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We experimentally study the Mn substitution effect on thermoelectric properties in a full-Heusler alloy, ${\mathrm{Fe}}_{2}\mathrm{TiSi}$. By employing molecular beam epitaxy, homogeneous and $L{2}_{1}$-ordered ${\mathrm{Fe}}_{2}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Si}$ epilayers are achieved. The lattice constant, the saturation magnetic moment, and electrical resistivity are intentionally controlled with increasing Mn substitution. Notably, we find that the sign of the Seebeck coefficient is varied from positive to negative at around $x=0.2$. On the basis of the first-principles calculations, we qualitatively understand that the observed variation in the electrical and thermoelectric properties arises from the change in the electronic band structure near the Fermi level, dominated by Fe and Mn atoms in ${\mathrm{Fe}}_{2}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Si}$.

We experimentally find that the substitution of Si for Al in Heusler-type ${\mathrm{Fe}}_{2}\mathrm{VAl}$ epilayers contributes to the significant reduction in the thermal conductivity, in which ${\mathrm{Fe}}_{2}\mathrm{VAl}$ is one of the next-generation thermoelectric materials without using toxic elements. Because of the low-temperature growth of the epilayers, the Si substitution induces the decrease in the degree of $L{2}_{1}$ ordering, giving rise to the formation of V-Si antisite defects in the epilayers. For ${\mathrm{Fe}}_{2}{\mathrm{VAl}}_{0.57}{\mathrm{Si}}_{0.43}$ epilayers, we can obtain a low thermal conductivity of $\ensuremath{\sim}3.9\phantom{\rule{0.16em}{0ex}}\mathrm{W}/(\mathrm{m}\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, one-third less than Si-substituted ${\mathrm{Fe}}_{2}\mathrm{VAl}$ bulk samples [Lue et al., Phys. Rev. B 75, 064204 (2007)]. We discuss that a possible origin of the low thermal conductivity is related to the low lattice thermal conductivity due to the presence of the V-Si antisite defects.

Fe 2TiSi has been expected to be one of the high-performance thermoelectric full-Heusler alloys. Here, we experimentally clarify the room-temperature Seebeck coefficient ( S) and thermal conductivity ( κ) for the Fe 2TiSi films. Using a molecular beam epitaxy technique, we demonstrate the high degree of L 2 1-ordering and the homogeneous single-phase structure in the Fe 2TiSi films. We find that the intrinsic conduction carriers of the Fe 2TiSi films are holes and impurity-induced carrier scattering is indicated, derived from weak magnetic moments due to the presence of D 0 3-type (Fe ⇔ Ti) structural disorder in the Fe 2TiSi film. From reliable measurements for thin films, the values of S and κ are estimated to be 101 μV/K and 5.6 W/(m K), respectively, at room temperature. This study will open a new way for full-Heusler alloy thermoelectric materials other than those in Fe 2VAl systems.

We study the effect of Fe–V nonstoichiometry on electrical and thermoelectric properties of Fe2VAl films. We find that temperature dependence of electrical resistivity and carrier type for Fe2− x V1+ x Al films are similar to those for bulk samples reported previously. In addition, the electrical and thermoelectric properties can be modulated by varying x. These results indicate that the electronic band structure having a pseudo gap at around the Fermi level is demonstrated even in thin-film Fe2VAl samples. This study will lead to further improvement in thermoelectric properties of the thin-film Fe2VAl.