Study of Spin Configuration in Mn Formate Di-Urea by Proton NMR

Antiferromagnetic spin arrangement of Mn formate di-urea is studied by proton NMR measurements below T N (3.78 K). Observed spectra are compared with possible spin arrangements. The antiferromagnetic moments within the layer point to the direction rotating 15 degrees around the c -axis from the middle direction between the [001]- and [\bar100]-directions. The layer including antiferromagnetic moments rotates following to the stacking of four-fold screw axis along the c -axis. The origin of this spin arrangement is discussed.