1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 Department of Physics, Technical University of Denmark3 unknown4 Department of Wind Energy, Technical University of Denmark
We present neutron and high-energy synchrotron X-ray diffraction studies to show that the anomalous antiferromagnetic (AF) phase diagram of TmNi2B2C in an applied field along [1 0 0] is governed by a quadrupolar ordering of the Tm ions. The ordering is revealed by a distortion of the lattice with the Tm ions displaced along the c-axis and modulated with the same wave vector QA=(0.484, 0, 0) as the AF phase induced by fields larger than ≈10 kOe. In zero field, the quadrupolar ordering temperature is TQ≅13.5 K but increases to about 20 K in a field of 100 kOe. The Tm displacements are also significantly enhanced, by a factor of 10 at 60 kOe. A model is presented that accounts for the quadrupolar as well as the low-field QF=(0.94, 0.94, 0) and high-field QA=(0.483, 0, 0) AF ordering previously observed in this compound.