MAGNETIC-STRUCTURES OF ALPHA-FEMOO4 AND ALPHA-COMOO4

The crystallographic and magnetic structures of alpha-FeMoO4 and alpha -CoMoO4 were determined by elastic neutron scattering. Both compounds were prepared by subsolidus reaction and crystallize in space group C2 /m. For the first compound the magnetic unit cell has doubled b- and c -axes with respect to the crystallographic cell, i.e. k = (0, 1/2, 1/2 ) and the magnetic space group is P2'. The magnetic moments are collin ear within each sublattice for the two different crystallographic site s and aligned along [010] for the ions on screw axes and along [100] f or the other site. The absolute values at T = 1.5 K are \mu(Fe2+)\ = 2 .1(2)mu(B), and 2.0(2)mu(B),. In a specific heat measurement with a po wder sample of alpha-CoMoO4 two phase transitions at T-1 = 6 K and T-2 =12 K are observed. We have determined the low temperature phase with the magnetic cell doubled along the a- and c-axes, i.e. k = (1/2, 0, 1 /2) and the magnetic space group is Pm', The magnetic moments are coll inear aligned along [001] for both sites and the absolute values at T = 1.5 K are \mu(Co2+)\ = 1.50(20)mu(B) and 1.35(20)mu(B) for the two d ifferent crystallographic sites.