Magnetic ground state of the 2D S=1/2 square antiferromagnet copper formate tetrahydrate

I performed a series of NMR experiments to investigate the magnetic ground state properties of the 2D S = l/2 square antiferromagnet copper formate tetrahydrate. This material contains planes of copper atom s which are linked by formate groups and are arranged in a square lattice. I measured the NMR frequency shifts for the deuterium nuclei in the formate groups as a function of field orientation. At 10 K, the shifts depend on the orientation of the local spins at the Cu sites. The orientation of these spins is governed by the magnetic free energy which contains the anisotropic Heisenberg exchange, the Dzyaloshinskii-Moriya interaction and the anisotropic Zeeman coupling to the external field. I created a T = 0 classical model of the free energy and determined the orientation of the sublattice magnetizations for different strengths of the various interactions. By calculating the resulting NMR shifts and comparing them to the experimental data, I showed th a t both the Dzyaloshinskii-Moriya interaction and the anisotropic Zeeman coupling need to be included in the free energy in order to model the data successfully. This point has not been appreciated in the literature to date. Furthermore, I determined the various parameters needed to describe the magnetic behavior of copper formate tetrahydrate. The Heisenberg exchange coupling J is 4.0 ± 0 .4 meV or 4 7 ± 4 K and the anisotropies in this were found to be negligible. The average moment of the antiferromagnetically ordered spins is 0.571±0.004 (j,b and the two Dzyaloshinskii-Moriya vectors are D + = (—0.03 ± 0 .01,0.0 ,-0.18 ± 0.02) meV and D “ = (0.0,0.00 ± 0.06,0.0) meV.Ph.D