Neutron-diffraction study of the metamagnetic phases in HoNi2B2C

Neutron diffraction has been used to determine the magnetic phase diagram o f HoNi2B2C, in the plane of temperature and magnetic field up to 4.6 T, for two different crystallographic orientations relative to the applied magnet ic field. For both orientations two first-order metamagnetic transitions ar e observed below 5.5 K as the applied field is increased. In both cases the first transition is to a state in which the magnetic structure is modulate d along the c axis with wave vector delta=[0,0,0.667]. With the field appli ed parallel to the [110] direction the second transition is to a saturated paramagnetic state in which the spins are aligned ferromagnetically by the applied magnetic field. With the field parallel to the [0 (1) over bar 0] d irection, the second transition is into a state in which the magnetic struc ture is modulated along the b axis with wave vector delta= [0,0.610,0], in disagreement with a simplified theoretical model.