Magnetic properties of the coupled ladder systemMgV2O5

We present magnetic-susceptibility measurements on ${\mathrm{MgV}}_2{\mathrm{O}}_5$, a compound in which the vanadium oxide planes have the same topology as in ${\mathrm{CaV}}_2{\mathrm{O}}_5$. The most striking property is that there is an energy gap of about 15 K, much smaller than in ${\mathrm{CaV}}_2{\mathrm{O}}_5$, where the values reported are of the order of 500 K. We show that this may be understood naturally in terms of the phase diagram of the frustrated coupled ladder system. This analysis leads to the prediction that ${\mathrm{MgV}}_2{\mathrm{O}}_5$ should have incommensurate dynamic spin fluctuations.