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

The phase transformation and magnetic properties of (Nd0.95La0.05)(9.5)FebalCo5Nb2B10.5 nanocomposite have been investigated systematically via thermomagnetic analysis, vibration sample magnetometer, x-ray diffraction, and conventional transmission electron microscopy. The Henkel plot was employed to 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 found that remanence B-r, coercivity H-ci, and maximum energy product BHmax obtained were affected by the magnetic phases present as well as the grain size of constituent phases and their distribution. The Henkel plot successfully 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 inadequate for samples treated above 750 degrees C. Although similar shapes of Delta 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 increased 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 precipitates. 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 processing temperature. (C) 1999 American Institute of Physics. [S0021-8979(99)47508-5].