Magnetic phase diagram of GdNi 2 B 2 C : Two-ion magnetoelasticity and anisotropic exchange couplings

Extensive magnetization and magnetostriction measurements were carried out on a single crystal of ${\mathrm{GdNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ along the main tetragonal axes. Within the paramagnetic phase, the magnetic and strain susceptibilities revealed a weak anisotropy in the exchange couplings and two-ion tetragonal-preserving $\ensuremath{\alpha}$-strain modes. Within the ordered phase, magnetization and magnetostriction revealed a relatively strong orthorhombic distortion mode and rich field-temperature phase diagrams. For the $H||(100)$ phase diagram, three field-induced transformations were observed: namely, at ${H}_{D}(T),$ related to the domain alignment; ${H}_{R}(T),$ associated with reorientation of the moment towards the c axis; and ${H}_{S}(T),$ defining the saturation process wherein the exchange field is completely counterbalanced. On the other hand, for the $H||(001)$ phase diagram, only two field-induced transformations were observed: namely, at ${H}_{R}(T)$ and ${H}_{S}(T).$ For both phase diagrams, ${H}_{S}(T)$ follows the relation ${H}_{S}[1\ensuremath{-}{(T/T}_{N}{)}^{2}{]}^{1/2}\mathrm{kOe}$ with ${H}_{S}(\stackrel{\ensuremath{\rightarrow}}{T}0)=128.5(5)\mathrm{kOe}$ and ${T}_{N}(H=0)=19.5\mathrm{K}.$ In contrast, the thermal evolution of ${H}_{R}(T)$ along the c axis (much simpler than along the a axis) follows the relation ${H}_{R}[1\ensuremath{-}{T/T}_{R}{]}^{1/3}\mathrm{kOe}$ where ${H}_{R}(\stackrel{\ensuremath{\rightarrow}}{T}0)=33.5(5)\mathrm{kOe}$ and ${T}_{R}(H=0)=13.5\mathrm{K}.$ It is emphasized that the magnetoelastic interaction and the anisotropic exchange coupling are important perturbations and therefore should be explicitly considered if a complete analysis of the magnetic properties of the borocarbides is desired.