Exotic antiferromagnetic transition in deformed pyrochlore lattice Ni₂(OH)₃Cl of atacamite-structure

As a part of an extensive study on the new geometric frustration series M₂(OH)₃X on deformed pyrochlore lattice, we have found an exotic antiferromagnetic order in atacamite-structure Ni₂(OH)₃Cl. Strong frustration exists in Ni₂(OH)₃Cl with antiferromagnetic transition below T_N=4 K. While neutron diffraction witnessed unambiguously a long-range order, there is no signature of an order as detected by μSR. Instead, the detected local field is dynamically fluctuating at zero field. The present material may exemplify a novel type of antiferromagnetic order recently predicted for Heisenberg spins on pyrochlore lattice, wherein the squared Fourier amplitude orders but the amplitude itself fluctuates rapidly.