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

The neutron-scattering technique has been used to measure the staggered magnetization of the S = 1/2, d = 3 antiferromagnet CuCl2 x 2D2O as a function of temperature and magnetic field. The critical exponents β/sub perpendicular/, β/sub 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 β/sub perpendicular/ = 0.321 +- 0.01 and β/sub parallel/ = 0.324 +- 0.013. These values are within experimental error of the theoretically expected value of 0.325 for the d = 3 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 a axis, the direction of the staggered magnetization was found to rotate continuously from the a to the 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