B. Taliashvili

Ferromagnetic Transition in Ge_{1-x}Mn_{x}Te Layers

Ferromagnetic transition temperature in thin layers of diluted magnetic (semimagnetic) semiconductor Ge1−xMnxTe was studied experimentally by SQUID magnetometry method and analyzed theoretically for a model Ising-type diluted magnetic system with Ruderman–Kittel–Kasuya–Yosida indirect exchange interaction. The key features of the experimentally observed dependence of the Curie temperature on Mn content (x ≤ 0.12) and conducting hole concentration p = (1–10)× 10 cm−3 were reproduced theoretically for realistic valence band and crystal lattice parameters of p-Ge1−xMnxTe taking into account short carrier mean free path encountered in this material and Ruderman–Kittel–Kasuya–Yosida mechanism with both delta-like and diffused character of spatial dependence of the exchange coupling between magnetic ions and free carriers.

Epitaxial Growth and Characterization of PbGeEuTe Layers

The structural and electrical properties of Pb1−yGeyTe and Pb1−x−yGeyEuxTe (0≤y≤0.4 and x≤0.05) monocrystalline layers grown by molecular beam epitaxy technique on BaF2 (111) substrate were studied by X-ray diffraction, Hall effect, and electrical conductivity measurements. Based on the temperature dependence of the lattice parameter the structural (ferroelectric) transition temperature was found in the temperature range before 100 to 250 K in layers with varying Ge and Eu content. Electrical measurements indicates that incorporation of Eu ions in the PbGeTe crystal matrix decreases the electrical conductivity in p-type PbGeEuTe layers by 1–2 orders of magnitude.

Electric and thermoelectric properties of CdTe/PbTe epitaxial nanocomposite

The electric and thermoelectric properties of novel, CdTe/PbTe layered nanocomposite material are investigated. The molecular beam epitaxy (MBE) method was used for preparation of samples with well controlled distances (from 20 to 70 nm) between the layers of CdTe nanograins embedded in PbTe thermoelectric matrix as well as with number of these layers from 2 to 10. The Hall effect measurements performed in temperature range from 4–300 K revealed that carrier mobility is strongly affected by scattering on CdTe grain boundaries. The observation of Shubnikov-de Haas oscillations confirms high quality of the samples and allows determination of effective mass of conducting electrons m* = 0.04m0. The measurements of the room temperature Seebeck coefficient together with electrical conductivity lead to the power factors which are comparable to those reported in PbTe/CdTe polycrystalline solid solutions.

Magnetic and Structural Properties of Ferromagnetic GeMnTe Layers

Ferromagnetic Ge1−xMnxTe thin films with x≤0.19 were deposited on (111) oriented BaF2 monocrystals using molecular beam epitaxy technique. X-ray diffraction carried out at high temperatures for samples with x≤0.05 revealed ferroelectric transition from rock-salt to rhombohedral structure at T=625–675 K. The magnetic properties investigated with SQUID magnetometry and ferromagnetic resonance technique exhibit an easy magnetization direction normal to the plane in as grown samples. We attribute this finding to lattice strain due to mismatch of thermal expansion coefficients or to the crystalline stress related to inhomogeneous distribution of Mn ions in the sample volume. Thermal treatment changes the easy axis into in-plane direction which can be associated with distinct improvement of the structural properties.