Fragile magnetic ground state of a spin- 12 metal-organic kagome lattice

Abstract

We report thermodynamic and neutron diffraction measurements on a $S=1/2$ metal-organic kagome compound ${{[{\mathrm{Cu}}_{3}({\mathrm{CO}}_{3}){}_{2}{(\mathrm{bpe})}_{3}]\ifmmode\cdot\else\textperiodcentered\fi{}{2\mathrm{ClO}}_{4}}}_{n}$ [bpe=1,2-bis(4-pyridyl)ethane] with a Curie-Weiss temperature of $\ensuremath{\sim}\ensuremath{-}2.9\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The dc and ac magnetization measurements as well as neutron diffraction study show no long-range magnetic order down to 1.5 K. The temperature dependence of specific heat also shows no signature of long-range magnetic order down to 86 mK, as expected for an ideal Heisenberg kagome lattice with a nearest-neighbor antiferromagnetic exchange interaction. The frequency-independent flattening behavior observed in the real part of the ac susceptibility in a zero applied magnetic field below $\ensuremath{\sim}6$ K rules out the presence of a spin glass state and indicates a slowing down of the spin dynamics at low temperatures. Our results suggest that strong quantum fluctuations enhanced by geometrical frustration suppress the long-range magnetic order and a ground state with quasistatic short-range canted antiferromagnetic-type order is realized in the ${{[{\mathrm{Cu}}_{3}({\mathrm{CO}}_{3}){}_{2}{(\mathrm{bpe})}_{3}]\ifmmode\cdot\else\textperiodcentered\fi{}{2\mathrm{ClO}}_{4}}}_{n}$ [bpe=1,2-bis(4-pyridyl)ethane]. The ground state is fragile against external magnetic fields. An applied magnetic field of 125 Oe (and higher) induces ferromagneticlike spin-spin correlations.