Magnetostructural phase transitions of DyFe4Ge2: Part II Neutron diffraction

We present the temperature magnetic phase diagram of the compound DyFe 4 Ge 2 determined from neutron diffraction data for the entire magnetically ordered regime. DyFe 4 Ge 2 undergoes at T N , T c = 55 K a simultaneous structural and magnetic transition of second order (or weakly first order) followed by two subsequent isostructural first-order magnetic transitions at T ic 2 = 45 K and T ic 1 = 28 K : P 4 2 / mnm , T N , T c = 55 K → Cmmm , q 1 = ( 0 1 2 0 ) T ic 2 = 45 K → q 2 = ( 0 q y 0 ) T ic 1 = 28 K → q 1 . The re-entrant lock-in magnetic phase q 1 = ( 0 1 2 0 ) is stable in the high-temperature range T ic2 – T N and in the low-temperature range 1.5 K– T ic1 while the incommensurately modulated q 2 = ( 0 q y 0 ) magnetic phase is sandwiched in the intermediate range T ic1 – T ic2 between the two commensurate phases. The wave vector q 2 has a temperature-dependent length with a minimum in the middle of the incommensurate range and corresponds to a multiaxial amplitude modulated phase. Symmetry analysis leads for both propagation vectors in Cmmm to a twofold and fourfold splitting of the tetragonal Dy 2 b site and the Fe 8 i sites, respectively. The low temperature q 1 = ( 0 1 2 0 ) and the q 2 = ( 0 q y 0 ) phases correspond to 3D canted magnetic structures described by the irreducible representations (Irreps) Γ 2 + Γ 3 while the high-temperature q 1 phase to 2D canted magnetic structures described by a single Irrep Γ 2 . The T ic2 transition is connected with reorientations of both Fe and Dy moments.