Thermodynamic properties of a layered S $=$ 7/2 Heisenberg magnet Gd(OH)CO$_{3}$

Thermodynamic quantities and ESR spectra of Gd(OH)CO$_{3}$ (I) are reported. The material may be considered to consist of weakly coupled layers with potentially triangular arrangement of exchange paths within each layer. Different bridging groups and distances among Gd$^{3+}$ ions may be responsible for spatial anisotropy of magnetic coupling. Preliminary analysis of magnetic susceptibility using Curie-Weiss law yielded $\theta =$-1.05 K indicating weak antiferromagnetic coupling and consequently, spin frustration in (I). More detailed simultaneous analysis of specific heat, susceptibility and magnetization studied down to nominally 0.45 K revealed non-negligible role of single-ion anisotropy. Using the model of weakly interacting S$=$7/2 trimers, the gross features of measured data may be explained while assuming single-ion anisotropy $D/k_{B}\approx $0.6 K and effective \textit{intra}trimer magnetic coupling \textbar $J/k_{B}$\textbar $\approx $0.3 K. The obtained $D$ value reasonably reproduces the position and shape of ESR line. The performed analysis suggests that magnetism in (I) is governed predominantly by crystal field effects and frustration plays a minor role.