Successive antiferromagnetic transitions with multi-kand noncoplanar spin order, spin fluctuations, and field-induced phases in deformed pyrochlore compoundCo2(

Structure and magnetic properties of the rhombohedral-structure compound ${\text{Co}}_{2}{(\text{OH})}_{3}\text{Br}$, a member of the geometrically frustrated series of the compounds ${M}_{2}{(\text{OH})}_{3}X$ where the magnetic ions form a deformed pyrochlore lattice, were studied using dc and ac magnetic susceptibilities, heat-capacity, neutron powder-diffraction, and muon-spin-rotation/-relaxation $(\ensuremath{\mu}\text{SR})$ measurements. The structure of ${\text{Co}}_{2}{(\text{OH})}_{3}\text{Br}$ is featured by alternatively stacked layers of perfect kagome-lattice planes and triangular-lattice planes with a 10% distortion along the stacking direction (the $c$ axis). Despite a very small difference in the distortion [0.42% larger in ${\text{Co}}_{2}{(\text{OH})}_{3}\text{Br}$], ${\text{Co}}_{2}{(\text{OH})}_{3}\text{Br}$ was found to show contrasting antiferromagnetism that is strikingly different from the previously reported ferromagnetic ${\text{Co}}_{2}{(\text{OH})}_{3}\text{Cl}$. Successive antiferromagnetic transition was observed at ${T}_{\text{N}1}=6.2\text{ }\text{K}$ and ${T}_{\text{N}2}=4.8\text{ }\text{K}$, respectively. The antiferromagnetic ground state is metastable and an intermediate magnetic phase was induced by applying a relatively low magnetic field of $H\ensuremath{\sim}5\text{ }\text{kOe}$. When the field was further increased above $H\ensuremath{\sim}20\text{ }\text{kOe}$ spin reorientation occurred to form a configuration similar to ferromagnetic ${\text{Co}}_{2}{(\text{OH})}_{3}\text{Cl}$. The successive antiferromagnetic transitions in zero field were found to occur with propagation vector of ${\mathbit{k}}_{1}=(0\text{ }\ensuremath{-}1/2\text{ }1/2)$ at ${T}_{\text{N}1}$ and an additional ${\mathbit{k}}_{2}=(0\text{ }0\text{ }3/2)$ at ${T}_{\text{N}2}$. Refinement of the neutron powder-diffraction patterns revealed an unconventional multi-$\mathbit{k}$ and noncoplanar spin structure for the antiferromagnetic phases. Multiple measurements, in particular, the $\ensuremath{\mu}\text{SR}$ study, consistently demonstrated magnetic coupling at high temperatures, and persistent fluctuations well below the ${T}_{\text{N}}$. This work presents a unique system to investigate the orbital effect and the critical role of lattice distortion in geometric frustration, and provides a single material system to study multiple phase transitions and competing exchange interactions.