Helical antiferromagnetic ordering in Lu1-xScxMnSi

Polycrystalline samples of Lu_1-xSc_xMnSi _{(x =0, 0.25, 0.5) are studied using powder x-ray diffraction, heat capacity Cp, magnetization, magnetic susceptibility χ, and electrical resistivity ρ measurements versus temperature T and magnetic field H. This system crystallizes in the primitive orthorhombic TiNiSi-type structure (space group Pnma) as previously reported. The ρ (T) data indicate metallic behavior. The Cp(T), χ (T), and ρ (T) measurements consistently indicate long-range antiferromagnetic (AF) transitions with AF ordering temperatures TN=246, 215, and 188 K for x =0, 0.25, and 0.5, respectively. A second transition is observed at somewhat lower T for each sample from the χ (T) and ρ (T) measurements, which we speculate are due to spin reorientation transitions; these second transitions are completely suppressed in H =5.5 T. The Cp data below 10 K for each composition indicate an enhanced Sommerfeld electronic heat capacity coefficient for the series in the range γ =24-29 mJ/mol K². The χ (T) measurements up to 1000 K were fitted by local-moment Curie-Weiss behaviors which indicate a low Mn spin S ∼1. The χ data below TN are analyzed using the Weiss molecular field theory for a planar noncollinear cycloidal AF structure with a composition-dependent pitch, following the previous neutron diffraction work of Venturini et al. [J. Alloys Compd. 256, 65 (1997), 10.1016/S0925-8388(96)03009-5]. Within this model, the fits indicate a turn angle between Mn ordered moments along the cycloid axis of ∼100^∘ or ∼145^∘, either of which indicate dominant AF interactions between the Mn spins in the Lu1-x}Sc_xMnSi series of compounds.