The metal-insulator transition temperature Tc in VO2 is experimentally shown to be almost the same as a magnetic transition temperature Tm characterized by an abrupt decrease in susceptibility, suggesting the evidence of the same underlying origin for both transitions. The measurement of susceptibility shows that it weakly increases on cooling for temperature range of T > Tm, sharply decreases near Tm and then unusually increases on further cooling. A theoretical approach for such unusual observations in susceptibility near Tm or below is performed by modeling electrons from each two adjacent V4+ ions distributed along V-chains as a two-electron system, which indicates that the spin exchange between electrons could cause a level splitting into a singlet (S = 0) level of lower energy and a triplet (S = 1) level of higher energy. The observed abrupt decrease in susceptibility near Tm is explained to be due to that the sample enters the singlet state in which two electrons from adjacent V4+ ions are paired into dimers in spin antiparallel. By considering paramagnetic contribution of unpaired electrons created by the thermal activation from singlet to triplet levels, an expression for susceptibility is proposed to quantitatively explain the unusual temperature-dependent susceptibility observed at low temperatures. Based on the approach to magnetic features, the observed metal-insulator transition is explained to be due to a transition from high-temperature Pauli paramagnetic metallic state of V4+ions to low-temperature dimerized state of strong electronic localization.

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Experiments for vanadium dioxide show a magnetic transition characterized by a sharp variation in susceptibility associated with the metal-insulator transition at the temperature Tc. The sample is shown to be of Pauli paramagnetism above Tc but of anomalous paramagnetism below Tc. Considering co-contributions of Pauli paramagnetism of residual V4+ ions and Curie-like paramagnetism of V-V dimers, we propose a phenomenological expression for the temperature-dependent susceptibility below Tc, which yields an excellent agreement with the experimental data. The unusual temperature dependence of Curie-like susceptibility below Tc is explained to be due to the fact that every dimer is formed by spin paring at an angle θ close to 180° and hence has an effective spin expressed as Seff=cos(θ/2) which decreases with lowering temperature. The observed metal-insulator and magnetic transitions are argued to be due to a transition from the high-temperature Pauli paramagnetic state of V4+ ions to the low-temperature dimerized state.

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