Low-Temperature Magnetic Susceptibility and Heat Capacity of GdAsO4

We report measurements obtained in zero applied magnetic field of the magnetic susceptibility and heat capacity of GdAs${\mathrm{O}}_4$ in the temperature range 0.3-20 K. This material is antiferromagnetic below 1.262 K. A detailed comparison of the experimental results is made with those reported for the isomorphic compound GdV${\mathrm{O}}_4$. The entropy and energy of GdAs${\mathrm{O}}_4$ above and below the Néel temperature are in close agreement with the predictions of the Heisenberg model suggesting that the interactions are not only isotropic, but predominantly with the nearest neighbors as well. The magnetic measurements, on the other hand, indicate considerable anisotropy such that GdAs${\mathrm{O}}_4$ does not undergo a spin-flop transition. Attempts to explain this discrepancy are given. We have derived a value of the exchange-interaction constant $\frac{J}{k}\approx -0.04$ K, assuming a coordination number of 4 where $J$ is defined by the Hamiltonian $\mathcal{H}=-2\mathrm{}\Sigma \stackrel{}{i<j}{J}_{\mathrm{ij}}{\overrightarrow{\mathrm{S}}}_i·{\overrightarrow{\mathrm{S}}}_j-g{\mu }_B\Sigma \stackrel{}{i}{\overrightarrow{\mathrm{H}}}_0·{\overrightarrow{\mathrm{S}}}_i.$