Magnetic structures of successively ordered phases in a triangular-lattice antiferromagnetCsNiBr3

The magnetic structures of the ordered phases of a triangular-lattice Heisenberg antiferromagnet (AF) ${\mathrm{CsNiBr}}_3$ have been studied by NMR. The successive magnetic transitions at ${\mathit{T}}_{\mathit{N}1}$=14.06 K and ${\mathit{T}}_{\mathit{N}2}$=11.51 K are confirmed by the growing of the internal fields at Cs sites. Below ${\mathit{T}}_{\mathit{N}2}$ the ${\mathrm{Ni}}^{2+}$ moments form a triangular structure in a plane including the c axis, in which the spins in one of the AF chains are parallel to the c axis and the spins in the other two chains have canting angles of ±39°. When the external field is parallel to the c axis, the spin plane rotates around the c axis. The triangular spin structure has no freedom of rotation in the spin plane, in contrast with the case of the isomorphic ${\mathrm{CsNiCl}}_3$. This difference is attributed to the value of D/${\mathit{J}}_1$ of 2.1 for ${\mathrm{CsNiBr}}_3$, which is larger by 1 order of magnitude than that for ${\mathrm{CsNiCl}}_3$.