Low-temperature incommensurate magnetic order in R 2 BaCoO 5 [R = rare earth]

R2BaCoO5 oxides, with R = Er, Tm, Lu and Y, show incommensurate (I) magnetic order below TN=3.5 K. The Er compound remains as an I-magnetic structure down to T=40 mK and the Y one shows the coexistence of both commensurate (C) and I structures in the vicinity of a Lifshitz point. Both facts are quite uncommon. A devil’s staircase behaviour in helimagnetic Tm2BaCoO5 is observed at 1.4 K<T<2.5 K. A sudden collapse of a sine-wave modulation of Co moments to a C-magnetic structure is evidenced in Lu2BaCoO5 at T=2.5 K with Co magnetic moments μ∼3.9 μB lying in the xy plane.