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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.

Neutron-diffraction experiments under high pressure have been done to study the magnetic structure of the ground state and the nature of the magnetic order of the pressure-induced superconductor CeRh 2 Si 2 . Two models for the magnetic structure of the ground state, namely, a 4-q superposed structure and a multidomain structure, have been tested and the former was found to be more realistic, though both of them have some inconsistency with the results of the previous NMR experiment. The pressure dependence of both the transition temperature and magnitude of the ordered magnetic moment indicate that the magnetism of this compound is basically itinerant despite its high transition temperature. Our studies also revealed that, when the pressure is close to the critical magnitude to destroy the antiferromagnetism, the compound can have a ''tiny ordered moment'' which is widely observed in the metallic Kondo-lattice compounds.

We have performed elastic neutron scattering experiments under uniaxial stress σ applied along the tetragonal [100], [110] and [001] directions for the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with σ along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment µo continuously increases from 0.02 µB/U (σ = 0) to 0.22 µB/U (0.25 GPa). The rate of increase ∂µo/∂σ is ∼ 1.0 µB/GPa, which is four times larger than that for the hydrostatic pressure (∂µo/∂P ∼ 0.25 µB/GPa). Above 0.25 GPa, µo shows a tendency to saturate, similar to the hydrostatic pressure behavior. For σ || [001], µo shows only a slight increase to 0.028 µB/U (σ = 0.46 GPa) with a rate of ∼ 0.02 µB/GPa, indicating that the development of the AF state highly depends on the direction of σ. We have also found a clear hysteresis loop in the isothermal µo(σ) curve obtained for σ || [110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the σ-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.

A quantum critical point of the heavy fermion Ce(Ru(1-x)Rh(x))2Si2, (x = 0,0.03) has been studied by single-crystalline neutron scattering. By accurately measuring the dynamical susceptibility at the antiferromagnetic wave vector k3 = 0.35c*, we have shown that the inverse energy width gamma(k3), i.e., the inverse correlation time, depends on temperature as gamma(k3) = c1 + c2T((3/2)+/-0.1), where c1 and c2 are x dependent constants, in a low temperature range. This critical exponent 3/2 +/- 0.1 proves that the quantum critical point is controlled by that of the itinerant antiferromagnet.