Crystal-field interactions and spin reorientation in (Er1−xDyx)2Fe14B

Spin reorientation in (${\mathrm{Er}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Dy}}_{\mathrm{x}}$ ${)}_2$ ${\mathrm{Fe}}_{14}$B alloys has been investigated by means of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectroscopy on magnetically aligned samples. For 0<x<0.5, the magnetization direction is found to change continuously from the basal plane to the tetragonal c axis with increasing temperature. Reorientation temperatures decrease with increasing Dy concentration, as expected from competition between the uniaxial anisotropy of Dy and the basal anisotropy of Er in the $R_2$ ${\mathrm{Fe}}_{14}$B (R a rare-earth atom) structure. Data are quantitatively interpreted with a model incorporating crystal-field and exchange interactions as well as Fe-sublattice and R dipolar anisotropies. The stability range of the basal-plane magnetization extends up to $x_c$≃0.5 at low temperatures, while the model predicts $x_c$=0.26 if only $B_2^0$ $O_2^0$ and $B_2^2$ $O_2^2$ terms are considered; inclusions of $B_4^0$ $O_4^0$ and $B_6^0$ $O_6^0$ terms provide a satisfactory fit to our $T_r$ versus x data.