  The monoclinic double-perovskite Sr2YRuO6 has recently gained a renewed
interest in order to get a deeper insight into the exotic magnetic ground
states associated with geometric frustration. Striking discrepancies between
the spin order derived from the neutron diffraction refinements and the
macroscopic magnetic and thermal responses is a major challenge that must be
addressed. In this work, detailed neutron diffraction measurements as a
function of temperature yield a completely different interpretation of the
patterns. We show that at low temperatures a spin structure of the K2NiF4-type
is an accessible configuration for the magnetic ground state. In the
neighborhood of the magnetic transition this configuration evolves into a
canted super-structure. The deduced temperature dependence of the canting angle
exhibits two closely spaced peaks, which are in excellent agreement with the
double peaks in the magnetic contribution to the specific heat and in the
thermal expansion coefficient. We explain these features in terms of
reorientation of the net ferromagnetic moment of the noncollinear spin state,
due to the local breaking of the inversion symmetry promoted by the monoclinic
distortions, with structural changes acting as the driving force.
