Magnetic and Thermal Properties of the Spin $S$~=~1/2 Zig-Zag Spin Chain Compound In$_2$VO$_5$ $^*$

The structure of In$_2$VO$_5$ consists of zig-zag V$^{4+}$ (spin $S$~=~1/2) chains along the $b$-axis. Prior to our work, there were two theoretical reports on this material.$^{1,2}$ One report suggested that the nearest-neighbor and next-nearest-neighbor interactions between the V$^{4+}$ moments would be anti-ferromagnetic and frustrated,$^1$ while the second report suggested that both these interactions should be ferromagnetic.$^2$ An experimental study of the physical properties of this material had not been reported. We measured magnetic susceptibility $\chi$, ac susceptibility $\chi_{\rm ac}$ and specific heat $C$ versus temperature $T$ on In$_2$VO$_5$ and $\chi$ and $C$ versus $T$ on the isostructural, nonmagnetic compound In$_2$TiO$_5$. The $\chi(T)$ data for In$_2$VO$_5$ showed that the dominant magnetic exchange between the V$^{4+}$ moments was ferromagnetic above 150~K\@. However, the $\chi(T)$ and the frequency dependence of the $\chi_{\rm ac}(T)$ data indicate that below 3~K the system is in a spin-glass state indicating the presence of disorder and frustrated interactions at these temperatures. Our $C$ and entropy $S$ data suggest that there may be a structural change below 140~K in In$_2$VO$_5$ which could possibly change the interactions between the V$^{4+}$ moments.\\ 1. I. M. Volkova, J. Phys.: Condens. Matter {\bf 19}, 176208 (2007).\\ 2. U. Schwingenschlogl, Phys. Rev. B {\bf 75}, 212408 (2007).\\ $^*$Supported by the USDOE under Contract No.\ DE-AC02-07CH11358.