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)${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$. For $2H\ensuremath{-}{\mathrm{Fe}}_{x}\mathrm{Ta}{\mathrm{Se}}_{2}$ 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 $x=0.10$. 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 $2H\ensuremath{-}{\mathrm{Fe}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ 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 $x=0.05$. The Fe-doped $4Hb$ phases of Ta${\mathrm{S}}_{2}$ and Ta${\mathrm{Se}}_{2}$ show distinctly different behavior with $4Hb\ensuremath{-}{\mathrm{Fe}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ showing a strong-local-moment formation and a susceptibility maximum while $4Hb\ensuremath{-}{\mathrm{Fe}}_{x}\mathrm{Ta}{\mathrm{Se}}_{2}$ 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.