Helical antiferromagnetic ordering in Lu1-xScxMnSi

Polycrystalline samples of Lu1-xScxMnSi (x = 0, 0.25, 0.5) are studied using powder x-ray diffraction, heat capacity C-p, magnetization, magnetic susceptibility chi, and electrical resistivity rho 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 rho(T) data indicate metallic behavior. The C-p(T), chi(T), and rho(T) measurements consistently indicate long-range antiferromagnetic (AF) transitions with AF ordering temperatures T-N = 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 chi(T) and rho(T) measurements, which we speculate are due to spin reorientation transitions; these second transitions are completely suppressed in H = 5.5 T. The C-p data below 10 K for each composition indicate an enhanced Sommerfeld electronic heat capacity coefficient for the series in the range gamma = 24-29 mJ/mol K-2. The chi(T) measurements up to 1000 K were fitted by local-moment Curie-Weiss behaviors which indicate a low Mn spin S similar to 1. The chi data below T-N 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)]. Within this model, the fits indicate a turn angle between Mn ordered moments along the cycloid axis of similar to 100 degrees or similar to 145 degrees, either of which indicate dominant AF interactions between the Mn spins in the Lu1-xScxMnSi series of compounds.