High-Temperature Magnetic Correlations in the 2DS=1/2Antiferromagnet Copper Formate Tetradeuterate

The temperature dependence of the structure factor $S\left(k\right)$ of the instantaneous magnetic fluctuations in copper formate tetradeuterate has been investigated using neutron scattering techniques. Our data extend to unprecedently high temperatures ( $T\sim J$) compared to the 2D exchange coupling. The correlation length $\xi$ and amplitude $S_0$ are in good agreement with the predictions for a 2D $S=1\mathrm{}/2$ Heisenberg antiferromagnet on a square lattice.