Magnetic, specific heat and electrical transport properties of oxygen-deficient nanosized rutile TiO_2−δ

An oxygen-deficient nanosized ${TiO}_{2-\delta }$, $\delta \sim 0.7$ sample was synthesized by a solvothermal method, and was characterized to have both ∼3 nm amorphous solid and ∼36–46 nm diameter rutile nanowires. Physical properties of the sample were investigated by measuring magnetic, specific heat, electrical resistance and magnetoresitance properties. DC magnetization M(H) data confirm ferromagnetic behavior previously reported for undoped TiO₂. Furthermore, M(T) dependence follows the power-law relation $M{\left(T\right)\propto (1-T/T_C)}^{\beta }$ in the near-critical regime, yielding Curie temperature $T_C\sim 415$ K and critical exponent β = 0.2. Moreover, our results of AC magnetic susceptibility measurements suggest an additional phase transition at $T^{\ast }\sim 310$ K, presumably due to spin orientation. The metallic-like electrical resistance exhibits a distinct drop below $T^{\ast }$ with a strong thermal hysteresis in the temperature range 225–275 K. Specific heat in the temperature range 20–300 K is well described by the sum of contributions from acoustic phonons with Debye temperature 605 K and optical phonons with Einstein temperature 113 K. Below 10 K the specific heat divulges a large excess, which can be interpreted as an additional contribution originating from soft potentials. (paper)