Soft and hard natures of Nd_2Fe_(14)B permanent magnet explored by first-order-reversal-curves

摘要

Two commercial Nd_2Fe_(14)B samples, MQP-B and sintered-NdFeB were investigated using synchrotron-based x-ray diffraction and first-order-reversal-curves (FORCs). Despite differing in magnetic and structural properties, the two samples were found to comprise two major ferromagnetic components in FORCs. For the sintered-NdFeB case, the soft component may originate from the intrinsically soft Nd-f site which was coupled with its local Fe atomic environment that differs in magnetic anisotropy from the Nd-g site (intrinsically hard). It may directly originate from the Nd-rich phase or microstructural imperfection, while the former possibility (Nd-f site) appears greater than the latter. While for the MQP-B, the minor second phase resulting from high structural disorder was likely in charge of the presence of the soft component. Sophisticated FORCs analyses revealed the natures of the soft and hard components, soft-hard coupling and switching reversibility of the two cases, irrespective of the origins of their two components. This provides insights to the origin of magnetic stability and reversal dynamics of Nd_2Fe_(14)B that have not been fully understood by conventional magnetic analyses. The coexistence of the two components led to an incoherent reversal undermining the magnetic stability of Nd_2Fe_(14)B. This is a fundamental problem as to why the performance extremity can only be improved finitely through extrinsic tuning. From FORCs simulation we understand that the soft-hard coupling was moderate in a real Nd_2Fe_(14)B compound. A stronger soft-hard coupling is necessary to conquer the anisotropic competition to enable a coherent reversal that will promote the magnetic hardness.