Magnetic properties of iron-doped layer-structure dichalcogenides

Magnetic-susceptibility measurements have been made on dilute excess Fe alloys of the layer-structure crystals Ta${\mathrm{Se}}_2$, Nb${\mathrm{Se}}_2$, and Ta${\mathrm{S}}_2$. The $2H$ selenides show local-moment formation and obey a Curie-Weiss law in the range 300-30 K with effective moments in the range (2.5-4.5)${\mathrm{\mu }}_{\mathrm{B}}$. For $2H-{\mathrm{Fe}}_x\mathrm{Ta}{\mathrm{Se}}_2$ a strong susceptibility maximum is observed at low temperature and at low fields the maximum has a cusplike shape characteristic of a "spin-glass" type transition. The temperature of the maximum scales approximately linearly with Fe concentration and reaches 13 K for $x=0.10\mathrm{}$. The presence of the magnetic impurities decreases the onset temperature of the charge-density wave and smears the transition. At the same time the magnetic impurities can stabilize a spin-density wave and the data are reviewed in terms of such a possibility. Crystals of $2H-{\mathrm{Fe}}_x\mathrm{Ta}{\mathrm{S}}_2$ do not show local-moment formation and the susceptibility measurements show a smearing of the charge-density wave but no depression of the onset temperature up to $x=0.05\mathrm{}$. The Fe-doped $4Hb$ phases of Ta${\mathrm{S}}_2$ and Ta${\mathrm{Se}}_2$ show distinctly different behavior with $4Hb-{\mathrm{Fe}}_x\mathrm{Ta}{\mathrm{S}}_2$ showing a strong-local-moment formation and a susceptibility maximum while $4Hb-{\mathrm{Fe}}_x\mathrm{Ta}{\mathrm{Se}}_2$ shows only a weak-local-moment formation and no magnetic anomalies. The susceptibility behavior of the various phases also correlates with magnetic anomalies observed in the transport properties of the Fe-doped layer structures and these correlations will also be reviewed.