The alternating-sign magnetoresistance of polycrystalline manganese chalcogenide films

The correlation between the dc and ac electrical resistance and the structural, magnetic, and thermoelectric properties of polycrystalline MnSe_1−XTe_Х (0.3 ≤ X ≤ 0.4) manganese chalcogenide films in the temperature range of 80–400 K has been investigated. Inhomogeneous electronic states and transitions between them accompanied by structural lattice deformations have been found at temperatures including the Neel temperature region. The magnetic susceptibility maximum above the Neel temperature in a magnetic field of 8.6 kOe has been observed. Temperature ranges of the coexistence of two types of electrically inhomogeneous states have been established by impedance spectroscopy in the frequency range of 0.1–1000 kHz and the change of the hopping conductivity for diffusion one accompanied by the magnetic susceptibility minimum has been found. The magnetoresistance in magnetic fields of up to 12 kОе has been established. It has been revealed that the thermopower and magnetoresistance change its sign upon heating. The experimental data are explained using a spin polaron model with the localization of polarons and formation of the electron phase-separation. The alternation of magnetoresistance in sign is attributed to the ferromagnetic orbital ordering of electrons and the negative magnetoresistance is explained by suppression of spin fluctuations in a magnetic field. (paper)