A number of publications, reporting ferromagnetism at low temperatures (about 10 K) in Bi₂Te₃ doped with manganese appeared recently. The work presented in this report was targeted at preparing a good quality Bi_2-XMn_XTe₃ crystals and studying the mechanisms of formation of ferromagnetism in this material. The samples were studied by means of x-ray diffraction, energy dispersive x-ray analysis (EDX), AC susceptibility measurements, magnetization measurements, electronic transport measurements, magnetoresistance measurements and positron annihilation lifetime spectroscopy.

Crystals containing up to x=0.02 of manganese were grown by Bridgman technique using material synthesized in advance as the ampule charge. The concentration of manganese was estimated from EDX, magnetic susceptibility and magnetization measurements. The obtained values of concentration are consistent. This and the analysis of x-ray diffraction data suggest that manganese is scattered in the Bi₂Te₃ matrix and Bi_2-XMn_XTe₃ is a true diluted magnetic semiconductor.

In heavily doped samples the ferromagnetic phase transition was observed. From the temperature dependence of AC magnetic susceptibility the temperature of the phase transition was estimated to be about 5.3 K. Comparison with other reports shows that the temperature of phase transition depends on manganese concentration. The hysteresis curves measured at 4.5 K have zero coercive field which is explained by closeness of the measurement temperature to Curie temperature.

The phase transition was also observed in electron transport measurements. An increase of resistivity, with the maximum at about 6 K, is observed in the ferromagnetic sample in the vicinity of the phase transition temperature. Such increase of resistivity, related with the increase of scattering, is typical for ferromagnetic materials. It is a further clue that manganese is randomly distributed in the crystal lattice. Anomalous Hall effect however was not observed in the ferromagnetic samples. Since a strong spin-orbit interaction is expected in bismuth telluride, this means that there is no spin polarization of the free carriers in the ferromagnetic state of the material.

The magnetoresistance was measured in the magnetic fields up to 13 T. The magnetoresistance of the ferromagnetic sample is linear above 2 T, reaches 30 % at 13 T and does not show signs of saturation both above and below phase transition temperature. The linear dependence of the magnetoresistance is typical for nonuniform materials in which the adjacent microscopic regions have slightly different properties. An additional component of magnetoresistance which appears at low temperatures and saturates at about 0.3 T is attributed to weak antilocalization phenomenon.

In positron annihilation lifetime spectroscopy two lifetime components of 250 ps and 480 ps were observed. The short lifetime component is attributed to annihilation events of the delocalized positrons in the bulk of the crystal. The 480 ps component should be related with clusters of vacancies or larger void structures. It may also be related with the layered structure of the Bi₂Te₃ crystal lattice.

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