Phase diagram of the effective Ising spin-1/2 chain compound CoNb2O6 in transverse magnetic field

CoNb2O6 is a model system to investigate the quantum phase transition of Ising spins in a transverse magnetic field. The interesting physics exclusively takes place within the magnetic CoO6 layers, separated by non-magnetic NbO6 layers. The edge-sharing oxygen octahedrons link the Co2+ spins and form 1D ferromagnetic chains along the c axis. Due to crystal field effects an easy-axis anisotropy is present, which leads to an effective spin-1/2 system described by the Ising model. Small inter-chain couplings J parallel ∼ 0.01 . J perpendicularto cause long-range antiferromagnetic order, which is incommensurate below TN1 = 2.95 K and becomes commensurate at TN2 = 1.97 K. A magnetic field parallel to the b axis is normal to the easy axis and allows to study the quantum phase transition in transverse field. Above 5 T the system is driven through its quantum critical point to a quantum paramagnet. Only few studies of the transverse field case are available. We present measurements of specific heat and magnetization in a temperature range from about 0.3 up to 10 K and discuss the phase diagram. We compare our measurements with the theoretical predictions of the 1D Ising model in a transverse field.