A study on the exchange coupling of NdFeB-type nanocomposites using Henkelplots

The phase transformation and magnetic properties of (Nd0.95La0.05)(9.5)Feba lCo5Nb2B10.5 nanocomposite have been investigated systematically via thermo magnetic analysis, vibration sample magnetometer, x-ray diffraction, and co nventional transmission electron microscopy. The Henkel plot was employed t o quantify the strength of the exchange coupling between the hard and soft magnetic phases in the as-spun and the thermally treated samples. It was fo und that remanence B-r, coercivity H-ci, and maximum energy product BHmax o btained were affected by the magnetic phases present as well as the grain s ize of constituent phases and their distribution. The Henkel plot successfu lly interpreted the effect of the exchange coupling on B-r, H-ci, and BHmax obtained for samples treated below 750 degrees C. However, it became inade quate for samples treated above 750 degrees C. Although similar shapes of D elta M-H curves were obtained in the Henkel plot, severe degradation in B-r , H-ci, and BHmax was found when the thermal treatment temperature was incr eased from 750 to 850 degrees C. This degradation may be attributed to the grain growth of the main phases, from 45 to 68 nm, and the development of p recipitates. In conjunction with the mass balance, the precipitated phases (presumably borides) may explain the increase in the T-c of Nd-2(FeCo)(14)B and the decreased amount of alpha-Fe and Fe3B with increasing thermal proc essing temperature. (C) 1999 American Institute of Physics. [S0021-8979(99) 47508-5].