Neutron-diffraction study of the staggered magnetization of CuCl2·2D2O

The neutron-scattering technique has been used to measure the staggered magnetization of the $S=\frac{1}{2}$, $d=3\mathrm{}$ antiferromagnet Cu${\mathrm{Cl}}_2$·2${\mathrm{D}}_2$O as a function of temperature and magnetic field. The critical exponents ${\beta }_{\perp }$, ${\beta }_{\parallel }$ describing the temperature variation of the order parameter in the regions well above and well below the bicritical point, respectively, were found to be ${\beta }_{\perp }=0.321\mathrm{}\pm 0.01$ and ${\beta }_{\parallel }=0.324\mathrm{}\pm 0.013$. These values are within experimental error of the theoretically expected value of 0.325 for the $d=3\mathrm{}$ Ising model. At the spin-flop transition the staggered magnetization changed 8% in magnitude, the reduction being due to the anisotropy of the $g$ factor. The magnetic form factor in the spin-flop region was found to be the same as that previously determined in zero field. In addition, with the applied field tilted in the $a-b$ plane away from the $\overrightarrow{\mathrm{a}}$ axis, the direction of the staggered magnetization was found to rotate continuously from the $\overrightarrow{\mathrm{a}}$ to the $\overrightarrow{\mathrm{b}}$ direction as is expected theoretically. The observed range in field over which the rotation takes place, however, is narrower than the predictions of mean-field theory.