Magnetotransport and structural properties of the Ni-doped FeTe_0.65 Se_0.35 crystals

The structural disorder, frequently present in crystals of iron chalcogenides, sometimes leads to unexpected improvement of superconducting properties, as reported in the case of FeTe_0.65Se_0.35 crystals [1]. In an effort to find the origin of such behavior, here we study the structure, the Hall effect, and the angle-dependent magnetoresistance (AMR) of the Fe_1−yNi_yTe_0.65Se_0.35 crystals, with y in the range from 0 to 0.08, grown by Bridgman’s method with different cooling rates, slow (S) and fast (F). The S crystals with single, tetragonal phase show inferior superconducting properties to these shown by F crystals, in which an admixture of monoclinic phase is found. The Hall effect of crystals with y>0.03, in which superconductivity is suppressed, confirms the electron doping of both types of crystals, while the AMR is an order of magnitude larger in F crystals, with y-dependent anisotropy at low temperatures (T < 8K), which disappears on warming. The analysis of the AMR suggest that it may originate in the magnetism of monoclinic inclusions, most likely of the type Fe₃(Se-Te)₄. The relation of these findings to superconductivity will be discussed.
Ref.: [1] D. J. Gawryluk, et al., Supercond. Sci. Technol. 24, 065011 (2011).