Neutron Diffraction Investigation of the Spiral Magnetic Structure in Cr2BeO4

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Abstract
Cr2BeO4 is orthorhombic with the olivine (Mg2SiO4) type of crystal structure. Previous magnetic susceptibility, neutron diffraction and AFMR measurements have revealed that the compound undergoes an antiferromagnetic transition at about 28°K, probably to a noncollinear spin arrangement. More accurate neutron powder data have now been obtained at 4.5°K with 1.1 and 2.5 Å neutrons which reveal that the magnetic structure is a cycloidal spiral having a propagation vector directed along the c axis (Pbnm orientation). The spiral periodicity is about 65 Å, which is roughly 12 times the unit cell length in this direction. The observed intensities are accounted for reasonably well by a simple model in which there is modulation of a basic antiferromagnetic arrangement consisting of a G mode on the Cr(I) inversion sites coupled with a G mode on the Cr(II) mirror sites. The moments lie in the ac plane, and there is an additional phase factor of 90° between the moments of the Cr ions on the two kinds of site.

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