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Unusual magnetic transition near metal-insulator transition and paramagnetic anomaly in VO2

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 S e f f = 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-temper...