Neutron study of the magnetism in NiCl2 center dot 4SC NH2 2

We study the strongly anisotropic quasi one dimensional S 1 quantum magnet NiCl2 amp; 8901;4SC NH2 2 using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field Hc1. For Hc1 lt; H lt; Hc2, where Hc2 indicates the transition to a fully magnetized state, three dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature magnetic field phase diagram, and we find that the total ordered magnetic moment reaches mtot 2.1 amp; 956;B at the field amp; 956;0H 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for H gt; Hc2, and we demonstrate the presence of an AF interaction between Ni2 on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin waves that develop from the lower energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices