Double-Q magnetic structures and strong planar anisotropy in tetragonal ErRu2Ge2 and ErRu2Si2

Single crystal magnetization measurements and powder neutron diffraction on tetragonal ErRu2Ge2 as well as anisotropy of the paramagnetic susceptibili ty and specific heat measurements on ErRu2Si2 are presented. Besides the hu ge crystal field contribution to the uniaxial anisotropy, which favors the basal plane, a strong in-plane anisotropy is evidenced. From these features and neutron diffraction experiments it is shown that magnetic structures o f these materials are double-Q and accordingly non-colinear below their Nee l temperature (5.2 and 6.0 K for Ge and Si based compounds, respectively). The magnetic structures induced during the metamagnetic processes are discu ssed.