Abstract :
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.
