ИСТИНА

The small gap Fe-based semiconductors FeSi, FeSb2 and FeGa3 attracted much interest because of their prospective thermoelectric applications and intriguing low-temperature properties. They represent a rare class of non-magnetic and semiconducting Fe-based intermetallic compounds characterized by a small hybridization gap of about 0.5 eV at the Fermi level [1,2]. The formation of the energy gap in these 3d-materials is reminiscent of that in strongly correlated 4f Kondo-insulators. Because of the small value of the energy gap and narrow energy bands, these compounds are considered as potential thermoelectric materials demonstrating extremely high Seebeck coefficient values of S ~ 500 μV/K at 50 K and S ~ 45 mV/K at 10 K for FeSi [3] and FeSb2 [4], respectively. Hear we present an overview of application of NMR, NQR and spin-lattice relaxation measurement technique in study of electronic and magnetic properties of strongly correlated Fe-based intermetallic compounds FeSb2 and Fe1-xCoxGa3. Special attention is focused on NMR evidence of the in-gap states formation in FeSb2 and FeGa3. Also nuclear resonance spectroscopy enables to track an evolution of crystal structure, electronic and magnetic properties with Co substitution for Fe in Fe1-xCoxGa3 solid solution [5,6]. In particular, in the parent compound FeGa3 the 69,71Ga spin-lattice relaxation rate 1/T1(T) reveals an unexpected huge maximum at low T with an essentially magnetic relaxation mechanism indicating the existence of the in-gap states. The other end binary compound, CoGa3, is a band metal. It demonstrates the metallic Korringa behavior of the spin-lattice relaxation with 1/T1 ~ T. Surprisingly, in the intermediate Fe0.5Co0.5Ga3 compound 1/T1(T) is strongly (by ~ 2 orders) enhanced due to antiferromagnetic (AF) spin fluctuations with 1/T1 ~ T1/2 in perfect agreement with Moriya’s spin-fluctuation theory [7] for itinerant magnetic systems. Such a 1/T1(T) behavior is a unique feature of weakly and nearly AF metals. This work was partially supported by the RFBR Grant №14-03-31181 mol_a; joint Russian-Taiwan Grant RFBR-NSC № 12-03-92002-NSC_a (101-2923-M-006-001-MY2) and Deutsche Forschungsgemeinschaft (DFG) via TRR 80 (Augsburg-Munich).