Magnetic Properties and Microstructure Studies of alpha-Fe/R_2Fe_14B-type Nanocomposite Ribbons with Low Boron and High Boron Concentration

摘要

The magnetic properties and microstructures of exchange-coupled alpha-Fe/R_2Fe_14B-type nanocomposites, made from Nd_2Fe_14B, Nd_9.5Fe_85.5B_5, (Nd_0.95La_0.05)_xFe_88-xCr_2B_10(x=7.5, 9.5 and 11), and (Nd_0.95La_0.05)_10Fe_79.8-yCr_yB_10.2 (y=2 and 4) alloys, were investigated. Even though the composition is different, almost all the ribbons, with the exception of (Nd_0.95La_0.05)_7.5Fe_80.5Cr_2B_10, contained merely two magnetic phases, i.e. alpha-Fe and R_2Fe_14B, with different volume fraction. An optimum combination of the intrinsic coercivity, _iH_c, of 9.5-12.6 kOe and maximum energy product, (BH)_max, of 13.8-17.5 MGOe have been successfully produced from melt spun ribbons with compositions of boron enriched (Nd_0.95La_0.05)_xFe_88-xCr_2B_10 (x=9.5 and 11) and (Nd_0.95La_0.05)_10Fe_79.8-yCr_yB_10.2 (y=2 and 4). However, high remanence of 9.9-10.7 kG, but low intrinsic coercive force of 5.9-7.9 kOe were achieved in the ribbons made from Nd_2Fe_14B and Nd_9.5Fe_85.5B_5 ribbons with low boron concentration. From transmission electron microscopy, it is verified that the coercive force of the studied ribbons is not only dependent upon the volume fraction of the R_2Fe_14B phase, but also influenced by the grain size of the ribbons. The magnetic domain pattern, evidenced by magnetic force microscopy, is well dependent upon the grain size and the magnetization of the magnetic phases.