Microscopic spin Hamiltonian for a dipolar Heisenberg magnet LiGdF$_4$ from EPR measurements

Low-temperature electron paramagnetic resonance measurements are performed on single crystals of LiY$_{1-x}$Gd$_x$F$_4$ with low $x = 0.005$ and moderate $x = 0.05$ concentration of Gd ions. Modeling of the experimental spectra allows us to precisely determine microscopic parameters of the spin Hamiltonian of the parent LiGdF$_4$ material, including the nearest-neighbor exchange constant. The obtained parameters are further tested by comparing a strongly anisotropic Curie–Weiss temperature obtained for LiGdF$_4$ in our static magnetization measurements with theoretically computed values. We find a fine balance between principal magnetic interactions in LiGdF$_4$, which results in a hidden magnetic frustration presumably leading to a delayed magnetic ordering and an enhanced magnetocaloric effect at low temperatures.