties in nanocomposites by magnetic annealing

An enhancement of the exchange coupling and hard magnetic properties of melt-spun Nd2Fe14B/α-Fe-type nanocomposites was achieved by optimization of their nanostructured morphology via magnetic annealing. Compared with the Nd2.4Pr5.6Dy1Fe84Mo1B6 sample annealed without a magnetic field, the magnetic annealing results in a noticeable improvement in the coercivity iHc, the remanence 4πMr, and energy product (BH)max. (BH)max at 50 K was enhanced by 43.7% after magnetic annealing in a field of 19 T. The kink in the demagnetization curve disappeared and, additionally, a much better squareness of the demagnetization curves was observed in the magnetically annealed samples. The intergrain exchange coupling was evaluated by Henkel plots. Evidence for stronger intergrain exchange coupling was found in the magnetically annealed sample, which is due to nanostructure refinement promoted by the magnetic annealing. The nanostructure refinement was confirmed by X-ray diffraction, transmission electron microscope, low-temperature demagnetization curves and a modified Brown’s equation analysis. The improvement of hard magnetic properties of the magnetically annealed sample results mainly from the magnetic-field-induced exchange-coupling enhancement.