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Effects of pressure on tiny antiferromagnetic moments in the heavy-electron compound URu2Si2 Amitsuka, H.; Sato, M.; Metoki, N.; Yokoyama, M.; Kuwahara, K.; Sakakibara, T.; Morimoto, H.; Kawarazaki, S.; Miyako, Y.; Mydosh, J.A.

In this study, we performed powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd 2 Ir 2 O 7 , which exhibits a metal-insulator transition at a temperature T MI of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector q 0 of ð0; 0; 0Þ and that it grows with decreasing temperature below 15 K. This structure was analyzed to be of the all-in allout type, consisting of highly anisotropic Nd 3þ magnetic moments of magnitude 2:3 AE 0:4 B , where B is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd 3þ splits below T MI . This suggests the appearance of a static internal magnetic field at the Nd sites, which probably originates from a magnetic order consisting of Ir 4þ magnetic moments. Here, we discuss a magnetic structure model for the Ir order and the relation of the order to the metal-insulator transition in terms of frustration.

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.

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