with cobalt doping*

The compound NiBr₂ crystallizes in the trigonal space group R-3m. Below T_N= 44 K, NiBr₂ exhibits antiferromagnetic order characterized by antiferromagnetically stacked sheets of ferromagnetically aligned Ni spins. A transition to a helical magnetic structure occurs below T_IC=22.8 K with a propagation vector of at 4.2 K. Here we use neutron diffraction to study the tuning of the magnetic properties with cobalt substitution in single crystals for several compositions of Ni_1-xCo_xBr₂, where x = 0, 0.2, 0.25, 0.3, and 0.45. Our measurements show that the T_N decreases with doping while changes in T_IC are more subtle. The neutron scattering studies further reveal that the 6 incommensurate spots in the parent compound give way to a ring of magnetic scattering reminiscent of that observed previously for Ni_0.92Zn_0.08Br₂.