Magnetic ordering in the frustrated J1-J2 Ising chain candidate BaNd2 O4

The AR2O4 family (R=rare earth) has recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the c axis. We have investigated polycrystalline BaNd2O4 with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below TN=1.7K, and they can be indexed with a propagation vector of k- =(0,1/2,1/2). The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double Néel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for T=0.46K. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. One possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical J1-J2 Ising chain with a double Néel ground state in zero field.