Abstract
The magnetoresistance and the noise power of non-metallic phase-separatedmanganites are studied. The material is modelled by a system of small ferromagneticmetallic droplets (magnetic polarons or ferrons) in an insulating matrix. Theconcentration of metallic phase is assumed to be far from the percolation threshold.The electron tunnelling between ferrons causes the charge transfer in such asystem. The magnetoresistance is determined both by the increase in the volumeof the metallic phase and by the change in the electron hopping probability. In theframework of such a model, the low-field magnetoresistance is proportional toH2 and decreases withtemperature as T−n,where ncan vary from 1 to 5, depending on the parameters of the system. In the high-fieldlimit, the tunnelling magnetoresistance grows exponentially. Different mechanismsof the voltage fluctuations in the system are analysed. The noise spectrumgenerated by the fluctuations of the number of droplets with extra electrons has a1/fform over a wide frequency range. In the case of strong magnetic anisotropy, the1/fnoise can also arise due to fluctuations of the magnetic moments of ferrons. The1/f noisepower depends only slightly on the magnetic field in the low field range whereas it canincrease as H6in the high-field limit.
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