Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure.

The thermoelectric properties of the n-type semiconductor TiNiSn were optimized by partial substitution with metallic MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the Ti1-xMnxNiSn1-xSbx system. The alloys were prepared by arc-melting and annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing, the majority phase was TiNiSn with some Ni-rich sites, and the minority phases were primarily Ti6Sn5, Sn and MnSn2. The Ni-rich sites were caused by Frenkel defects; this led to metal-like behavior in the semiconductor specimens at low temperature. For x ≤ 0.05 the samples showed an activated conduction, whereas for x > 0.05 they showed metallic character. The figure of merit for x = 0.05 was increased by 61% (zT = 0.45) in comparison with the pure TiNiSn.

Evidence for localized moment picture in Mn-based Heusler compounds.

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) were used to probe the electronic structure and magnetic moment of Mn in Heusler compounds with different crystallographic structure. The results were compared with theoretical calculations of the magnetic and electronic properties, and it was found that in full and half Heusler alloys, Mn is metallic on both sublattices. The magnetic moment is large and localized when octahedrally coordinated by the main group element, consistent with previous theoretical work, and reduced when the main group coordination is tetrahedral. The magnetic and electronic properties of Mn in full and half Heusler compounds are strongly dependent on the structure and sublattice, a fact that can be exploited to design new materials.

Identifying the electronic character and role of the Mn states in the valence band of (Ga,Mn)As.

We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, for both low (1%) and high (13%) Mn doping values, the electronic character of the states near the top of the valence band. Magnetization and temperature-dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.