Exchange coupling and recoil loop area in Nd2Fe14B nanocrystalline alloys

The recoil behavior of nanocrystalline materials based on Nd2Fe14B has been investigated in melt-spun alloys with the stoichiometric composition Nd2Fe14B and in the nanocomposite Nd9Fe86B5. The onset of recoil hysteresis, or attainment of appreciable area of the recoil loop, was determined and followed as a function of the reverse applied magnetic field. Its behavior was correlated with that of two other independent parameters: the reversible portion of the recoil susceptibility and the recoil remanence, which both provide information on the ease of remagnetization of the materials. The simultaneous appearance of extrema in a restricted internal field range in all three parameters strongly suggests that the area of the recoil loops in these materials may be taken as a measure of the extent of the intergranular exchange. The temperature dependence of the results provides information about the thermal evolution of the interphase exchange coupling; this information is not evident in major loop measurements. Preliminary results indicate that the off-stoichiometric Nd9Fe86B5 maintains a higher degree of interphase exchange coupling but only at low reverse fields; the stoichiometric Nd2Fe14B maintains a lower degree of magnetic reversibility but sustains it to higher reverse fields. Comparison of the results obtained from different forms of the Nd2Fe14B-based nanocrystalline alloys highlights the origins of the differences in their technical magnetic behavior and suggests structural modification pathways to improve their properties. (C) 2003 Elsevier B.V. All rights reserved.