Effects of Band Filling on Magnetic Structures: The Case of $\mathit{R}{\mathrm{Ni}}_2{\mathrm{Ge}}_2$

We establish that strong Fermi surface nesting drives the Néel transition in the $R{\mathrm{Ni}}_2{\mathrm{Ge}}_2$ compounds. Generalized susceptibility, ${\chi }_0\left(\mathbf{q}\right)$, calculations found nesting to be responsible for both incommensurate wave vector, $\left(000.793\right)$, in ${\mathrm{GdNi}}_2{\mathrm{Ge}}_2$, and the commensurate structure, $\left(001\right)$, in ${\mathrm{EuNi}}_2{\mathrm{Ge}}_2$, as revealed by x-ray resonant exchange scattering. A continuous transition from incommensurate to commensurate magnetic structures via band filling is predicted. The surprisingly higher $T_N$ in ${\mathrm{EuNi}}_2{\mathrm{Ge}}_2$ than that in ${\mathrm{GdNi}}_2{\mathrm{Ge}}_2$ is also explained.