A high-resolution x-ray resaonant exchange scattering study of the magnetic structure of PrNi$_2$Ge$_2$*

The zero-field magnetic structure of PrNi$_2$Ge$_2$ has been determined using high-resolution x-ray resonant magnetic scattering at the Pr L$_2$ edge. Magnetic susceptibility measurements had previously indicated the presence of two distinct ordering transitions at T$_N$=20.4K and T$_t$=7.7K respectively. Below TN, magnetic Bragg peaks were observed of the form q=(0,0,q$_z$) consistent with Fermi-surface nesting vector in RNi$_2$Ge$_2$ series compounds. q$_z$ reaches a value of 0.8128$\pm$0.0002 c$^*$ for temperatures below $\sim$12K. Intensity measurements as a function of scattering wave vector Q are consistent with moments aligned along the c axis for all temperatures. No obvious changes in intensity or wave vector were observed at the T$_t$ transition.