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Thermal conductivity of a 300-nm-thick VO 2 thin film and its temperature dependence across the metal-insulator phase transition (T MIT ) were studied using a pulsed light heating thermoreflectance technique. The VO 2 and Mo/VO 2 /Mo films with a VO 2 thickness of 300 nm were prepared on quartz glass substrates: the former was used for the characterization of electrical properties, and the latter was used for the thermal conductivity measurement. The VO 2 films were deposited by reactive rf magnetron sputtering using a V 2 O 3 target and an Ar-O 2 mixture gas at 645 K. The VO 2 films consisted of single phase VO 2 as confirmed by X-ray diffraction and electron beam diffraction. With increased temperature, the electrical resistivity of the VO 2 film decreased abruptly from 6.3 ' 10 %1 to 5.3 ' 10 %4 Ω cm across the T MIT of around 325-340 K. The thermal conductivity of the VO 2 film increased from 3.6 to 5.4 W m %1 K %1 across the T MIT . This discontinuity and temperature dependence of thermal conductivity can be explained by the phonon heat conduction and the Wiedemann-Franz law.

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Short-range antiferromagnetic (speromagnetic) behavior was observed for amorphous BiFeO3 by static magnetic measurements, ac susceptibility (χac) measurement, and 57Fe Mössbauer spectra measurements. The magnetic behavior is classified into three temperature ranges: (i) paramagnet for T≳220 K; (ii) local clustering of spins for 20 K<T<220 K; and (iii) speromagnet in which all spins are frozen into random directions for T<20 K. In the spin freezing state the magnetization shows irreversibility and the χac−T curve has a cusp. The variation of the spin freezing temperature against measuring magnetic field and measuring frequency indicate that the speromagnetic order is very similar to that of spin glass.

Electrical conductivity and powder X-ray studies have been made on (Sr1-αNa[α])CoO x after firing at 980°C in air. The data are explainable in terms of the mixed phase of SrCoO x (x∼2.5) and NaCo2O4. We found a distinct metallic conductivity in NaCo2O4; the resistivity decreases linearly to 12 K from room temperature. This compound may be another candidate mother oxide for future study of high-temperature superconductivity.

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