1. Condensed Matter DepartmentNRC Kurchatov Institute—PNPI Orlova roshcha 1 Gatchina 188300 Russia
2. Faculty of PhysicsSaint Petersburg University Universitetskaya nab. 7-9 Saint Petersburg 198504 Russia
3. Science DivisionInstitut Laue-Langevin Avenue de Martyrs 71 Grenoble 38042 France
4. Laboratory for Quantum MagnetismÉcole polytechnique fédérale de Lausanne Lausanne CH-1015 Switzerland