The improvement of magnetic property by grain refinement using magnetic field annealing crystalline (Nd0.8Pr0.2)2.2Fe12Co2B ribbons

Abstract

The (Nd0.8Pr0.2)2.2Fe12Co2B ribbons were prepared by melt-spinning at a wheel speed of 15 m/s. The effect of magnetic field annealing treatment with 1 T in the temperature range of 573–723 K for 15 min on the magnetic properties including parameters of the coercivity, the remanence, and the maximum energy product ((BH)max) of (Nd0.8Pr0.2)2.2Fe12Co2B ribbons was investigated. The largest value of (BH)max among sample studied is obtained at 673 K for 15 min with 1 T. XRD result shows that samples are crystalline state and single Nd2Fe14B (2:14:1) phase is observed in both melt-spun ribbon and magnetic annealing sample at 673 K. The refined microstructure with an average grain size of 25 ± 3 nm is obtained in crystalline (Nd0.8Pr0.2)2.2Fe12Co2B melt-spun ribbons. The magnetic field annealing treatment with 1 T can further reduce the average grain size to 20 ± 1 nm. The mechanism of grain refinement is discussed from two aspects including the length-to-width ratio of a grain and the stress. The interaction domain structure is observed in both melt-spun ribbon and the sample annealed at 673 K with 1 T by Lorentz magnetic microscopy suggesting the presence of the exchange coupling between 2:14:1 grains. The grain-refined microstructure of magnetic annealing sample at 673 K strengthens the exchange coupling interaction leading to an enhancement of the remanence and the (BH)max, with a rise of 4% and 6%, respectively. The values of (BH)max of melt-spun (Nd0.8Pr0.2)2.2Fe12Co2B ribbon (141 kJ/m3) and annealed ribbon with 1 T magnetic field (150 kJ/m3) are both larger than that of some Dy-containing Nd-Fe-B permanent alloys with an improving range of 7%-74%.