Cycloidal Spin Configuration of Orthorhombic MnSO4

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Abstract
The crystal and magnetic structures of antiferromagnetic MnSO4 (a=5.260 Å, b=8.042 Å, c=6.847 Å, space group Cmcm) have been determined from x‐ray and neutron diffraction data collected from polycrystalline samples at 300° and at 77° and 4.2°K, respectively. The neutron diffraction pattern at 4°K exhibits additional peaks of magnetic origin which are slightly displaced from the positions expected on the basis of the original unit cell, or any simple multiple cell. In addition, the observed extinction rules cannot be satisfied by any combination of simple collinear magnetic structures. Complete indexing of the magnetic peaks can be achieved only on the basis of a cone spiral arrangement related to a simple antiferromagnetic structure of CrVO4 type,1 in which ferromagnetic (001) sheets are coupled antiparallel to adjacent sheets. The propagation vector of the spiral is along the a axis with a periodicity of 30 Å (almost 6a). The spin plane of the spiral component is the a‐b plane. It is believed that this is the first case in which a spiral has been observed in an orthorhombic compound, and it is of particular interest that the spiral is cycloidal. Good agreement of observed and calculated magnetic intensities is obtained with a fundamental spin component of 1.7 μB directed along the c axis, and a spiral component of 4.5 μB in the a‐b plane. The resultant moment of each Mn2+ ion is therefore 4.8 μB, and the cone half‐angle is about 78°. The temperature dependence of the satellite peaks indicates a Néel point of about 10°K.

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