Field-tuned magnetic structure and phase diagram of the honeycomb magnet YbCl₃

We report thermodynamic and neutron diffraction measurements on the magnetic ordering properties of the honeycomb lattice magnet YbCl₃. We find YbCl₃ exhibits a Néel type long-range magnetic order at the wavevector (0, 0, 0) below T_N = 600 mK. This magnetic order is associated with a small sharp peak in heat capacity and most magnetic entropy release occurs above the magnetic ordering temperature. The magnetic moment lies in-plane, parallel to the monoclinic a-axis, whose magnitude m_Yb = 0.86(3) μ_B is considerably smaller than the expected fully ordered moment of 2.24 μ_B for the doublet crystal-field ground state. The magnetic ordering moment gradually increases with increasing magnetic field perpendicular to the ab-plane, reaching a maximum value of 1.6(2) μ_B at 4 T, before it is completely suppressed above ∼ 9 T. These results indicate the presence of strong quantum fluctuations in YbCl₃.