Magnetic structure of hexagonal RMnO3 (R = Y, Sc):: Thermal evolution from neutron powder diffraction data

The magnetic structures of the hexagonal YMnO3 and ScMnO3 oxides (space group P6(3)cm) have been studied by neutron-diffraction experiments on powder samples. This study has been complemented with magnetic susceptibility and specific-heat measurements. They confirm that YMnO3 and ScMnO3 become ordered below T-N approximate to 70 K and T-N approximate to 129 K, respectively, according to a triangular antiferromagnetic structure. The fatter compound undergoes a second magnetic transition below 75 K, corresponding to a spin reorientation. The presence of weak ferromagnetism of Dzialoshinskii-Moriya type is observed in all the ordered region for the Sc material. In both compounds the moments are contained in the (a,b) plane and oriented perpendicular (R = Y) or parallel (R = Sc) to the [100] directions. The low-temperature ordered moments are 2.90(2)mu(B) (R = Y) and 3.54(3)mu(B) (R = Sc) at T = 1.7 K. The different behavior observed in the thermal evolution of the magnetic structure of both compounds is discussed in terms of geometrical parameters determined from high-resolution structural data. The origin of the weak ferromagnetism in ScMnO3 is also analyzed.