We report experimental evidence for the charge-orbital ordering in magnetite below the Verwey transition temperature T(V). Measurements of O K-edge resonant x-ray scattering on magnetite reveal that the O 2p states in the vicinity of the Fermi level exhibit a charge-orbital ordering along the c axis with a spatial periodicity of the doubled lattice parameter of the undistorted cubic phase. Such a charge-orbital ordering vanishes abruptly above T(V) and exhibits a thermal hysteresis, correlating closely with the Verwey transition in magnetite.

The temperature dependence of the resistivity of ferromagnetic GaMnAs, as-grown or lowtemperature-annealed samples is measured from 2 to 290 K. A resistivity minimum is observed with a corresponding temperature T M around 10 K for each sample. Below T M , the resistivity exhibits logarithmic temperature dependence, as ␣ ln͑T͒, and ␣ is independent of the external magnetic field up to 9 T. Such behavior is explained in terms of the Kondo effect arising from the presence of Mn interstitials in the GaMnAs samples. In addition, a well-defined T-squared dependence of resistivity is found in the temperature range between T M and the Curie temperature ͑T C ͒, which is attributed to single magnon scattering.

No abstract

No abstract