The magnetic structures of RMgSn compounds (R = Ce, Pr, Nd, Tb)

Highlights: → The room-temperature crystal structure has been refined for five RMgSn phases adopting the CeScSi- and/or the TiNiSi-type structure. → The magnetic structure has been determined for RMgSn phases with R = Ce, Pr, Nd, Tb. → The CeScSi-type phases show commensurate (R = Nd, Pr) or incommensurate (R = Tb) antiferromagnetic ordering. → PrMgSn, high temperature phase, orders first ferromagnetically, then becomes spin-canted. - Abstract: The synthesis of the new compounds RMgSn (R = La-Nd, Sm, Gd-Tm, Lu and Y) has been recently reported. The compounds formed by La and Ce crystallise in the TiNiSi structure type (oP12, Pnma), while from Nd they adopt the CeScSi-type (tI12, I4/mmm); PrMgSn is dimorphic: its high-temperature form (HT) is TiNiSi-type while the low-temperature one (LT) is CeScSi-type. In this paper we now report the results of a neutron diffraction investigation which has been performed in order to refine the crystal as well as the magnetic structures for the RMgSn compounds with R = Ce, Pr, Nd and Tb. All these compounds see at low temperature the establishment of long range magnetic ordering with a predominantly antiferromagnetic interaction; only PrMgSn-HT orders ferromagnetically. These results agree with those from magnetic measurements recently reported. The magnetic structure of CeMgSn is of the amplitude-modulated type, the value of the magnetic propagation vector refined at 2 K is τ = [0, 0.1886(4), 0.3384(8)]. The PrMgSn-HT phase below T = 52 K adopts first a purely ferromagnetic structure, then at about T = 15 K a second magnetic coupling leads to a spin-canted magnetic structure. Both PrMgSn-LT and NdMgSn have the same antiferromagnetic commensurate magnetic structure. The TbMgSn compound below TN = 35 K orders antiferromagnetically with an equal moment cycloidal structure; however a second magnetic transition at a temperature corresponding to TN2 = 65 K is likely also present.