Double-Q magnetic structures and strong planar anisotropy in tetragonal ErRu2Ge2 and ErRu2Si2 A. Garnier, D. Gignoux, B. Ouladdiaf, D. Schmitt and T. Shigeoka Eur. Phys. J. B, 16 3 (2000) 423-427 Published online: 15 August 2000 DOI: 10.1007/PL00011079