  Cu-doped type-I germanium clathrate can exhibit dilute magnetism, including
ferromagnetism, antiferromagnetism, and metamagnetic transitions up to 90 K.
63Cu NMR measurements confirm that these transitions are due to a dilute
composition of magnetic defects coupled by conduction electrons, behavior
similar to that of magnetic semiconductors. Magnetic measurements indicate
localized magnetic moments, attributed to clusters of magnetic ions, with
competition between ferromagnetic and antiferromagnetic exchange, and also
indications of glassy behavior in the ferromagnetic phase. NMR Knight shifts
and relaxation times show that the conduction band is metallic with a large
Korringa ratio. Comparison to a mean-field theory for the ordering behavior
gives a good accounting for the ferromagnetic transition.
