MAGNETIC HARDENING MECHANISM IN NANOCRYSTALLINE ND2FE14B WITH 0.1 AT.PERCENT ADDITION OF CR, CU, OR ZR

The nanocrystalline hard magnetic Nd2Fe14B phase plays a crucial role in the development of exchange spring magnets. We report the hardening mechanism of nanocrystalline Nd2Fe14B, as well as the effect of small amounts of magnetic additions on the hardening behavior. It is shown that the hardening mechanism is of the pinning type in the pure Nd2Fe1 4B phase. Retention of pinning-type behavior, with a marginal increase in coercivity was observed for small additions of Cr or Cu. Addition of Zr changes the mechanism to nucleation type with a decrease in coer civity. Additions of Zr or Cu are thought to form precipitates at grai n boundaries. The precipitates containing Cu were found to be effectiv e in pinning the domains and enhancing the coercivity. The Zr-containi ng precipitates were found to be ineffective in pinning the domain wal ls and act as nucleation centers for reverse domains, resulting in nuc leation-type hardening. The magnetic behavior of Cr-containing samples above the Neel temperature of Cr remains unaltered. This lack of chan ge in the Cr-containing samples combined with the observed lowering of remanence indicates the presence of Cr in solution with Nd2Fe14B. Thu s, we conclude that not only the magnetic nature of the impurity, but also its location is important in controlling the coercivity mechanism . (C) 1998 American Institute of Physics. [S0021-8979(98)09201-9].