T. Koyanagi

Preparation of sintered degenerate n-type PbTe with a small grain size and its thermoelectric properties

Sintered degenerate n-type PbTe samples with small grain sizes ranging from 0.7 to 4 μm were prepared and the effects of grain size on their thermoelectric properties were then investigated. The Seebeck coefficient of the sintered samples increased almost double when the grain size decreased from 4 to 0.7 μm. On the other hand, their electrical and thermal conductivity decreased with decreasing grain size. Accordingly, decreasing their grain size increased their thermoelectric figure-of-merit. A maximum value of the figure-of-merit of the obtained small grain-size samples was significantly higher than that of large grain-size samples with the same carrier concentration reported. This favorable result was caused mainly by the increase in the Seebeck coefficient. The influences of grain boundaries on the increase in the Seebeck coefficient were discussed. It is concluded that the Seebeck coefficient was increased by potential barrier scattering, which occurred at the grain boundaries in the sintered samples.

Temperature dependence of the Seebeck coefficient and the potential barrier scattering of n-type PbTe films prepared on heated glass substrates by rf sputtering

Thermoelectric properties of polycrystalline degenerate n-type PbTe films have been investigated in order to understand potential barrier scattering. The Seebeck coefficients of the PbTe films obtained in this study were larger than those of bulk samples having the same carrier concentrations in the temperature range from 100 to 450 K. Some of their power factors were larger than those of bulk samples having the same carrier concentrations in the temperature range from 200 to 450 K, while their electrical conductivities were smaller than those of bulk samples. From a comparison of these results with those previously reported, we concluded that potential barrier scattering occurred at grain boundaries in our films, resulting in the above favorable changes in thermoelectric properties. By analyzing their properties using the energy filtering model, we estimated the height of grain-boundary potential barriers, which probably influenced the increases in the Seebeck coefficient. We also examined the origin of ...

Carrier-induced ferromagnetism in Ge0.92Mn0.08Te epilayers with a Curie temperature up to 190 K

IV-VI diluted magnetic semiconductor Ge0.92Mn0.08Te epilayers are grown on BaF2 substrates by molecular beam epitaxy. The ferromagnetic behaviors, such as the spontaneous magnetization, the coercive field, and the Curie temperature TC, are altered by the hole concentration p. In the Ge0.92Mn0.08Te layer with high p, strong magnetic anisotropy and the temperature dependence of the magnetization expected for homogeneous ferromagnets are observed, implying that long-range ordering is induced by the holes. The maximum TC reaches 190 K for 1.57×1021 cm−3.

Epitaxial growth of Cd1−xMnxTe films by ionized‐cluster beams and their magneto‐optical properties

Epitaxial growth of Cd1−xMnxTe films with x ranging from 0 to 0.76 has been investigated on sapphire (0001) substrates using the ionized‐cluster beam method. Magneto‐optical properties of the films are discussed using Kramers–Kronig relation. Epitaxial films in the form of a single crystal can be obtained at low substrate temperatures up to 300 °C by the reactive composing of an accelerated CdTe cluster beam and a neutral MnTe cluster beam. When CdTe clusters are only ionized and accelerated, the composition of grown films varies largely, depending on the acceleration voltage. Faraday rotation for Cd1−xMnxTe films represents a band‐edge dispersion, and the intensity increases as Mn concentration x increases; for the film with x=0.76, the maximum value of the Verdet constant was 0.72 °/cm G, which was two orders of magnitude larger than that of a CdTe (x=0) film. The magneto‐optical figure of merit θF/α (H=5 kOe) for a Cd1−xMnxTe (x=0.76) epitaxial film was estimated to be about 400 ° at 660 nm.

Magnetic properties of IV–VI compound GeTe based diluted magnetic semiconductors

Magnetic properties of IV–VI compound GeTe based diluted magnetic semiconductors with 3d transition metals from Ti to Ni have been investigated. Ferromagnetic order is observed for the Cr, Mn, and Fe doped GeTe films, whereas the Ti, V, Co, and Ni doped films are paramagnetic. The ferromagnetic order could give rise to p–d exchange interaction because amplitudes of negative magnetoresistance and the anomalous Hall effect are proportional to that of spontaneous magnetization. The Curie temperatures determined by extrapolating the steep linear part of the temperature dependence of the squared residual magnetization for the Cr, Mn, and Fe doped GeTe films are 12, 47, and 100 K, respectively.

Carrier-enhanced ferromagnetism in Ge1−xMnxTe

We have studied the dependence of magnetic properties on the carrier concentration of IV–VI diluted magnetic semiconductor Ge1−xMnxTe prepared by ionized-cluster beam technique. With increasing carrier concentration, the magnetic properties drastically change; saturation magnetization increases and coercive field decreases in hysteresis loop, and the Curie temperature increases. These results obviously show carrier-enhanced ferromagnetic order. The similar effect is also observed in magnetoresistance behavior.

Ferromagnetic properties of IV–VI diluted magnetic semiconductor Ge1−xMnxTe films prepared by radio frequency sputtering

IV–VI diluted magnetic semiconductor Ge1−xMnxTe films with various Mn compositions up to x=0.53 were prepared on glass substrates by a rf sputtering method. Magnetic measurements indicate that all the Ge1−xMnxTe films (0.07⩽x⩽053) show ferromagnetic order at low temperatures since hysteretic behavior is clearly observed. The effective spin Seff’s of Mn ions deduced from the spontaneous magnetization at 4.2 K are less than 1.0, indicating that Mn ions are partially aligned ferromagnetically. The value of the Seff depends on the carrier concentration as well as the Mn composition. The magnetic order of the Ge1−xMnxTe films is thus caused by the competition between the ferromagnetic Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and the antiferromagnetic superexchange mechanism. The Curie temperature based on the mean field approximation increases linearly with increasing Mn composition up to x=0.3 and above this point tends to saturate around 80 K. On the basis of a simple RKKY mechanism, the spin exchang...

Influences of Potential Barrier Scattering on the Thermoelectric Properties of Sintered n-Type PbTe with a Small Grain Size

Sintered degenerate n-type PbTe samples with a grain size down to about 0.2 ?m were prepared and their thermoelectric properties subsequently measured. Accordingly, the effects of grain size on thermoelectric properties were investigated. For samples with a grain size above ?0.5 ?m, the Seebeck coefficient and thermoelectric figure-of-merit increased with decreasing grain size. In particular, the room-temperature figure-of-merit reached a maximum value of ?4?10-4 K-1, which is about twice as large as the value of a large-grain-size sample with the same carrier concentration reported in the literature. For samples with a grain size below ?0.5 ?m, with decreasing grain size, the Seebeck coefficient increased but the figure-of-merit did not increase because of a substantial decrease in electrical conductivity. The increase in the Seebeck coefficient with decreasing grain size was probably caused by the potential barrier scattering at grain boundaries. On the other hand, the decrease in electrical conductivity is explained mainly by the porosity effect. Thus, we consider that if a sintered PbTe sample with a smaller grain size and a lower porosity is prepared, a larger improvement in the figure-of-merit due to the potential barrier scattering can be obtained.

Magneto-optical Kerr effect of rf-sputtered PtMnSb thin films

Thin films of the C1b‐phase Heusler alloy PtMnSb, prepared by rf sputtering and subsequent annealing, have been studied in order to assess the high magneto‐optical Kerr effect of this compound. The room‐temperature saturation magnetization (Ms) and coercive force (Hc) for PtMnSb thin films after being annealed at 500 °C were about 400 emu/cm3 and 110 Oe, respectively. The easy axis of magnetization was parallel to the film plane for all films. The polar Kerr rotation θK for the PtMnSb films with a C1b phase has a maximum value in excess of 1.8° at 710‐nm wavelength in a 5‐kOe external magnetic field perpendicular to the film plane. Thin films substituted with Ni for Pt, which are an admixture of PtMnSb and an isoelectronic compound NiMnSb, are also examined. The θK peak showed a red shift, in contrast with PtMnSb containing excess Pt concentration, where a blue shift was observed. The intensity reduced significantly with increasing Ni concentration in the films.

Synthesis and Thermoelectric Properties of Silicon Clathrates Sr8AlxGa16-xSi30 with the Type-I and Type-VIII Structures

Nominal composition Sr8AlxGa16-xSi30 (0≤x≤16) samples were prepared by powder metallurgy. The samples with x=0–7 exhibited the type-I clathrate structure (P m3n, No. 223), while the samples with x=8–13 almost exhibited the type-VIII clathrate structure (I43m, No. 217). These samples possessed the electrical conductivities and Seebeck coefficients typical of n-type degenerated semiconductors. The results on their thermoelectric properties indicate that the type-VIII SrAlGaSi clathrate would be a more efficient thermoelectric material than the type-I SrAlGaSi clathrate. For example, the type-VIII Sr8Al9Ga7Si30 sample had a maximum power factor of 11 µW cm-1 K-2 at 1000 K and a room temperature Hall mobility of 24 cm2 V-1 s-1, while those of the type-I Sr8Al6Ga10Si30 sample were 7 µW cm-1 K-2 and 5 cm2 V-1 s-1, respectively.