1/f noise in the half-metallic oxides CrO2, Fe3O4, and La2/3Sr1/3MnO3

The excess low-frequency (1/f ) electrical noise of three representative half-metallic oxides, CrO2, Fe3O4, and La2/3Sr1/3MnO3, has been studied as a function of the temperature ~5–300 K! and a magnetic field ~up to 12 T!. In CrO2 and La2/3Sr1/3MnO3, the resistance fluctuations are investigated in the metallic regime, below Tc . The magnetite noise is measured across the Verwey transition, in the hopping regime. We find, in all these oxides, a high normalized noise level which roughly follows the temperature dependence of the resistivity. For CrO2 and La2/3Sr1/3MnO3, the form of the density of state of the responsible excitations is inferred. Oxygen displacement around grain boundaries, magnetic fluctuations, and reversed-spin electronic excitations in the band structure are discussed as possible explanations of the low-frequency fluctuations. For Fe3O4, the 1/f noise and its temperature dependence are attributed to fluctuations of the number of carriers in a critical network. @S0163-1829~99!04219-8#