Dilute Fe-doping and Magnetic Properties of Type-I Germanium Clathrates

We have prepared samples of Ba$_{8}$Ge$_{30}$Ga$_{16-x}$Fe$_{x}$ with $x \leq$ 1. Fe substitutes on the type-I clathrate framework, with a lattice parameter decreasing with $x$. Ferromagnetic behavior was observed, with $T_{c}$ = 65 K for $x$ = 1, decreasing linearly to 58 K at $x$ = 0.2. Fe exhibits a low-spin moment: for example 1.9 $\mu _{B}$ per Fe for $x$ = 1, from a Curie fit above $T_{c}$. The field-cooled and zero-field-cooled magnetization exhibit a pronounced divergence below 60 K, and at 2 K the saturation magnetization is only 0.75 $\mu _{B}$ per Fe in a field of 7 T. These results suggests a noncollinear spin configuration. In ac susceptibility measurements, $\chi\prime$ and $\chi\prime\prime$ become frequency dependent below $T_{c}$, however with very little peak shift, rather different from the spin-glass behavior we previously observed in the more highly Fe-doped chiral clathrate. We conclude that the transition near 65 K is ferrimagnetic. At these doping levels, Fe occupancy is well below the percolation threshold. It is thus likely that the mechanism for magnetic ordering is conduction-electron mediation, as in diluted magnetic semiconductors. This work was supported by the Robert A. Welch Foundation, and by the NSF (DMR-0103455).