Magnetic properties of Fe nanodots are simulated using a scaling technique and Monte Carlo method, in good agreement with experimental results. For the 20-nm-thick dots with diameters larger than 60 nm, the magnetization reversal via vortex state is observed. The role of magnetic interaction between dots in arrays in the reversal process is studied as a function of nanometric center-to-center distance. When this distance is more than twice the dot diameter, the interaction can be neglected and the magnetic properties of the entire array are determined by the magnetic configuration of the individual dots. The effect of crystalline anisotropy on the vortex state is investigated. For arrays of noninteracting dots, the anisotropy strongly affects the vortex nucleation field and coercivity, and only slightly affects the vortex annihilation field.

The thermoelectric properties of materials with nominal compositions NaPb 18-x Sn x MTe 20 (M ) Sb, Bi) were investigated in the temperature range 300-650 K. All the members of NaPb 18-x Sn x MTe 20 have a cubic rock-salt (NaCl) type structure and exhibit p-type charge transport behavior between 300-650 K. The relative fraction of Sn strongly affects the physical, structural, and transport properties of the materials. Independent of the nature of the pnicogen atom (M), the electrical conductivity increases with decreasing Pb:Sn ratio, whereas the thermopower decreases. Hall effect data for selected samples, e.g., NaPb 15 Sn 3 BiTe 20 and NaPb 13 Sn 5 SbTe 20 , show high carrier concentrations of ∼1 × 10 20 cm -3 at room temperature. Comparing corresponding members from the antimony and bismuth series, we observed that the Sn-free compositions (x ) 0) exhibit the highest power factors, and as a consequence, the highest ZT, with NaPb 18 BiTe 20 reaching a ZT ≈ 1.3 at 670 K. The NaPb 18-x Sn x MTe 20 series exhibit increasing total thermal conductivity with increasing fraction of Sn with room temperature values between 1.37 W/(m K) for x ) 3 and 3.9 W/(m K) for x ) 16 for NaPb 18-x Sn x SbTe 20 . The lowest lattice thermal conductivity, ∼0.4 W/(m K), was observed for the composition NaPb 2 Sn 16 BiTe 20 at 650 K. High-resolution transmission electron microscopy on several members of the NaPb 18-x Sn x MTe 20 series reveal that they are inhomogeneous on the nanoscale with widely dispersed nanocrystals embedded in a Pb 1-y Sn y Te matrix. Also observed are lamellar features in these materials associated with compositional fluctuations and significant strain at the nanocrystal/matrix interface.

We report the synthesis, microstructure, and transport properties of composite thermoelectric materials based on the eutectic phase relationship between PbTe and Ge. When quenched, these eutectic mixtures exhibit considerably stronger mechanical strength and reduced brittleness compared to PbTe itself, while at the same time they possess lower lattice thermal conductivity. Thermal conductivity measurements show values lower than expected based on the law of mixtures and multiphase composites. We find that the thermoelectric performance in these composites can be tuned through the use of hypereutectic compositions and alloying of Ge with Si. PbI2 was used as an n-type dopant, and precise control of the carrier concentration was achieved to optimize the electrical transport and thermoelectric properties. ZT values approaching 1.3 at 778 K have been obtained in samples of PbTe–Ge0.8Si0.2(5%), which represent an ∼62% improvement over that of PbTe.

Thermoelectric (TE) properties of the coevaporated Bi2Te3 and Sb2Te3 films are measured from 100 to 300 K for Seebeck coefficient αS and from 5 to 300 K for electrical resistivity ρe, mobility μe, and Hall coefficient RH. For the low-temperature characterization of TE films, the conditions for coevaporation deposition of Bi, Te, and Sb to form Bi2Te3 and Sb2Te3 films are also investigated, including substrate material, substrate temperature Tsub, and elemental flux ratio (FR). The resublimation of Te occurring above 473 K significantly affects the film composition and quality. Our optimal deposition conditions for Bi2Te3 films are Tsub=533 K and FR=2.4, and those for Sb2Te3 films are Tsub=503 K and FR=3.0. The TE properties of both films are strongly temperature dependent, while Bi2Te3 films show a stronger temperature dependence than Sb2Te3 films due to different major scattering mechanisms. αS of both the coevaporated films are close to or higher than those of bulk materials, but ρe is much higher (due to lower carrier concentrations for Sb2Te3 films and lower μe for Bi2Te3 films). Also, no freeze-out regime is found for both Bi2Te3 and Sb2Te3 films at low temperatures. The room-temperature power factors of αS2/ρe for Bi2Te3 and Sb2Te3 films are 2.3 and 2.0 mW/K2 m, and the maxima are 2.7 mW/K2 m for Bi2Te3 at T=220 K and 2.1 mW/K2 m for Sb2Te3 at T=280 K. Shadow mask technique is successfully used for the micropatterning (20 μm) of TE films with no significant change in properties.

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