Magnetic hardening mechanism in nanocrystalline Nd₂Fe₁₄B with 0.1 at. % addition of Cr, Cu, or Zr

Abstract

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 Nd₂Fe₁₄B 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 coercivity. Additions of Zr or Cu are thought to form precipitates at grain boundaries. The precipitates containing Cu were found to be effective in pinning the domains and enhancing the coercivity. The Zr-containing precipitates were found to be ineffective in pinning the domain walls and act as nucleation centers for reverse domains, resulting in nucleation-type hardening. The magnetic behavior of Cr-containing samples above the Neel temperature of Cr remains unaltered. This lack of change in the Cr-containing samples combined with the observed lowering of remanence indicates the presence of Cr in solution with Nd₂Fe₁₄B. Thus, we conclude that not only the magnetic nature of the impurity, but also its location is important in controlling the coercivity mechanism.