High-field magnetization and electron spin resonance in the spin-gap systemη-Na1.286V2O5

Abstract

In this work, we report results of high-field magnetization and electron spin resonance (ESR) measurements performed on $\ensuremath{\eta}{\text{-Na}}_{1.286}{\text{V}}_{2}{\text{O}}_{5}$, a compound that shows a spin gap. This system may be regarded as an assembly of weakly interacting magnetic entities, each of which containing, at low temperature, 18 antiferromagnetically (AF)-coupled $S=\frac{1}{2}$ spins. The purpose of this work is to determine the gap value and to probe the low-lying energy levels in this compound. On the one hand, the high-field magnetization measurements, performed at temperatures down to 1.7 K on powder samples, suggest a spin gap, $\ensuremath{\Delta}$, of approximately 39 K, a value which is quite close to the earlier estimate of 35 K, which some of us deduced from the susceptibility measurements at low field [Duc et al., Phys. Rev. B 69, 094102 (2004)]. On the other hand, these measurements show a magnetization step at one ninth of the saturation magnetization, giving strong support to the assumption that this system may be regarded as a finite-size system composed of 18 AF-coupled $S=\frac{1}{2}$ spins. A theoretical fit of the magnetization indicates the necessity to include couplings to second or more distant neighbors. The ESR data at various frequencies (from 9.6--980 GHz) and at temperatures ranging from 4.2--150 K, obtained on powder samples as well as on single crystals, are in accordance with the gap value mentioned above. In addition, these ESR data indicate that the closure of the gap is not accompanied by any detectable mixing between the singlet level and the lowest-lying level of the first excited triplet. This implies the absence of any appreciable Dzyaloshinskii-Moriya interaction in this compound.