Anisotropy energy of Nd ion in Nd2Fe12-xCo2AlxB compounds

Ac initial susceptibility measurements are performed to determine the spin reorientation temperature Tsr. It is found that Co and/or Al substitutions for Fe in Nd2Fe14B result in a decrease of Tsr. The estimation of the exchange field (2μBHex) is made by fitting the calculations with experimental data of Tsr on the assumption that the crystal field acting on the Nd ion is unchanged due to the small amount of Al substitution for Fe. Neglecting the off-diagonal terms of BnmOnm with m ≠ 0 and the high order terms (all of which decrease rapidly with increasing temperature), the Hamiltonian of the Nd ion may be expressed by H = B20O20 + 2(gJ - 1)μBHex · J. According to this model the easy c-axis stability energy ΔF(T) = (F(90, T) - F(0, T)) may be estimated in the temperature range 200 to 450 K. It is assumed that Al partial substitution for Fe in Nd2Fe12Co2B compound may decrease the exchange field acting on Nd spin which in turn results in the decrease of Tsr and the deterioration of the thermostability of this type of compounds. (author)