Neutron diffraction study of the magnetic order in magnetic coordination polymer CuF$_{2}$(H$_{2}$O)$_{2}$(pyz) (pyz=pyrazine)

The new linear chain coordination polymer CuF$_{2}$(H$_{2}$O)$_{2}$(pyz)(pyz=pyrazine) provides an interesting example for the study of low dimensional physics. CuF$_{2}$(H$_{2}$O)$_{2}$(pyz) has a monoclinic structure where the Cu$^{2+}$ ions form a 2 dimensional (2D) square lattice in the bc-plane. Short range magnetic order appears below 10 K followed by a transition to long range antiferromagnetic order with T$_{N} \quad \sim $ 2.6 K. To further understand the magnetic behavior of CuF$_{2}$(H$_{2}$O)(pyz), we have performed neutron diffraction experiments on deuterated single crystals CuF$_{2}$(D$_{2}$O)$_{2}$(d$_{4}$-pyz). Below 2.6 K we observe magnetic Bragg peaks which are consistent with the propagation vector (1/2 1 0). Refinement of the data shows that the magnetic moment lies in the ac-plane. Fitting the temperature dependence of the magnetic order parameter to a power-law form in the reduced temperature range of 1-T/T$_{N}$ = 0.01-0.4 yields a critical exponent, $\beta $, of 0.25$\pm $0.01. This result is consistent with the expectation for a 2D XY model where $\beta $=0.23 [1]. \\[4pt] [1] S. T. Bramwell and P. C. W. Holdsworth, J. Phys.: Condens. Matter. \textbf{5} L53-L59 (1993).