Evaluation of Exchange Interactions of the Spin Doughnut Mo₇₅V₂₀

Abstract

The magnetic properties of the spin doughnut Mo₇₅V₂₀, which is a frustrated magnetic cluster with 20 spins, are studied by using the finite-temperature Lanczos method. Our model Hamiltonian consists of one ten-spin ring and two five-spin rings with nearest-neighbor antiferromagnetic couplings J and J^′′. The five-spin rings are coupled to the ten-spin ring by an antiferromagnetic exchange interaction J′ so as to form isosceles triangles of one J-bond and two J^′-bonds. By fitting the theoretical magnetic susceptibility with the experimental one measured by Müller et al., the exchange parameters are estimated as J=388 K, J^′=163 K, and J^′′=81 K. The most interesting feature of this system is that the two five-spin rings behave as two free spins at low temperatures, although J^′>J^′′. We also study the temperature dependence of the specific heat and find three peaks at T∼J, J^′′, and J^′⁄100.