Comparison of the improvement of thermal stability of NdFeB sintered magnets: Intrinsic and/or microstructural

Abstract

Temperature-dependent magnetic properties of nine NdFeB sintered magnets with various Co-Nb, Co-V, or Co-Mo additions have been measured up to 175 °C using both closed-loop and open-circuit methods. The irreversible loss of induction, reversible temperature coefficient of induction (α), and temperature coefficient of intrinsic coercivity β have been related to the Tc, the Br, and the Hci at 25 and 175 °C. The irreversible loss of induction is strongly affected by the Hci at 25 °C and the α has been found to be strongly dependent upon the Tc. Intrinsic coercivity of more than 25 and 7.5 kOe at 25 and 175 °C, respectively, are essential to bring the β to better than −0.5%/ °C. A magnet with a composition of Nd12Dy3Fe70Co5Nb2B8 has been found to exhibit an outstanding thermal stability: a Br of 8.5 kG (comparable to that of SmCo5 sintered magnet) and a Hci of 12.9 kOe when measured at 175 °C. An α of −0.10%/ °C and a β of −0.4%/ °C have been obtained.