Hexagonal α-Ru trichloride single crystals exhibit a strong magnetic anisotropy and we show that upon applying fields up to 14 T in the honeycomb plane the successive magnetic order at T1 = 14 K and T2 = 8 K could be completely suppressed whereas in the perpendicular direction the magnetic order is robust. Furthermore the field dependence of χ(T) implies coexisting ferro-and antiferromagnetic exchange between in-plane components of Ru 3+ -spins, whereas for out-of-plane components a strong antiferromagnetic exchange becomes evident. 101 Ru zero-field nuclear magnetic resonance in the ordered state evidence a complex (probably non coplanar chiral) long-range magnetic structure. The large orbital moment on Ru 3+ is found in density-functional calculations. PACS numbers: 75.30.Gw, 75.40.Cx, Low dimensional 4d-and 5d-magnets show a wide variety of magnetic ground states due to crystal electric field (CEF) splitting in combination with a strong spin-orbit coupling (SOC). Especially the 5d 5 -iridate compounds earned great attention because of the predicted topological Mott insulating state due to the strong SOC and the Coulomb correlation [1]. Furthermore the strong SOC favors the asymmetric Dzyaloshinskii-Moriya (DM) interaction that often results in chiral spin arrangements in the ordered phases [2,3]. In addition, for spin-1/2 systems geometrical frustration of the magnetic exchange interactions frequently leads to a quantum spin-liquid ground state[4]. Among 4d-and 5d-systems, the Heisenberg-Kitaev model (HKM) was established to describe the competing bond-dependent magnetic exchange interactions in the honeycomb type of lattice structures [5]. Prominent examples are the 2-1-3 iridates (Li 2 IrO 3 , Na 2 IrO 3 ) where the magnetism is associated to the 5d 5 electrons on the Ir 4+ ions. According to the HKM, the phase diagram provides a transition from a conventional Neel-type of antiferromagnetic (AFM) order to a AFM stripy-(or zigzag-) type of order towards a pure quantum spin liquid (QSL) phase as a function of control parameter [6]. Indeed Na 2 IrO 3 shows an AFM order of zigzag-type at T = 15 K, whereas the Li 2 IrO 3 system is more close to the QSL regime and a non coplanar spiral order is discussed [7].In order to search for new 4d-or 5d-model system with the honeycomb lattice arrangement as a platform of HKM α-RuCl 3 turns out to be an excellent candidate because the low spin 3+ state of Ru (4d 5 ) is equivalent to the low spin 4+ state of Ir (5d 5 ). However, lowtemperature magnetic properties of α-RuCl 3 were not studied in detail and, so far, on powders only. Recently, Plumb and co-workers have shown in a spectroscopic experiment that α-RuCl 3 is a magnetic insulator due to sizable Coulomb correlations accompanied by the spinorbit coupling [8].In this Rapid Communication, we report detailed studies on the magnetic anisotropy by magnetization and specific heat on single crystals over a wide temperature and field range. Furthermore, we applied 99,101 Ru zero-field nuclear magnetic resonance as a local and ...
