Neutron Scattering Study on Competition between Hidden Order and Antiferromagnetism in U(Ru₁₋ₓRhₓ)₂Si₂ (x <= 0.05)

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We have performed elastic and inelastic neutron scattering experiments on the solid solutions U(Ru₁₋ₓRhₓ)₂Si₂ for the Ru rich concentrations: x=0, 0.01, 0.02, 0.025, 0.03, 0.04 and 0.05. Hidden order is suppressed with increasing x, and correspondingly the onset temperature Tₘ (17.5 K at x=0) of weak antiferromagnetic (AF) Bragg reflection decreases. For x=0.04 and 0.05, no magnetic order is detected in the investigated temperature range down to 1.4 K. In the middle range, 0.02 <= x <= 0.03, we found that the AF Bragg reflection is strongly enhanced. At x=0.02, this takes place at 7.7 K (=Tₘ), which is significantly lower than Tₘ (13.7 K). Tₘ increases with increasing x, and seems to merge with Tₘ at x=0.03. If the AF state is assumed to be homogeneous, the staggered moment μₒ estimated at 1.4 K increases from 0.02(2) μB/U (x=0) to 0.24(1) μB/U (x=0.02). The behavior is similar to that observed under hydrostatic pressure (μₒ increases to 0.25 μB/U at 1.0 GPa), suggesting that the AF evolution induced by Rh doping is due to an increase in the AF volume fraction. We also found that the magnetic excitation observed at Q=(1,0,0) below Tₘ disappears as T is lowered below Tₘ.