Magnetotransport and superconductivity in dilute Fe alloys of Nb${\mathrm{Se}}_2$, Ta${\mathrm{Se}}_2$, and Ta${\mathrm{S}}_2$

Single crystals of dilute iron alloys of the layer structures Nb${\mathrm{Se}}_2$, Ta${\mathrm{Se}}_2$, and Ta${\mathrm{S}}_2$ have been studied using measurements of magnetoresistance and Hall effect in magnetic fields up to 220 kG. Iron concentrations in the range 0-10 at.% can induce spin-exchange scattering, metal-insulator transitions, or enhanced superconducting anisotropy depending on the phase of the crystal and the type of iron doping. Kondo-like resistance minima accompanied by negative magnetoresistance and anomalous Hall effects are observed in the $2H$ phase of ${\mathrm{Fe}}_x\mathrm{Ta}{\mathrm{Se}}_2$ and ${\mathrm{Fe}}_x\mathrm{Nb}{\mathrm{Se}}_2$. A $\ln H$ behavior at high fields and a $\ln T$ behavior at zero field or low fields have been fit to theoretical expressions and antiferromagnetic exchange constants on the order of -0.1 eV are obtained. Excess iron doping of $2H-\mathrm{Ta}{\mathrm{S}}_2$ produces enhanced superconductivity characterized by parameters similar to those observed for organic intercalation. The interlayer coupling strength is consistent with a tunneling model describing both the iron-doped material and the organic intercalates. Substitutional iron doping of Ta${\mathrm{S}}_2$ and Ta${\mathrm{Se}}_2$ stabilizes the $1T$ phase and produces a dramatic rise in resistivity at low temperature following a $T^{-\alpha }$ power-law behavior.