Polarons In Magnetic Semiconductors Research Articles

Giant negative magnetoresistance of spin polarons in magnetic semiconductors–chromium-doped Ti2O3 thin films

Epitaxial Cr-doped Ti2O3 films show giant negative magnetoresistance up to −365% at 2K. The resistivity of the doped samples follows the behavior expected of spin (magnetic) polarons at low temperature. Namely, ρ=ρ0exp(T0∕T)p, where p=0.5 in zero field. A large applied field quenches the spin polarons and p is reduced to 0.25 expected for lattice polarons. The formation of spin polarons is an indication of strong exchange coupling between the magnetic ions and holes in the system.

The Influence of Local Magnetization on Transport in Cd0.91Mn0.09Te:In

AbstractWe describe the influence of local magnetization on electron localization and transport properties on the insulating side of the metal insulator transition in the dilute magnetic persistent photoconductor Cd0.091Mn0.09Te:ln. Measurements of both the temperature dependence of the transport properties, and also the dielectric constant, are reported for just one sample in which the carrier concentration n was varied by photodoping. From these results we are able to extract the carrier concentration dependence of the localization length and the permitivity of the electrons. We also report onl a new transport effect which occurs at ultra low temperatures and/or carrier concentrations very close to the metal insulator transition. We find that this mechanism is totally magnetic in origin and are able to explain it in terms of the well devewloped ideas of magnetic polarons in magnetic semiconductors.

Kohn-Sham density-functional approach to dense magnetic polarons in magnetic semiconductors.

Kohn-Sham density-functional approach to dense magnetic polarons in magnetic semiconductors.

Magnetic Polarons in EuO

AbstractThe existence of spin‐polarized complexes or magnetic polarons in magnetic semiconductors has been suggested theoretically some time ago, but up to now there is little experimental evidence of this phenomenon. With this in view, electron paramagnetic resonance is investigated (g‐factor and linewidth temperature coefficient) and also several other electrical and magnetic parameters of EuO‐based magnetic semiconductors with various donor concentrations (oxygen vacancies or Gd3+ ions substituting Eu2+ ions). The results of the investigation show that in a limited region of donor concentrations and temperatures there occurs a localization of charge carriers which may be attributed to the formation of magnetic polarons.

Thermoelectric power of small polarons in magnetic semiconductors

The thermoelectric power (Seebeck coefficient) $\ensuremath{\alpha}$ of a small polaron in both ferromagnetic and antiferromagnetic semiconductors and insulators is calculated for the first time. In particular, we obtain the contribution to the Seebeck coefficient arising from exchange interactions between the severely localized carrier (i.e., small polaron) of charge $q$ and the spins of the host lattice. In essence, we study the heat transported along with a carrier. This heat, the Peltier heat, II, is related to the Seebeck coefficient by the Kelvin relation: $\ensuremath{\Pi}=qT\ensuremath{\alpha}$, where $T$ is the temperature. The heat per carrier is simply the product of the temperature and the change of the entropy of the system when a small polaron is added to it. The magnetic contribution to the Seebeck coefficient is therefore directly related to the change of the magnetic entropy of the system upon introduction of a charge carrier. We explicitly treat the intrasite and intersite exchange interactions between a small polaron and the spins of a spin-\textonehalf{} system. These magnetic interactions produce two competing contributions to the Seebeck coefficient. First, adding the carrier tends to provide extra spin freedom (e.g., spin up or spin down of the carrier). This effect augments the entropy of the system, thereby producing a positive contribution to the Peltier heat. Second, however, the additional exchange between the carrier and the sites about it enhances the exchange binding among these sites. This generally reduces the energetically allowable spin configurations. The concomitant reduction of the system's entropy provides a negative contribution to the Peltier heat. At the highest of temperatures, when $\mathrm{kT}$ exceeds the intrasite exchange energy, the first effect dominates. Then, the Peltier heat is simply augmented by $\mathrm{kT}$ ln2. Alternatively, at temperatures well below the magnetic transition temperature, the second effect dominates. The Peltier heat then garners a negative contribution. In the experimentally accessible range between these limits, both effects are comparable. There the magnetic contribution to the Seebeck coefficient is generally sizable, ~ 100 \ensuremath{\mu}V/K. Furthermore, this magnetic contribution to the Seebeck coefficient is distinguished from the usual nonmagnetic contribution by its temperature dependence; it rises with temperature. Thus, the exchange interactions between a small polaron and its magnetic environment produce a significant and distinctive contribution to the carrier's Seebeck coefficient.