Magnetic structure of hexagonalRMnO3(R=Y, Sc):Thermal evolution from neutron powder diffraction data

The magnetic structures of the hexagonal ${\mathrm{YMnO}}_3$ and ${\mathrm{ScMnO}}_3$ oxides (space group ${P6\mathrm{}}_{3}cm)$ 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 ${\mathrm{YMnO}}_3$ and ${\mathrm{ScMnO}}_3$ become ordered below $T_N\approx 70$ K and $T_N\approx 129$ K, respectively, according to a triangular antiferromagnetic structure. The latter 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 $\mathrm{}(a,b)\mathrm{}$ plane and oriented perpendicular $(R=\mathrm{Y})$ or parallel $(R=\mathrm{Sc})$ to the [100] directions. The low-temperature ordered moments are $2.90\left(2\right)\mathrm{}{\mu }_B$ $(R=\mathrm{Y})$ and $3.54\left(3\right)\mathrm{}{\mu }_B$ $(R=\mathrm{Sc})$ at $T=1.7\mathrm{}$ 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 ${\mathrm{ScMnO}}_3$ is also analyzed.