Neutron Diffraction Study of VF2

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Abstract
A neutron diffraction study of VF2 has been carried out at temperatures between 1.3° and 300°K with powdered and single‐crystal samples. Below its transition temperature, 7°K, associated with the development of the long‐range magnetic order, this rutile structured compound has a spiral spin structure in which the spins lie perpendicular to the tetragonal c axis and spiral along the c axis with a turn angle of 96.0°±0.5°. There is no detectable change in the magnitude of the turn angle between 4.2° and 1.3°K. Based on the magnetic susceptibility and magnetic anisotropy data of VF21 below 4.2°K and Yoshimori's model,2 we have calculated a value for the turn angle of VF2 98°, in good agreement with the neutron scattering result. The neutron diffraction data between 20° and 77°K confirm that there is a large spin correlation between the near neighbors. The parameter, u, designating the fluorine positions in the unit cell, ±(u, u, 0) and ±(½+u, ½−u, ½), was determined to be 0.308±0.001. Comparison of the scattering angles of the Bragg peaks indicates that the c axis shrinks about 0.5% while the a axis expands about 0.2% when the powdered sample is cooled from 300° to 4.2°K.

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