Spin-singlet dimerization in La2RuO5investigated using magnetic susceptibility and specific heat measurements

Abstract

The origin of spin-dimerization and concomitant spin-gap opening in the triclinic phase of poly- and single-crystalline La${}_{2}$RuO${}_{5}$ at unusually high temperatures was investigated using magnetic susceptibility and specific-heat measurements. From the low-temperature crystal structure the formation of antiferromagnetically coupled Ru${}^{4+}$ ($S=1$) dimers within the quasi-two-dimensional magnetic system can be deduced, resulting in a nonmagnetic singlet state. It was found that the antiferromagnetic coupling within the dimers is much stronger than the interaction with neighboring dimers. La${}_{2}$RuO${}_{5}$ exhibits a step-like change in the magnetic susceptibility at 161 K, indicating a first-order transition of combined magnetic and structural character. The size of the spin-gap has been estimated from the thermally activated behavior in the low-temperature dimerized phase and was found to be significantly different in the polycrystalline sample when compared to the results obtained from the single crystals. The magnetic entropy obtained from specific-heat measurements amounts to roughly $0.5R\mathrm{ln}(3)$, reflecting solely the contribution of spin degrees of freedom to the entropy change during the phase transition.