Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4

We report on magnetization and electron spin resonance (ESR) measurements of Sr${}_2$VO${}_4$ with orthorhombic symmetry. In this dimer system the $V^{4+}$ ions are in tetrahedral environment and are coupled by an antiferromagnetic intra-dimer exchange constant $J\mathrm{/}{k}_B\approx$ 100 K to form a singlet ground state without any phase transitions between room temperature and 2 K. Based on an extended-H"{u}ckel-Tight-Binding analysis we identify the strongest exchange interaction to occur between two inequivalent vanadium sites via two intermediate oxygen ions. The ESR absorption spectra can be well described by a single Lorentzian line with an effective g-factor $g$ = 1.89. The temperature dependence of the ESR intensity is well described by a dimer model in agreement with the magnetization data. The temperature dependence of the ESR linewidth can be modeled by a superposition of a linear increase with temperature with a slope $\alpha$ = 1.35 Oe/K and a thermally activated behavior with an activation energy $\mathrm{\Delta }\mathrm{/}{k}_B$ = 1418 K, both of which point to spin-phonon coupling as the dominant relaxation mechanism in this compound.