Complex magnetic order in the decorated spin-chain system Rb2Mn3(MoO4)3(OH)2

The macroscopic magnetic properties and microscopic magnetic structure of Rb2Mn3(MoO4)3(OH)2 (space group Pnma) are investigated by magnetization, heat capacity, and single-crystal neutron diffraction measurements. The compound's crystal structure contains bond-alternating [Mn3O11]∞ chains along the b axis, formed by isosceles triangles of Mn ions occupying two crystallographically nonequivalent sites (the Mn1 site on the base and Mn2 site on the vertex). These chains are only weakly linked to each other by nonmagnetic oxyanions. Both superconducting quantum interference device magnetometry and neutron diffraction experiments show two successive magnetic transitions as a function of temperature. On cooling, it transitions from a paramagnetic phase into an incommensurate phase below 4.5 K with a magnetic wave vector near k1=(0,0.46,0). An additional commensurate antiferromagnetically ordered component arises with k2=(0,0,0), forming a complex magnetic structure below 3.5 K with two different propagation vectors of different stars. On further cooling, the incommensurate wave vector undergoes a lock-in transition below 2.3 K. The experimental results suggest that the magnetic superspace group is Pnma.1′(0b0)s0ss for the single-k incommensurate phase and is Pn′ma(0b0)00s for the two-k magnetic phase. We propose a simplified magnetic structure model taking into account the major ordered contributions, where the commensurate k2 defines the ordering of the c-axis component of the Mn1 magnetic moment, while the incommensurate k1 describes the ordering of the ab-plane components of both Mn1 and Mn2 moments into elliptical cycloids.