Realization of the axial next-nearest-neighbor Ising model in U3Al2Ge3

Here we report small-angle neutron scattering (SANS) measurements and theoretical modeling of U3Al2Ge3. Analysis of the SANS data reveals a phase transition to sinusoidally modulated magnetic order at TN=63 K to be second order and a first-order phase transition to ferromagnetic order at Tc=48 K. Within the sinusoidally modulated magnetic phase (Tc<T<TN), we uncover a dramatic change, by a factor of 3, in the ordering wave vector as a function of temperature. These observations all indicate that U3Al2Ge3 is a close realization of the three-dimensional axial next-nearest-neighbor Ising model, a prototypical framework for describing commensurate to incommensurate phase transitions in frustrated magnets.