NEUTRON-DIFFRACTION STUDY OF THE HOMNSI, LUMNSI AND SC0.9LU0.1MNSI COMPOUNDS

The magnetic properties of the TiNiSi-type RMnSi compounds (R=Ho, Lu, Sc0.9Lu0.1) have been studied by magnetization and neutron diffraction measurements. In their whole ordered range, LuMnSi and Sc0.9Lu0.1MnSi exhibit hat spiral antiferromagnetic structures (T-N=255 and 125 K, r espectively) whereas HoMnSi displays three different magnetic regimes. Between 300 and 55 K, a collinear antiferromagnetic structure prevail s while between 55 and 15 K, a canted antiferromagnetic structure, due to a non-collinear arrangement of the Ho moments, takes place. At low er temperatures, incommensurate components appear on both Mn and Ho su blattices yielding a ''double-cone'' magnetic arrangement. Within the three compounds, the Mn moment value, at 2 K, strongly decreases with the R size from 2.67 mu(B), in HoMnSi to 2.18 mu(B) in LuMnSi and 1.53 mu(B) in Sc0.9Lu0.1MnSi. In HoMnSi, the magnetic structure is charact erized by antiferromagnetic Mn-Mn zigzag chains aligned along [010]. I n the LuMnSi and Sc0.9Lu0.1MnSi compounds, the angle between the Mn mo ment directions in a chain decreases with the lanthanide size from 121 degrees in LuMnSi to 52 degrees in Sc0.9Lu0.1MnSi. The magnetic inter actions in the Mn-Mn zigzag chains and the evolution of the Mn magneti c moment value are discussed and compared with those observed in the r elated CeFeSi-type RMnSi compounds. (C) 1997 Elsevier Science S.A.