Influence of Co substitution for Fe on the magnetic properties of nanocrystalline (Nd,Pr)-Fe-B based alloys

Abstract

The effects of Co substitution for Fe on the spin reorientation temperature, magnetic properties over a wide temperature range, the thermal stability and on exchange enhancement in nanocrystalline (Nd/Pr)z(Fe1−xCox)94−zB6 alloys have been comprehensively studied. Such Co substitution substantially increased the Curie temperature (TC) of the 2/14/1 phase and slightly decreased the spin reorientation temperature (TSR) for these alloys. Despite the smaller anisotropy field for the Nd2Co14B phase, in comparison with Nd2Fe14B, a small Co substitution did not greatly decrease the room temperature (RT) coercivity jHC, regardless of the rare earth to transition metal ratio. The remanence Jr at RT was generally slightly improved by Co substitutions up to ∼20%, though less so for the 8 at% rare earth (RE) series having a high volume fraction of soft magnetic phase. Similarly, substitution of up to 20% Co slightly enhanced the maximum energy product (BH)max at RT, except for the 8 at% RE alloys which showed a marked decline beyond 10 at% Co due to the strongly diminished jHC. In the case of the sub-ambient magnetic properties, Co substitution markedly decreased Jr and (BH)max, though the effect on jHC was much smaller. In contrast, Co additions, because of the enhancement of Tc, substantially improved the elevated temperature behaviour of all experimental alloys, by decreasing the values of the temperature coefficients for Jr and iHc, α and β, respectively, particularly for α. Moreover, the Co also reduced the irreversible losses in Jr and jHC measured on ribbon samples. Analysis of microstructural parameters governing the coercivity, using the modified Brown equation, suggested that Co substitution not only reduced the stray fields, by homogenizing the grain structure and smoothing the grain boundaries, but also improved the exchange coupling between grains.