Magnetic phase diagram of the two-dimensional antiferromagnetNi5(TeO3)4Br2

Abstract

Phase diagram of the two-dimensional antiferromagnet ${\text{Ni}}_{5}{({\text{TeO}}_{3})}_{4}{\text{Br}}_{2}$ with triangular arrangement of ${\text{Ni}}^{2+}$ $(S=1)$ magnetic moments within the $[{\text{Ni}}_{5}{\text{O}}_{17}{\text{Br}}_{2}]$ subunits has been investigated by temperature and magnetic field dependent heat-capacity, magnetization, and magnetic-torque measurements down to 1.5 K and up to 23 T. A nonzero magnetic contribution to the heat capacity observed up to $2.3{T}_{N}$ is consistent with short-range magnetic ordering and the two-dimensional nature of the system. Below the N\'eel temperature ${T}_{N}=29\text{ }\text{K}$ several antiferromagnetic phases were identified. The zero-field phase is characterized by a planar antiferromagnetic arrangement of the two in-layer neighboring $[{\text{Ni}}_{5}{\text{O}}_{17}{\text{Br}}_{2}]$ magnetic clusters within the magnetic unit cell. When the magnetic field is applied along the ${a}^{\ensuremath{\ast}}$ crystal axis, a spin-flop-like transition to a phase with a complex out-of-plane arrangement of ${\text{Ni}}^{2+}$ $(S=1)$ magnetic moments occurs at $\ensuremath{\sim}10\text{ }\text{T}$. Using a molecular-field approach we predict that this transition will shift to higher fields with increasing temperature and that a magnetic phase with ferromagnetic ordering of $[{\text{Ni}}_{5}{\text{O}}_{17}{\text{Br}}_{2}]$ magnetic clusters will occur above 24 T. We ascribe the richness of the magnetic phases to strongly exchange-coupled clusters, being the basic building blocks of the investigated layered system.