Determination of exchange energies in the sawtooth spin ring {Mo${}_{75}$V${}_{20}$} by ESR

The magnetism of the polyoxometalate cluster {${\text{Mo}}_{75}{\text{V}}_{20}$}, containing a sawtooth ring of 10 corner-sharing triangles located on the equator of the barrel-shaped molecule, has remained debatable since it is masked by contributions from impurities as well as temperature-independent paramagnetism. In this paper, we demonstrate the usefulness of electron spin resonance (ESR) measurements, since the temperature dependence of the ESR intensity can discriminate between impurity and molecular contributions. We determine the exchange parameters and therefore also the low-lying spectrum of {${\text{Mo}}_{75}{\text{V}}_{20}$}, especially the low-lying singlet states which so far have been probed solely by specific-heat measurements.

Figure 1

Temperature dependence of $\chi T$ from Ref. 3. The inset shows a scheme of the metal skeleton of {${\text{Mo}}_{75}{\text{V}}_{20}$} highlighting the nearest-neighbor exchange pattern between the V${}^{4+}$ ions.

Figure 2

Temperature dependence of ESR spectra for 190 GHz. The very narrow peak observed around 6.8 T is a field marker. The inset shows the spectrum for 190 K. The solid and dashed lines are fitting curves for resonance $\alpha$ and $\beta$, respectively (see text for details).

Figure 3

Solid circles are the integrated intensities for resonance $\beta$ coming from paramagnetic impurities; they are fitted using Eq. (1) (thin solid curve). Open circles are the normalized integrated intensities for resonance $\alpha$ coming from the intrinsic {${\text{Mo}}_{75}{\text{V}}_{20}$}. Thick solid and dashed curves are calculated curves of the normalized integrated intensity for various values of $J_1$. The calculated curves are obtained from the singlet and triplet energy levels (inset), assuming an equilateral triangle model. The energy is normalized by $J_1$.

Figure 4

Normalized integrated intensities for resonance $\alpha$ coming from the intrinsic {${\text{Mo}}_{75}{\text{V}}_{20}$} for various ratios $J_2/J_1$. The respective exchange parameters are shown in the legend.

Figure 5

Bond lengths and angles for an individual triangle of {${\text{Mo}}_{75}{\text{V}}_{20}$}.