Magnetic ordering in SrEr$_2$O$_4$ and SrHo$_2$O$_4$

Single crystal neutron diffraction reveals two distinct components to the magnetic ordering in geometrically frustrated compounds $\rm SrEr_2O_4$ and $\rm SrHo_2O_4$. One component, a long-range ordered ${\bf k}=0$ structure, is associated with the appearance of resolution limited Bragg peaks below the ordering temperature. Another component is a quasi 1D short-range structure which manifests itself by the presence of a strong diffuse scattering signal forming {\it planes} in reciprocal space. On cooling from higher temperatures down to 0.06~K, the partially ordered component develops gradually and does not undergo a pronounced phase transition. The magnetic moments in the long-range structure are pointing along the [001] axes in both compounds. In the short-range structure (which is incommensurate in $\rm SrEr_2O_4$ and ${\bf k} \approx 1/2$ in $\rm SrHo_2O_4$) the moments are predominantly pointing along the [001] and [010] axes in these two compounds respectively. The unusual coexistence of two magnetic structures is probed using XYZ-polarised neutron scattering techniques [1,2]. [1] T.J. Hayes {\it et al.}, to appear in Phys. Rev. B (2011). [2] O. Young {\it et al.}, in preparation (2011).