Magnetic ordering of the linear chain system manganese oxalate dihydrate investigated by means of neutron diffraction and bulk magnetic measurements

Neutron diffraction experiments were carried out in the temperature range from 1.5 to 295 K on a polycrystalline sample of manganese(II) oxalate dihydrate MnC 2 O 4 .2D 2 O, a system composed of linear chains of oxalato-metal groups. In the course of this study the monoclinic space group C2/c has been confirmed and structural parameters have been refined, using the Rietveld profile-fitting procedure. An appreciable change ( approximately=2%) of the lattice constant a in the temperature range from 100 to 295 K has been observed. At 1.5 K the neutron diagram reveals a group of weak and closely spaced Bragg reflections arising from the magnetic ordering of Mn 2+ ions. Indexed on the basis of a magnetic unit cell with lattice constants a m =6a, b m =2b and c m =c the observed structure is best described as an antiferromagnetic cone spiral, which the cone axis perpendicular to the b-c plane and the value of the magnetic moment equal to (3.7+or-0.4) mu B . The resulting fundamental and spiral components are equal to (1.6+or-0.1) mu B and (3.4+or-0.4) mu B , respectively. The temperature dependence of the magnetic reflection 511 yields T N =2.60+or-0.05 K. On the other hand, the temperature dependence of bulk magnetic susceptibility exhibits a rounded maximum in the vicinity of 10 K. A consistent description of the magnetic behaviour of MnC 2 O 4 .2D 2 O at low temperatures has been achieved by assuming that intra-chain interactions between magnetic ions are much stronger than the inter-chain couplings. The fit of the magnetic susceptibility based on Fisher's results (1964) for a linear Heisenberg chain yielded J/k B =-1.22+or-0.01 K.