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

Abstract

The phase transformation and magnetic properties of (Nd0.95La0.05)9.5FebalCo5Nb2B10.5 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 Br, coercivity Hci, 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 Br, Hci, and BHmax obtained for samples treated below 750 °C. However, it became inadequate for samples treated above 750 °C. Although similar shapes of ΔM–H curves were obtained in the Henkel plot, severe degradation in Br, Hci, and BHmax was found when the thermal treatment temperature was increased from 750 to 850 °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 Tc of Nd2(FeCo)14B and the decreased amount of α-Fe and Fe3B with increasing thermal processing temperature.