Origin of Soft Spin Structure in Mn Formate Di-Urea

Proton NMR measurements at low temperature show that Mn moment direction in Mn formate di-urea easily changes by temperature as well as the replacement of hydrogen with deuterium atom in urea. The change in the moment direction is due to the competition of the magnetic anisotropy caused by the dipole interaction with the anisotropy caused by the surrounding ligand ions. The observed spin structure of four-fold screw rotations of antiferromagnetic layers is well understood by very small interlayer exchange interactions and magnetic anisotropies. The possibility is pointed out that the dipole interaction between the first nearest neighboring layers is the important origin of the transition to the three-dimensional long range order of this compound.