Ferromagnetism and spin-dependent transport in n -type Mn-doped bismuth telluride thin films

We describe a detailed study of the structural, magnetic, and magnetotransport properties of single-crystal, n-type, Mn-doped bismuth telluride thin films grown by molecular beam epitaxy. With increasing Mn concentration, the crystal structure changes from the tetradymite structure of the Bi2Te3 parent crystal at low Mn concentrations towards a BiTe phase in the (Bi2Te3)m(Bi2)n homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temperature in the range 13.8-17 K in films with ∼2%-∼10% Mn concentration. Magnetization hysteresis loops reveal that the magnetic easy axis is along the c axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68-nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain-wall scattering. Ordinary Hall effect measurements show that the carrier density is n type, increases with Mn doping, and is high enough (≥2.8×1013 cm-2) to place the chemical potential in the conduction band. Thus the observed ferromagnetism is likely associated with both bulk and surface states. Surprisingly, the Curie temperature does not show any clear dependence on the carrier density but does increase with Mn concentration. Our results suggest that the ferromagnetism probed in these Mn-doped bismuth telluride films is not mediated by carriers in the conduction band or in an impurity band.