First-order transition to a noncollinear antiferromagnetic structure in U2Rh3Si5

We have determined the antiferromagnetic structure of monoclinic U2Rh3Si5 and the temperature dependence of the magnetically ordered moment μ via single-crystal neutron diffraction. The value of μ exhibits a sharp discontinuity at the ordering temperature TN=25.5 K, jumping from 1.6μB to zero within a narrow temperature interval [ΔTN=0.2 K, μ(9 K)=2.35μB]. This demonstrates the first-order nature of the transition. The magnetically ordered structure is a noncollinear arrangement of the eight uranium magnetic moments within the crystallographic unit cell. The moments are oriented parallel to nearest-neighbor U-Rh bonds, creating uniaxial (Ising-type) magnetic behavior, with canted local easy axes. The temperature dependence of the lattice parameters, as determined via high-resolution x-ray diffraction, exhibit pronounced discontinuities at TN. These data demonstrate a strong magnetoelastic coupling.