Temperature dependence of the sublattice magnetization in the quasi-two-dimensional S=1/2 Heisenberg antiferromagnet La₂CuO₄

We report the measurement of the Cu nuclear magnetic resonance (NMR) spectra up to high temperatures (160 K) in the quasi-two-dimensional spin-1/2 Heisenberg antiferromagnet La₂CuO₄ (T_N=312 K). The normalized NMR frequency, which is directly proportional to the decrease of the sublattice magnetization, follows the power law AT^α with A=(6.9±0.5)×10^-6 and α=(2.00±0.01) below about 100 K, while it is slightly suppressed above 100 K. The sublattice magnetization has been calculated numerically using a linear-spin-wave model. Good agreement with the experimental data is obtained by using the following coupling constants which were reported previously: The intraplane exchange coupling J=1590 K, the intraplane x-y symmetric exchange coupling J_xy/J=1.5×10^-4, and the interplane exchange coupling J^′/J=5×10^-5. In addition, we have determined the quadrupole frequency, the angle between the internal magnetic field, and the axis of the maximum component of the electric field gradient tensor, and this tensor’s asymmetry parameter.