Neutron diffraction study of metamagnetic phases in ErNi2B2C

Neutron diffraction has been used to determine the magnetic order in the metamagnetic phases of ErNi2B2C, with the magnetic field applied parallel to the [0 1 0] and [1 1 0] crystallographic axes. Below 6 K, ErNi2B2C has an incommensurate antiferromagnetic structure with a modulation along the a-axis of wavevector δ=(0.55,0,0). For both orientations three first-order metamagnetic transitions are observed below 6 K as the applied field is increased. The first two transitions are to incommensurate antiferromagnetic states with different values of the a-axis modulation, and the third transition is to a saturated paramagnetic state in which the spins are aligned ferromagnetically by the applied field. With the field applied parallel to the [0 1 0] direction, which is the magnetically easy axis, the upper incommensurate state has an additional ferromagnetic component which is not observed with the field applied parallel to the [1 1 0] direction.