Neutron diffraction studies of metamagnetic phase transitions in TbRu2Si2 and DyRu2Si2

Investigations of metamagnetic phase transitions in TbRu2Si2 and DyRu2Si2 using single-crystal neutron diffraction under magnetic fields up to 5 T along the c-axis are reported.
Without a magnetic field, TbRu2Si2 and DyRu2Si2 exhibit a one-dimensional magnetic order of a squared wave with a wavevector τ = (0.23, 0,0) and τ = (0.222, 0, 0) respectively, with magnetic moments parallel or antiparallel to the c axis. The τ = 0.23 wave consists of 26 ferromagnetic (100) planes with a 5 + 4 − 4 + 5 − 4 + 4 − sequence along the a axis, where 5+ and 4− denotes five and four consecutive ferromagnetic (100) planes with up and down spins respectively. The τ = 0.222 structure can be expressed by a 5 + 5 − 4 + 4 − sequence. Applied fields induce a gradual change in this one-dimensionality up to a critical field Hc1 = 21 kOe; from this value up to Hc2 = 29 kOe, TbRu2Si2 becomes a two-dimensionally modulated ferrimagnet with the wavevectors (0.23, 0, 0), (0.23, 0.23, 0) and a ferromagnetic component. Just below Hc1 the structure becomes ferrimagnetic with a 5 + 4 − 5 + 3 − 5 + 4 − sequence. Above Hc2 = 29 kOe all the moments are ferromagnetically aligned in the field direction. DyRu2Si2 shows a similar metamagnetic behaviour with Hc1 = 10 kOe and Hc2 = 17 kOe. The metamagnetic phase is two dimensional with wavevectors (0.222, 0, 0), (0.222, 0.222, 0) and a ferromagnetic component.