Type: Article
Publication Date: 2015-10-07
Citations: 11
DOI: https://doi.org/10.1103/physrevb.92.134406
The magnetic ground state of two isostructural coordination polymers, (i) the quasi-two-dimensional $S=1/2$ square-lattice antiferromagnet $[\mathrm{Cu}({\mathrm{HF}}_{2})({\mathrm{pyrazine})}_{2}]{\mathrm{SbF}}_{6}$ and (ii) a related compound $[\mathrm{Co}({\mathrm{HF}}_{2})({\mathrm{pyrazine})}_{2}]{\mathrm{SbF}}_{6}$, was examined with neutron powder diffraction measurements. We find that the ordered moments of the Heisenberg $S=1/2$ Cu(II) ions in $[\mathrm{Cu}({\mathrm{HF}}_{2})({\mathrm{pyrazine})}_{2}]{\mathrm{SbF}}_{6}$ are $0.6(1){\ensuremath{\mu}}_{b}$, while the ordered moments for the Co(II) ions in $[\mathrm{Co}({\mathrm{HF}}_{2})({\mathrm{pyrazine})}_{2}]{\mathrm{SbF}}_{6}$ are $3.02(6){\ensuremath{\mu}}_{b}$. For Cu(II), this reduced moment indicates the presence of quantum fluctuations below the ordering temperature. We show from heat capacity and electron spin resonance measurements that due to the crystal electric field splitting of the $S=3/2$ Co(II) ions in $[\mathrm{Co}({\mathrm{HF}}_{2})({\mathrm{pyrazine})}_{2}]{\mathrm{SbF}}_{6}$, this isostructual polymer also behaves as an effective spin-half magnet at low temperatures. The Co moments in $[\mathrm{Co}({\mathrm{HF}}_{2})({\mathrm{pyrazine})}_{2}]{\mathrm{SbF}}_{6}$ show strong easy-axis anisotropy, neutron diffraction data, which do not support the presence of quantum fluctuations in the ground state, and heat capacity data, which are consistent with 2D or close to 3D spatial exchange anisotropy.