Strongly disordered Heisenberg spin-1 chains: Vanadium warwickites

The structure of the compound MgVOBO3, is characterized by the presence of ribbons where the spin-1 V3+ ions are randomly located. We present X-ray, susceptibility chi(T) as a function of temperature and high-field magnetization measurements m(H) (up to 45 T) on powdered samples. These give evidence that in this system, with strong disorder on the magnetic chains, there are arbitrarily low-energy excitations which are responsible for the power-law dependence of chi(T) and m(H) that we observe. We find that the magnetic behaviour is reminiscent of the disordered spin-1/2 antiferromagnetic chain. Our theoretical results for the strongly disordered spin-1 antiferromagnetic Heisenberg chain substantiate the experimental observations.