Antiferromagnetic Resonance in Copper Formate Tetrahydrate

It has been suggested 1–3 that monoclinic Cu (HCOO)2·4H2O may closely approximate a two‐dimensional antiferromagnetic. Magnetic susceptibility and magnetization measurements have indicated that antiferromagnetic ordering occurs at 17°K and have also shown evidence for weak ferromagnetism.2,3 An additional broad peak in the susceptibility has been observed near 65°K.3,4 Recent ESR measurements5 have shown a unique linear temperature dependence of the paramagnetic linewidth. In order to obtain more information on the magnetic parameters characterizing this crystal, we have made AFMR measurements between 1.4° and 15°K. Resonance transitions have been observed at 9 GHz, from 32 to 36 GHz, from 50 to 80 GHz, and from 107 to 124 GHz with magnetic fields up to 26 kOe. The minimum resonance field for the low‐frequency mode in the ac plane is observed at a position (called the z axis) 3° from the c axis. The AFMR modes (frequency vs magnetic field) for the field along the z and b axis suggest that z and b are resp...