Low temperature metamagnetic transitions in single crystal ErNi$_{2}$B$_{2}$C: torque magnetometry study*

The phase diagram of metamagnetic transitions in single-crystal rare-earth nickel borocarbide ErNi$_2$B$_2$C has been determined at 1.9 K with a Quantum Design torque magnetometer. The critical fields of the transitions depend crucially on the angle between applied field and the easy axis [100] in the $ab$-plane. Torque measurements have been made while sweeping the magnitude of the magnetic field at a constant angular direction (parallel to basal tetragonal $ab$-planes) over an angular range greater than two quadrants. Sequences of metamagnetic transitions with increasing field differ for the fields along (or close enough to) the easy [100] axis from those near the hard [110] axis. These torque measurements reveal new metamagnetic states in ErNi$_2$B$_2$C which were not apparent in previous longitudinal-magnetization and neutron studies. Their nature is considered and clarified. In the low-field range influences of superconductivity are observed and interpreted.