Abstract
Using X-band electron spin resonance (ESR) spectroscopy, we investigate the magnetic properties of thelow-dimensional quantum-spin systems VO(HPO4)·0.5H2O, KZn(H2O)(VO)2PO4)2(H2PO4), and CsV2O5, where the magnetic V4+(=1/2) ions are arranged in pairs. By studying the temperature dependence as well asthe angular dependence of the ESR spectra, we find that these systems, which may be well described as spin- 1/2dimer systems, show a change in behavior as a function of temperature. At room temperature, in accordancewith the Kubo-Tomita prediction, an exchange narrowing phenomenon takes place, whereas linewidth broadening and then the appearance of a fine structure are instead observed at helium temperatures where spins arestrongly correlated.