Antiferromagnetic Resonance in Copper Formate Tetrahydrate

Abstract

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 respectively the easy and intermediate axes for the spins. However, the following differences from the behavior of a normal antiferromagnet are observed: (1) Along the z direction, the high and lowfield branches intersect at a nonzero frequency. (2) Along the b axis a discontinuity is observed at 5.3 kOe at which a weak signal is observed at various frequencies between 9 and 72 GHz; (3) Above 5.3 kOe, an additional resonance is observed in the bc′ plane. We have attempted to fit the data with a two-sublattice model with z as the easy axis and weak moment along the b axis. The model contains isotropic exchange, symmetric and antisymmetric exchange, and g-tensor anisotropy. Good agreement between the calculation and the data is obtained for the low-frequency mode along the z and b axes above 5.3 kOe. From this fit and the static measurements2,3 consistent values of the exchange parameters are obtained. However, below 5.3 kOe the calculated resonance frequencies are considerably lower than the experimental ones. Furthermore, this two-sublattice model is inadequate to explain the discontinuity along the b axis and the additional resonance in the bc′ plane. A detailed paper will be published elsewhere.