Magnetic Structure of Phase II in $\mathbf{U}({\mathrm{Ru}}_{0.96}{\mathrm{Rh}}_{0.04}{)}_2{\mathrm{Si}}_2$ Determined by Neutron Diffraction under Pulsed High Magnetic Fields

We report neutron diffraction measurements on $\mathrm{U}({\mathrm{Ru}}_{0.96}{\mathrm{Rh}}_{0.04}{)}_2{\mathrm{Si}}_2$ single crystal under pulsed high magnetic fields up to 30 T applied along the tetragonal $c$ axis. The high-field experiments revealed that the field-induced phase II above 26 T corresponds to a commensurate up-up-down ferrimagnetic structure characterized by the wave vector $\mathit{q}=(2/3,0,0)$ with the magnetic moments parallel to the $c$ axis, which naturally explains the one-third magnetization plateau and the substantially changed Fermi surface in phase II. This $a$-axis modulated magnetic structure indicates that the phase II near the hidden order phase is closely related to the characteristic incommensurate magnetic fluctuations at ${\mathbf{Q}}_1=(0.6,0,0)$ in the pure system ${\mathrm{URu}}_2{\mathrm{Si}}_2$, in contrast to the pressure-induced antiferromagnetic order at ${\mathbf{Q}}_0=(1,0,0)$.