Neutron Diffraction Studies of Antiferromagnetism in CrF2and CrCl2

Abstract

Neutron diffraction observations have been made on powder samples of Cr${\mathrm{F}}_2$ and Cr${\mathrm{Cl}}_2$ from 298° to 4.2°K. These materials, which have crystal structures similar to rutile but distorted from tetragonality, become antiferromagnetic at low temperatures with different magnetic structures. For Cr${\mathrm{F}}_2$ the magnetic unit cell is identical with the chemical cell and the moments at the corner sites are directed oppositely to those at the center of the cell. The magnetic unit cell of Cr${\mathrm{Cl}}_2$ requires doubling of the $b$ and $c$ axes of the orthorhombic chemical cell and the structure consists of ferromagnetic (011) planes with adjacent planes antiparallel. In neither case were the intensities compatible with a magnetic axis directed along a simple crystallographic direction. Unique magnetic axes could not be definitely established but the data suggest that they lie parallel to the longest Cr-F and Cr-Cl bonds. Néel temperatures of 53° and 20°K were observed for Cr${\mathrm{F}}_2$ and Cr${\mathrm{Cl}}_2$, respectively.