Influence of cobalt substitution on the magnetism of NiBr2

Abstract

Co-substituted ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Br}}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}1$) single crystals were synthesized using vapor transport. The physical properties of the crystals were characterized by x-ray powder diffraction, magnetization, and specific-heat measurements. Room-temperature x-ray powder diffraction data indicate that a change from the ${\mathrm{CdCl}}_{2}$ structure type to the ${\mathrm{CdI}}_{2}$ structure type occurs within $0.56<x<0.76$. ${\mathrm{NiBr}}_{2}$ has a commensurate antiferromagnetic phase below ${T}_{\mathrm{N}}\ensuremath{\approx}46\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and an incommensurate magnetic ground state below ${T}_{\mathrm{IC}}\ensuremath{\approx}20\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Both magnetic transitions are affected by cobalt substitution, and the incommensurate phase transition is present up to at least $x=0.56$. The evolution of magnetism has been studied as a function of cobalt content and is summarized in the temperature-composition phase diagram.