Triplet dimerization crossover driven by magnetic frustration inIn2VO5

Abstract

${\mathrm{In}}_{2}\mathrm{V}{\mathrm{O}}_{5}$, containing magnetically frustrated zigzag chains, shows a remarkable crossover at $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ between paramagnetic states with positive $(17\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and negative $(\ensuremath{-}70\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ Weiss temperatures. Magnetic moment and entropy data show that the ${\mathrm{V}}^{4+}$ $S=1∕2$ spins condense into $S=1$ triplet dimers below the crossover. A further freezing of the antiferromagnetically coupled triplet dimers into a global singlet state is observed at $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with no long range magnetic order down to $0.42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in fields up to $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. No structural V-V dimerization is observed by high-resolution synchrotron x-ray diffraction down to $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but a subtle lattice anomaly evidences a spin-lattice coupling in the triplet dimer state. This is assigned to longitudinal oxygen displacement modes that reduce frustration within the chains and so couple to the spin dimer fluctuations.